the physics of the universe-ii - siddhant singh

7/27/2019 The Physics of the Universe-II - Siddhant Singh

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THE PHYSICS OF THE UNIVERSE

1

The Big Bang and the Big

Crunch—

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Topic Index: Page no.

- Introduction………………………………………………………………………………………(2)

- The Expanding Universe and Hubble’s Law……………………………………..(4)

- Cosmic Background Radiation………………………………………………………….(7)

- Dark Matter……………………………………………………………………………………….(9)

- Cosmic Inflation………………………………………………………………………………..(13)

- Timeline of the Big Bang……………………………………………………………….....(19)

- Accelerating Universe and Dark Energy…………………………………………..(22)

- Antimatter………………………………………………………………………………………..(28)

- The Big Crunch, the Big Freeze and the Big Rip………………………………(30)

- Superstrings and Quantum Gravity…………………………………………………(34)

- Conclusion………………………………………………………………………………………..(39)

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INTRODUCTION

Most scientists now believe that we live in a finite expanding universe which has not existed

forever, and that all the matter, energy and space in the universe was once squeezed into an

infinitesimally small volume, which erupted in a cataclysmic "explosion" which has become

known as the Big Bang.

Thus, space, time, energy and matter all came into being at an infinitely dense, infinitely hot

gravitational singularity, and began expanding everywhere at once. Current best estimates

are that this occurred some 13.7 billion years ago, although you may sometimes see

estimates of anywhere between 11 and 18 billion years.

The Big Bang is usually considered to be a theory of the birth of the universe, although

technically it does not exactly describe the origin of the universe, but rather attempts to

explain how the universe developed from a very tiny, dense state into what it is today. It is

just a model to convey what happened and not a description of an actual explosion, and the

Big Bang was neither Big (in the beginning the universe was incomparably smaller than the

size of a single proton), nor a Bang (it was more of a snap or a sudden inflation).

(The Big Bang and the expansion of the universe)




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In fact, “explosion” is really just an often-used analogy and is slightly misleading in that it

conveys the image that the Big Bang was triggered in some way at some particular centre. In

reality, however, the same pattern of expansion would be observed from anywhere in the

universe, so there is no particular location in our present universe which could claim to be

the origin.

It really describes a very rapid expansion or stretching of space itself rather than an

explosion in pre-existing space. Perhaps a better analogy sometimes used to describe the

even expansion of galaxies throughout the universe is that of raisins baked in a cake

becoming more distant from each other as the cake rises and expands, or alternatively of a

balloon inflating.

Neither does it attempt to explain what initiated the creation of the universe, or what came

before the Big Bang, or even what lies outside the universe. All of this is generally considered

to be outside the remit of physics, and more the concern of philosophy. Given that time and

space as we understand it began with the Big Bang, the phase “before the Big Bang” is as

meaningless as “north of the North Pole”.

Therefore, to those who claim that the very idea of a Big Bang violates the First Law of

Thermodynamics (also known as the Law of Conservation of Energy) that matter and energy

cannot be created or destroyed, proponents respond that the Big Bang does not address the

creation of the universe, only its evolution, and that, as the laws of science break down

anyway as we approach the creation of the universe, there is no reason to believe that the

First Law of Thermodynamics would apply.

The Second Law of Thermodynamics, on the other hand, lends theoretical (albeit

inconclusive) support to the idea of a finite universe originating in a Big Bang type event. If

disorder and entropy in the universe as a whole is constantly increasing until it reaches

thermodynamic equilibrium, as the Law suggests, then it follows that the universe cannot

have existed forever, otherwise it would have reached its equilibrium end state an infinite

time ago, our Sun would have exhausted its fuel reserves and died long ago, and the

constant cycle of death and rebirth of stars would have ground to a halt after an eternity of

dissipation of energy, losses of material to black holes, etc.

The Big Bang model rests on two main theoretical pillars: the General Theory of Relativity

(Albert Einstein’s generalization of Sir Isaac Newton’s original theory of gravity) and the

Cosmological Principle (the assumption that the matter in the universe is uniformly

distributed on the large scales, that the universe is homogeneous and isotropic).

The Big Bang (a phrase coined, incidentally, by the English astronomer Fred Hoyle during a

1949 radio broadcast as a derisive description of a theory he disagreed with) is currently

considered by most scientists as by far the most likely scenario for the birth of universe.

However, this has not always been the case, as the following discussion illustrates.




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THE EXPANDING UNIVERSE AND HUBBLE'S LAW

When Albert Einstein was formulating his ground-breaking theory of gravity in the early 20th

Century, at a time when astronomers only really knew of the existence of our own galaxy, he

necessarily used the simplifying assumption that the universe has the same gross properties

in all parts, and that it looks roughly the same in every direction wherever in the universe anobserver happens to be located. Like Sir Isaac Newton two hundred years before him, he

assumed an infinite, static or “steady state” universe, with its stars suspended essentially

motionless in a vast void.

However, when Einstein tried to apply his General Theory of Relativity to the universe as a

whole, he realized that space-time as whole must be warped and curved back on itself,

which in itself would cause matter to move, shrinking uncontrollably under its own gravity.

Thus, as early as 1917, Einstein and others realized that the equations of general relativity

did not describe a static universe. However, he never quite came to terms with the idea of adynamic, finite universe, and so he posited a mysterious counteracting force of cosmic

repulsion (which he called the “cosmological constant”) in order to maintain a stable, static

universe. Adding additional and arbitrary terms to a theory is not something that scientists

do lightly, and many people argued that it was an artificial and arbitrary construct and at

best a stop-gap solution.

As we have noted, up until that time, the assumption of a static universe had always been

taken for granted. To put things into perspective, for most of history (see the section on

Cosmological Theories Through History), it had been taken for granted that the static earth

was the centre of the entire universe, as Aristotle and Ptolemy had described. It was only in

the mid-16th Century that Nicolaus Copernicus showed that we were not the centre of the

universe at all (or even of the Solar System for that matter!). It was as late as the beginning

of the 20th Century that Jacobus Kapteyn’s observations first suggested that the Sun was at

the centre of a spinning galaxy of stars making up the Milky Way. Then, in 1917, humanity

suffered a further blow to its pride when Curtis Shapely revealed that we were not even the

centre of the galaxy, merely part of some unremarkable suburb of the Milky Way (although

it was still assumed that the Milky Way was all there was).

Some years later, in 1925, the American astronomer Edwin Hubble stunned the scientific

community by demonstrating that there was more to the universe than just our Milky Way

galaxy and that there were in fact many separate islands of stars - thousands, perhaps

millions of them, and many of them huge distances away from our own.

Then, in 1929, Hubble announced a further dramatic discovery which completely turned

astronomy on its ear. With the benefit of improved telescopes, Hubble started to notice that

the light coming from these galaxies was shifted a little towards the red end of the spectrum

due to the Doppler effect (known as “redshift”), which indicated that the galaxies were

moving away from us. After a detailed analysis of the redshifts of a special class of starscalled Cepheids (which have specific properties making them useful as “standard candles” or

distance markers), Hubble concluded that the galaxies and clusters of galaxies were in fact




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flying apart from each other at great speed, and that the universe was therefore definitively

growing in size. In effect, all the galaxies we see are slighly red in colour due to redshift.

Hubble showed that, in our expanding universe, every galaxy is rushing away from us with a

speed which is in direct proportion to its distance, known as Hubble’s Law, so that a galaxy

that is twice as far away as another is receding twice as fast, one ten times as far away if receding ten times as fast, etc. The law is usually stated as v = H0D, where v is the velocity of

recession, D is the distance of the galaxy from the observer and H0 is the Hubble constant

which links them. The exact value of the Hubble constant itself has long been the subject of

much controversy: Hubble's initial estimates were of the order of approximately 500

kilometres per second per megaparsec (equivalent to about 160 km/sec per million light

years); the most recent best estimates, with the benefit of the Hubble Telescope and the

WMAP probe, is around 70 kilometres per second per megaparsec.

This expansion, usually referred to as the "metric expansion" of space, is a “broad-brush

effect” in that individual galaxies themselves are not expanding, but the clusters of galaxies

into which the matter of the universe has become divided are becoming more widely

separated and more thinly spread throughout space. Thus, the universe is not expanding

"outwards" into pre-existing space; space itself is expanding, defined by the relative

separation of parts of the universe. Returning to the image of the expanding universe as a

balloon inflating, if tiny dots are painted on the ballon to represent galaxies, then as the

balloon expands so the distance between the dots increases, and the further apart the dots

the faster they move apart.




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(Artist's impression of the "metric expansion" of the universe)

In such an expansion, then, the universe continues to look more or less the same from every

galaxy, so the fact that we see all the galaxies receding from us does not necessarily mean

that we are at the very centre of the universe: observers in all other galaxies would also see

all the other galaxies flying away according to the same law, and the pattern of galactic

dispersal would appear very much the same from anywhere in the cosmos.

The old model of a static universe, which had served since Sir Isaac Newton, was thus proved

to be incontrovertibly false, but Hubble’s discovery did more than just show that the

universe was changing over time. If the galaxies were flying apart, then clearly, at some

earlier time, the universe was smaller than at present. Following back logically, like a movie

played in reverse, it must ultimately have had some beginning when it was very tiny indeed,

an idea which gave rise to the theory of the Big Bang. Although now almost universally

accepted, this theory of the beginnings of the universe was not immediately welcomed by

everyone, and several strands of corroborating evidence were needed, as we will see in the

following sections.

In the face of Hubble’s evidence, E instein was also forced to abandon his idea of a force of

cosmic repulsion, calling it the “biggest blunder” he had ever made. But others, notably the




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Russian physicist Alexander Friedmann and the Belgian priest and physicist Georges

Lemaître, had already used Einstein’s own theory of prove that the universe was in fact in

motion, either contracting or expanding. It is now recognized that Einstein’s description of

gravity as the curvature of space-time in his General Theory of Relativity was actually one of

the first indications of a universe which had grown out of much humbler beginnings.

And, as we will see later, Einstein’s “biggest blunder” may actually turn out to have been one

of his most prescient predictions.

COSMIC BACKGROUND RADIATION

The Ukrainian-American physicist George Gamow was the first to realize that, because the

universe is all there is, the huge heat from a hot Big Bang could not dissipate in the same

way as the heat from a regular explosion and therefore it must still be around today.

Gamow's research students, Ralph Alpher and Robert Herman, moreover, argued in 1948

that, because the Big Bang effectively happened everywhere simultaneously, that energy

should be equally spread as cosmic microwave background radiation (or CMB for short)

throughout the universe.

This radiation was emitted approximately 300,000 years after the Big Bang, before which

time space was so hot that protons and electrons existed only as free ions, making theuniverse opaque to radiation. It should be visible today because, after this time, when

temperatures fell to below about 3,000°K, ionized hydrogen and helium atoms were able to

capture electrons, thus neutralizing their electric charge (known as “recombination”), and

the universe finally became transparent to light.




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("Black body" spectrum of the cosmic microwave background radiation)

In 1965, Arno Penzias and Robert Wilson, two young employees of Bell Telephone

Laboratories in New Jersey, discovered, although totally by accident, exactly that. The

mysterious microwave static they picked up on their microwave antenna seemed to be

coming equally from every direction in the sky, and eventually they realized that thismicrowave radiation (which has a temperature of about -270°C, marginally above absolute

zero, and the coldest thing found in nature) must indeed be the “afterglow” of the Big Bang.

Penzias and Wilson received the 1978 Nobel Prize in Physics for their discovery (although,

strangely, Gamow’s contribution was never recognized).

It was later confirmed that the intensity of these microwaves at different wavelengths traces

out a “black body” or “thermal” curve, consistent with radiation that has been brought into

balance with its environment - just what would be expected if they were indeed a relic of an

early hot “fireball” stage. This discovery, perhaps the most important cosmological discovery

since Edwin Hubble had shown that we live in an expanding universe, was powerful evidence

that our universe had indeed begun in a hot, dense state and had been growing and cooling

ever since.

(WMAP colour-enhanced picture of cosmic microwave background radiation - colours

indicate 'warmer' (red) and 'cooler' (blue) spots)

The same photons that were around in the early stages of the Big Bang, then, have been

propagating ever since, though growing fainter and less energetic as they fill a larger and

larger universe. So ubiquitous is this cosmic microwave background radiation that, even

though each cubic centimetre contains just 300 photons of it, in total it makes up 99% of all

the photons in the universe (the remaining 1% being in starlight). It has been estimated that

1% of the “snow” which appears on a TV screen tuned betwe en stations is attributable to

cosmic background radiation!




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In view of the importance of cosmic microwave background radiation to the Big Bang model

of the universe (no other model has explained CMB quite so neatly), efforts were redoubled

in an attempt to definitively prove the connection, first in the form of the Cosmic

Background Explorer (COBE) satellite in 1989, and then the Wilkinson Microwave Anisotropy

Probe (WMAP) in 2001. Both probes have confirmed the predicted data with increasing

accuracy, as well as providing the most detailed picture we have of how the universe looked

soon after the Big Bang, and establishing the age of the universe with much greater accuracy

at 13.7 billion years.

Another indirect indication that the universe began with a Big Bang is wrapped up in the

very fact that the night sky we see from Earth is black. Olbers’ Paradox, named after the 19th

Century German astronomer, Heinrich Wilhelm Olbers, who was one of the first to start to

think of the universe as a whole. Olbers (who definitively stated the problem in 1823,

although several others, dating back to the time of Newton, had previously posed similar

ideas in various ways) asked why, if the universe was studded with billion upon billions of stars, the night sky was not completely lit up with the light from all these stars.

The answer (first pointed out, interestingly enough, by the author Edgar Allen Poe in 1848)

lies in the fact that the light from the more distant stars, in fact from the majority of the

objects in the universe, has still to reach us. The only stars and galaxies we see are those

close enough that their light has taken less than the 13.7 billion years since the Big Bang to

reach us. For the same reason, the most distant objects visible (those recorded with

sensitive equipment like the Hubble Space Telescope) appear to consist of much younger

galaxies, only recently formed, or consisting mainly of glowing diffuse gas not yetfragmented into stars.

Another apparent paradox is the question of why, given that the universe started off as

much hotter than the centre of the hottest star, all the primordial nuclei of hydrogen were

not instantly transmuted into the tightly-bound and ultra-stable nuclei of iron (the final state

of fusion process). In that case, no long-lived stars could ever have existed in our present

universe as all the available fuel would have been used up in the initial fireball, and the

universe as we know it would have been a non-starter. In fact, the ultra-hot conditions of

the first few minutes of the expansion only lasted long enough to turn about 23% of the

hydrogen into helium and tiny traces of lithium. It turns out that even the oldest objects in

the universe contain about 23-24% of helium, and this confirms calculations which predict

that hydrogen and helium are the only elements which would be created prolifically in a Big

Bang event.

DARK MATTER

The simple Big Bang theory is, however, not without its potential problems, and some

aspects require further investigation and explanation. One such problem is the rather




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unfortunate fact that about 90% of the matter which is predicted to exist in the universe

appears to be invisible or otherwise unaccounted for!

The evenness of the cosmic microwave background radiation (the afterglow of the initial Big

Bang) suggests that the matter emitted from the Big Bang should have been spread around

very smoothly. But we know that the universe is in fact clumpy, with clusters of galaxies andgreat voids of empty space in between. Actually, in 1992, NASA’s Cosmic Background

Explorer (COBE) satellite did discover some variations or ripples in the brightness of the

afterglow, which probably resulted from a period about 450,000 years after the Big Bang,

when some parts of the universe became just a few thousandths of a per cent denser than

others. These barely noticeable clumps of matter grew to become bigger clumps due to the

cumulative effects of gravity, and the denser regions (the “seeds” of structure) became ever

denser over time, leading to the great clusters of galaxies we see today.

However, the modelling of this theory revealed that the 13.7 billion years which has elapsed

since the Big Bang is actually nowhere near long enough for the huge structures of today’s

universe to have developed, by the gradual process of gravity and increasing density, out of

the tiny imperfections and clumps indicated by the COBE satellite. This could only have

happened if there was, and/or is, much more matter in the universe than our current

estimates of the matter tied up in visible stars. This has led to speculation about so-called

"dark matter", an unknown substance which emits no light, heat, radio waves, nor any other

kind of radiation (thus making extremely hard to detect).




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(Ring of dark matter in the galaxy cluster Cl 0024+17)

The idea of dark matter, though, goes back much further than that. The stars in spiralgalaxies like our own Milky Way whirl about the galactic centre, prevented from flying off

into intergalactic space by gravity. However, calculations of the speed of the whirling, dating

back to work by maverick astronomer Fritz Zwicky in the 1930s, suggest that the galaxy is

actually spinning much faster than it theoretically should be in order to maintain its current

equilibrium. Zwicky hypothesized that the only way this could occur was if galaxies, ours and

all the others, actually contained much more matter (he estimated at least ten times as

much) as is visible in stars, spread reasonably evenly thoughout the galaxy.

Zwicky's observations were backed up by more accurate data gathered by Vera Rubin in the

1960s,. and by Jim Peebles and Jerry Ostricker in the 1970s. Rubin noted that stars right out

near the edge of the galaxy were orbiting around the galactic centre at the same speed as

stars much closer in (whereas in our solar system, for example, the innermost planets orbit




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much faster than those further out). It therefore appeared that the force of gravity did not

get weaker the further a star was from the centre of the galaxy, which flew in the face of all

that was known about gravity. Other more recent studies have shown that even hydrogen

gas out on the fringes of the galaxy is still orbiting just as fast as the inner stars.

Thus, it appears that around 90% of the mass making up galaxies must be composed of anunknown, invisible substance which came to be known as dark matter. The same thing also

applies on a larger scale to entire clusters of galaxies, millions of light years across, which

would also need to contain about ten times more material than we can see in order to hold

together. This is almost exactly the factor of additional matter required by the models to

allow the structures we see in today’s universe to have developed from the ripples in the

cosmic microwave background radiation discovered by the COBE satellite.

(Super-Kamiokande, a neutrino observatory in Japan)




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More recent studies, using modern gravitational lensing techniques, have added further

confirmations, if any were needed, and have allowed the creation of a kind of "map" of dark

matter which shows how galaxies and clusters of stars tend to form around, and within, the

largest blobs of dark matter, which forms a kind of all-pervading halo around the visible

objects of the universe. In this way, Carlos Frenk has produced a dramatic 3D simulation of

the dark matter throughout the whole visible universe, showing what he calls the "skeleton"

of the universe, or the "scaffolding" around which galaxies and clusters of galaxies have

formed. It seems that everything we know is ultimately dependent on dark matter - without

dark matter there would be no galaxies; without galaxies there would be no stars; without

stars there would be no planets, and therefore no life.

The problem is that dark matter, whatever it may be, is invisible and extremely hard to

detect. It is affected by gravity, but not by any of the other fundamental forces; it has no

electrical charge; it does not seem to stick or clump together but floats freely; and it passes

through atoms of normal matter without any kind of interference we can detect. In fact, itappears not even to interact with itself: colliding galaxies have been observed, where the

normal matter of the two galaxies re-coalesces together as expected, but the dark matter

just keep on going along its original path regardless.

So, despite its apparent ubiquity, no-one really knows what dark matter is. Among the

possible candidates are so-called MACHOs (short for MAssive Compact Halo Objects), such

as small brown and black dwarf stars, cold unattached planets, comet-like lumps of frozen

hydrogen, tiny black holes, possibly even mini dark galaxies. Astronomers are using a

technique known as gravitational lensing to try to spot where such matter might lie.

Scientists are also investigating another kind of exotic particle, known as WIMPs (short for

Weakly Interacting Massive Particles), hypothetical super-symmetrical particles which may

be all around us but which pass through normal matter without stopping and without

interacting in any way. Experiments to look for WIMPs are being carried out in highly-

shielded, super-cooled facilities deep down in rocky mines where other interfering cosmic

rays cannot penetrate.

Neutrinos and other so-called “exotic particles” are another possibility. Neutr inos are tiny

elementary particles which have no electric charge and hardly interact at all with ordinary

atoms, and which mysteriously may even move faster than the speed of light. It is

hypothesized that they could have come into existence during the first second after the Big

Bang as part of the reaction with the photons that were created at that time, and it is

calculated that there could be hundreds of millions of them for every atom in the universe,

with millions of them passing through you and I and everything around us every second. So,

even if each neutrino weighed a hundred-millionth as much as an atom, they could

theoretically still be the dominant matter in the universe.

COSMIC INFLATION




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Another conundrum thrown up by the basic Big Bang theory is how to explain the relative

homogeneity and evenness of the temperature of the cosmic microwave background

radiation. How did large-scale structures such as galaxies and clusters of galaxies develop out

of what should have been a rather boringly amorphous and featureless fireball?

This would appear to be in direct violation of the Second Law of Thermodynamics, whichdescribes an inexorable tendency towards entropy and uniformity and away from patterns

and structures. If our universe had started out completely smooth, then it should have

continued that way, and the universe today would contain nothing more than thinly spread

dark matter along with less than one atom per cubic metre of hydrogen and helium gas, with

no sign of the texture and complexity we see around us (stars, galaxies, a multitude of

elements, life).

However, even very slight irregularities in the early phases of expansion would have become

amplified as slightly dense patches are affected by additional gravity until they condensed

into self-contained structures held together by their own gravity. Galaxies crashed and

merged and cannibalized their neighbours, and larger scale structures like clusters and

super-clusters formed by a continuing process of gravitational aggregation working on these

newly formed galaxies.

(The horizon problem of the Big Bang model)

Heat tends to travel from a hot body to a cold one so that the temperatures of both bodies

eventually even out (a result of the Second Law of Thermodynamics itself), like hot coffee in

a cold cup. The microwave background radiation discovered by Arno Penzias and Robert




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Wilson in the 1960s appeared to be extremely uniform throughout the observable universe,

with almost no variance. But if, as the evidence suggests, the last time the cosmic

background radiation had any contact with matter was about 450,000 years after the Big

Bang (by which time the universe had cooled to around 3,000°C), then this presents a

paradox, because the universe at that time would already have had a diameter of around 90

million light years, and just not enough time would have elapsed for radiation or heat to

have flown around the whole universe and equalized itself, and the horizons could never

have actually been in causal contact with each other (known as the “horizon problem”).

So, in theory, there actually ought to be even more variation today than there is. That is,

unless the very early universe was in fact much smaller than the models were predicting. The

most widely accepted theory as to how this might have been possible is known as cosmic

inflation, which was first proposed in 1980 by the American physicist Alan Guth, developed

out of Steven Weinberg’s Electroweak Theory and Grand Unified Theory.

As we will see, the addition of inflation to the Big Bang model claimed to solve the horizon

problem, as well as one or two other potential problems that had been identified with the

standard Big Bang theory, such as the “flatness problem” (why the density of matter in the

universe appears “fine-tuned” to be very close to the critical value at which space is

perfectly flat rather than a non-Euclidean hyperbolic or spherical shape) and the “magnetic

monopole problem” (why the magnetic monopoles which theory suggests should have been

produced in the high temperatures of the early universe appear not to have persisted to the

present day).




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(Graph of the expansion of the observable universe with inflation)

Cosmic inflation is the idea that the very early universe went through a period of

accelerated, exponential expansion during the first 10-35 of a second before settling down

to the more sedate rate of expansion we are still experiencing, so that all of the observable

universe originated in a small (indeed, microscopic) causally-connected region.

Although the universe has been expanding since the initial Big Bang, inflation refers to the

hypothesis that, for a very short time, the universe expanded at a sharply INCREASING rate,

rather than at the decreasing rate it followed before inflation and has followed since. By

some calculations, inflation increased the size of the universe by a factor of around 1026

during that tiny fraction (far less than a trillionth) of a second, expanding it from smaller than

the size of a proton to about the size of a grapefruit.

Technically, the expansion during this period of inflation (and even the somewhat slower

expansion which succeeded it) proceeded faster than the speed of light. To explain how this

is possible (the speed of light being supposedly the maximum speed it is possible to travel),

an analogy may help. If two airplanes are flying directly away from each other at their

maximum speed of, say, 500 kilometres per hour, they are actually flying apart at 1,000

kilometres per hour even though neither individual plane is exceeding 500km per hour.

Thus, "expansion", in terms of the expanding universe, is not the same thing as "travel".




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It is still not clear to scientists, however, exactly what caused the inflationary phase, the best

guess being some kind of a negative "vacuum energy density" (or positive "vacuum

pressure") triggered by the separation of the strong nuclear force from the other elementary

forces at this time. It is hypothesized that this separation caused a kind of symmetry

breaking or phase transition (analagous to the phase transition when water turns to ice),

which left the universe in a highly unstable state with much more energy than it would

otherwise have had, causing a sharp outward antigravitational effect, smoothing out most of

the irregularities in the existing matter and creating vast quantities of particles in a very

short time.

(Under inflation, the observable universe is merely a tiny part of the whole that lies within

our horizon)

This theory allows for some kind of very slight unevenness (so-called quantum fluctuations)on a sub-atomic scale at a very early stage in the growth of the universe which provided

starting points for the large-scale structures we see in today’s universe. This suggests the




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rather bizarre possibility that the largest structures in the universe, the great clusters of

galaxies, may actually have been spawned by sub-microscopic seeds.

Guth hypothesized that the reason why the universe appears to be flat is because it is

actually fantastically big (in the same way that the spherical Earth appears flat to those on its

surface), and that the observable universe is actually only a very small part of the actualuniverse. In fact, Guth’s calculations suggest that the entire universe may be at least 1023

times bigger than the size of the observable universe (the part within the horizon, that we

are able, at least in principle, to see), roughly equal to the ratio of the size of the observable

universe to the planet Earth. Thus, although the observable universe may appear to be

effectively flat, the entire universe may be completely different in nature. Also, although an

enormous number of magnetic monopoles could well have arisen in the inflationary early

universe, the chances of actually observing even one magnetic monopole are infinitesimally

small in a universe of such immense size.

Thus, the incredibly vast and fast expansion of the universe caused by inflation “solved” both

Robert Dickes flatness problem and Guth’s own monopole problem. But it also solved the

horizon problem: according to the inflation theory, the universe blew up so quickly that

there was no time for the essential homogeneity to be broken, and the universe after

inflation would therefore have been very uniform, even though the parts of it were not still

in touch with each other.

In an attempt to prove the inflation theory, the Cosmic Background Explorer (COBE) probe

was launched in 1992, and its initial results confirmed almost exactly the amount of variation

in the cosmic microwave background radiation that was predicted by inflationary theory. In

2003, the Wilkinson Microwave Anisotropy Probe (WMAP) demonstrated the existence of

these non-uniformities with even greater precision.

Guth’s theory has been very influential, even if he himself could find no way to end inflation

so that stars and galaxies could form (known the "graceful exit" problem), and he considered

his own theory something of a failure because of this. There have been many other

refinements and revisions since Guth's original model, such as the “new inflationary model”

of Russian physicist Andrei Linde, who had been working on an inflation theory

independently (as had Paul Steinhardt and Andreas Albrecht). This new model hypothesized

a slow (as opposed to Guth’s fast) breaking of symmetry, and the creation of many "bubble

universes" (just one of which contains our own observable universe). A later proposal by

Linde, known as the “chaotic inflationary model”, hypothesized that the repulsive antigravity

effect was caused by a “spin-0 field” rather than any kind of phase transition as Guth had

thought.

Linde's work, and that of fellow Russian Alex Vilenkin, has also given rise to the idea of

“eternal inflation”, where the inflation as a whole actually never stops, but small localized

energy discharges within the overall energy field - almost like sparks of static electricity, buton on a cosmic scale - create small points of matter in the form of tiny particles. Such a

process may represent the birth of a new universe, such as our own. Beginning in this way

with what we have called a Big Bang, this new universe then itself proceeds to expand,




19

although at a much slower rate than the continuing inflation outside of it. The rest of space

outside of that universe is still full of undischarged energy, still expanding at enormous

speed, and new universes, new Big Bangs, are occurring all the time.

The theory of cosmic inflation, then, supports the scenario in which our universe is just one

among many parallel universes in a multiverse. As we will see in later sections, somecorroborating evidence for such a scenario also arises from work on dark energy, on

superstring theory and on quantum theory. However, the idea of a hypothetical multiverse,

which we can never see or prove, is anathema to many physicists, and many critics still

remain.

TIMELINE OF THE BIG BANG

Since the Big Bang, 13.7 billion years ago, the universe has passed through many different

phases or epochs. Due to the extreme conditions and the violence of its very early stages, it

arguably saw more activity and change during the first second than in the all the billions of

years since.

From our current understanding of how the Big Bang might have progressed, taking into

account theories about inflation, Grand Unification, etc, we can put together an approximate

timeline as follows:

Planck Epoch (or Planck Era), from zero to approximately 10 -43 seconds (1 Planck

Time):

This is the closest that current physics can get to the absolute beginning of time, and

very little can be known about this period. General relativity proposes a gravitational

singularity before this time (although even that may break down due to quantum

effects), and it is hypothesized that the four fundamental forces (electromagnetism,

weak nuclear force, strong nuclear force and gravity) all have the same strength, and

are possibly even unified into one fundamental force, held together by a perfect sym-

metry which some have likened to a sharpened pencil standing on its point (i.e. too

symmetrical to last). At this point, the universe spans a region of only 10-35 metres (1

Planck Length), and has a temperature of over 1032°C (the Planck Temperature).

Grand Unification Epoch, from 10 –43 seconds to 10 –36 seconds:

The force of gravity separates from the other fundamental forces (which remain

unified), and the earliest elementary particles (and antiparticles) begin to be created.

Inflationary Epoch, from 10 –36 seconds to 10 –32 seconds:

Triggered by the separation of the strong nuclear force, the universe undergoes an

extremely rapid exponential expansion, known as cosmic inflation. The linear dimen-

sions of the early universe increases during this period of a tiny fraction of a second

by a factor of at least 1026 to around 10 centimetres (about the size of a




20

grapefruit). The elementary particles remaining from the Grand Unification Epoch (a

hot, dense quark-gluon plasma, sometimes known as “quark soup”) become distrib-

uted very thinly across the universe.

Electroweak Epoch, from 10 –36 seconds to 10 –12 seconds:

As the strong nuclear force separates from the other two, particle interactions create

large numbers of exotic particles, including W and Z bosons and Higgs bosons (the

theoretical Higgs field slows some of the particles down and confers mass on them,

allowing a universe made entirely out of radiation to support things that have mass).

Quark Epoch, from 10 –12 seconds to 10 –6 seconds:

Quarks, electrons and neutrinos form in large numbers as the universe cools off to below

10 quadrillion degrees, and the four fundamental forces assume their present forms.

Quarks and antiquarks annihilate each other upon contact, but, in a process known

as baryogenesis, a surplus of quarks (about one for every billion pairs) survives, which

will ultimately combine to form matter.

(Timeline and major events since the Big Bang)

Hadron Epoch, from 10 –6 seconds to 1 second:

The temperature of the universe cools to about a trillion degrees, cool enough to al-

low quarks to combine to form hadrons (like protons and neutrons). Electrons collid-

ing with protons in the extreme conditions of the Hadron Epoch fuse to form neutrons

and give off massless neutrinos, which continue to travel freely through space today,

at or near to the speed of light. Some neutrons and neutrinos recombine into new

proton-electron pairs. The only rules governing all this apparently random combining

and re-combining are that the overall charge and energy (including mass-energy) be

conserved.




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Lepton Epoch, from 1 second to 3 minutes:

After the majority (but not all) of hadrons and antihadrons annihilate each other at

the end of the Hadron Epoch, leptons (such as electrons) and antileptons (such as

positrons) dominate the mass of the universe. As electrons and positrons collide and

annihilate each other, energy in the form of photons is freed up, and colliding

photons in turn create more electron-positron pairs.

Nucleosynthesis, from 3 minutes to 20 minutes:

The temperature of the universe falls to the point (about a billion degrees) where

atomic nuclei can begin to form as protons and neutrons combine through nuclear

fusion to form the nuclei of the simple elements of hydrogen, helium and lithium.

After about 20 minutes, the temperature and density of the universe has fallen to the

point where nuclear fusion cannot continue.

Photon Epoch (or Radiation Domination), from 3 minutes to 240,000 years:

During this long period of gradual cooling, the universe is filled with plasma, a hot,

opaque soup of atomic nuclei and electrons. After most of the leptons and

antileptons had annihilated each other at the end of the Lepton Epoch, the energy of

the universe is dominated by photons, which continue to interact frequently with the

charged protons, electrons and nuclei.

Recombination/Decoupling, from 240,000 to 300,000 years:

As the temperature of the universe falls to around 3,000 degrees (about the same

heat as the surface of the Sun) and its density also continues to fall, ionized hydrogen

and helium atoms capture electrons (known as “recombination”), thus neutralizing

their electric charge. With the electrons now bound to atoms, the universe finallybecomes transparent to light, making this the earliest epoch observable today. It also

releases the photons in the universe which have up till this time been interacting

with electrons and protons in an opaque photon-baryon fluid (known as

“decoupling”), and these photons (the same ones we see in today’s cosmic

background radiation) can now travel freely. By the end of this period, the universe

consists of a fog of about 75% hydrogen and 25% helium, with just traces of lithium.

Dark Age (or Dark Era), from 300,000 to 150 million years:

The period after the formation of ther first atoms and before the first stars is

sometimes referred to as the Dark Age. Although photons exist, the universe at thistime is literally dark, with no stars having formed to give off light. With only very

diffuse matter remaining, activity in the universe has tailed off dramatically, with

very low energy levels and very large time scales. Little of note happens during this

period, and the universe is dominated by mysterious “dark matter”.

Reionization, 150 million to 1 billion years:

The first quasars form from gravitational collapse, and the intense radiation they

emit reionizes the surrounding universe, the second of two major phase changes of

hydrogen gas in the universe (the first being the Recombination period). From this

point on, most of the universe goes from being neutral back to being composed of

ionized plasma and galaxies.

Star and Galaxy Formation, 300 - 500 million years onwards:




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(The process of star formation)

Gravity amplifies slight irregularities in the density of the primordial gas and pockets

of gas become more and more dense, even as the universe continues to expand

rapidly. These small, dense clouds of cosmic gas start to collapse under their own

gravity, becoming hot enough to trigger nuclear fusion reactions between hydrogenatoms, creating the very first stars.The first stars are short-lived supermassive stars, a

hundred or so times the mass of our Sun, known as Population III (or “metal -free”)

stars. Eventually Population II and then Population I stars also begin to form from the

material from previous rounds of star-making. Larger stars burn out quickly and

explode in massive supernova events, their ashes going to form subsequent

generations of stars. Large volumes of matter collapse to form galaxies and

gravitational attraction pulls galaxies towards each other to form groups, clusters and

superclusters.

Solar System Formation, 8.5 - 9 billion years:Our Sun is a late-generation star, incorporating the debris from many generations of

earlier stars, and it and the Solar System around it form roughly 4.5 to 5 billion years

ago (8.5 to 9 billion years after the Big Bang).

Today, 13.7 billion years:

The expansion of the universe and recycling of star materials into new stars

continues.

ACCELERATING UNIVERSE AND DARK ENERGY




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Like dark matter, cosmic inflation (even if it is not actually proven beyond all doubt) is now

usually seen as part of the standard Big Bang theory, and to some extent the two additional

concepts rescue the Big Bang theory from being completely untenable. However, other po-

tential problems still remain.

The universe has continued to expand since the Big Bang, albeit at a slower rate since theperiod of inflation, while at the same time the gravity of all the matter in the universe is

working to slow down and eventually reverse the expansion. Two main possibilities therefore

present themselves: either the universe contains sufficient matter (known as the "critical

mass") for its gravity to reverse the expansion, causing the universe to collapse back to what

has become known as the “Big Crunch”, a kind of mirror image of the initial Big Bang; or it

contains insufficient matter and it will go on expanding forever.

According to General Relativity, the density parameter, Omega, which is defined as the aver-

age density of the universe divided by the critical density (i.e. that required for the universe

to have zero curvature) is related to the curvature of space. If Omega equals 1, then the

curvature is zero and the universe is flat; if Omega is greater than 1, then there is positive

curvature, indicating a closed or spherical universe; if Omega is less than 1, then there is

negative curvature, suggesting an open or saddle-shaped universe.

The cosmic inflation model hypothesizes an Omega of exactly 1, so that the universe is in fact

balanced on a knife’s edge between the two extreme possibilities. In that case, it will contin-

ue expanding, but gradually slowing down all the time, finally running out of steam only in

the infinite future. For this to occur, though, the universe must contain exactly the critical

mass of matter, which current calculations suggest should be about five atoms per cubic

metre (equivalent to about 5 x 10-30

g/cm3).

This perhaps sounds like a tiny amount (indeed it is much closer to a perfect vacuum than

has even been achieved by scientists on Earth), but the actual universe is, on average, much

emptier still, with around 0.2 atoms per cubic metre, taking into account visible stars and

diffuse gas between galaxies. Even including dark matter in the calculations, all the matter in

the universe, both visible and dark, only amounts to about a third of the required critical

mass, suggesting a continuously expanding universe.




24

(Graph of how critical density affect the expansion of the universe)

However, in 1998, two separate teams of astronomers observing distant type 1a supernovas

(one led by the American Saul Perlmutter and the other by the Australians Nick Suntzeff and

Brian Schmidt) made parallel discoveries which threw the scientific community into disarray,

and which also has important implications for the expanding universe and its critical mass.

The faintness of the supernova explosions seemed to indicate that they were actually further

away from the Earth than had been expected, suggesting that the universe’s expansion had

actually speeded up (not slowed) since the stars exploded. Contrary to all expectations,

therefore, the expansion of the universe actually seems to be significantly speeding up - welive in an accelerating universe!

The only thing that could be accelerating the expansion (i.e. more than countering the

braking force of the mutual gravitational pull of the galaxies) is space itself, suggesting that

perhaps it is not empty after all but contains some strange “dark energy” or “antigravity”

currently unknown to science. Thus, even what appears to be a complete vacuum actually

contains energy in some currently unknown way. In fact, initial calculations (backed up by

more recent research such as that on the growth of galaxy clusters by NASA's Chandra x-ray

space telescope and that on binary galaxies by Christian Marinoni and Adeline Buzzi of the

University of Provence) suggest that fully 73 - 74% of the universe consists of this dark

energy.




25

Given that around 22% of the universe has been attributed to dark matter (see the section

on Dark Matter for more discussion of this), this suggests that only around 4% of the

universe consists of what we think of as "normal", everyday, atom-based matter (such as

stars, intergalactic gas, etc). However, nowadays this is generally accepted as the "standard

model" of the make-up of the universe. So, for all our advances in physics and astronomy, it

appears that we can still only see, account for and explain a small proportion of the totality

of the universe, a sobering thought indeed.

(Estimated distribution of dark matter and dark energy in the universe)

Incorporating dark energy into our model of the universe would neatly account for the"missing" three-quarters of the universe required to cause the observed acceleration in the

revised Big Bang theory. It also makes the map of the early universe produced by the WMAP

probe fit well with the currently observed universe. Carlos Frenk's beautiful 3D computer

models of the universe resembles remarkably closely (taking dark matter and dark energy

into account) the actual observed forms in the actual universe, even if not all scientists are

convinced by them. Alternative theories, such as Mordehai Milgrom's idea of "variable

gravity", are as yet poorly developed and would have the effect of radically modifying all of

physics from Newton onwards. So dark energy remains the most widely accepted option.

Further corroboration of some kind of energy operating in the apparent vacuum of space

comes from the Casimir effect, named after the 1948 experiments of Dutch physicists

Hendrik Casimir and Dirk Polder. This shows how smooth uncharged metallic plates can

move due to energy fluctuations in the vacuum of empty space, and it is hypothesized that

dark energy, generated somehow by space itself, may be a similar kind of vacuum

fluctuation.

Unfortunately, like dark matter, we still do not know exactly what this dark energy is, how it

is generated or how it operates. It appears to produce some kind of a negative pressure

which is distributed relatively homogeneously in space, and thereby exerts a kind of cosmic

repulsion on the universe, driving the galaxies ever further apart. As the space between the

galaxies inexorably widens, the effects of dark energy appears to increase, suggesting that




26

the universe is likely to continue expanding forever, although it seems to have little or no

influence within the galaxies and clusters of galaxies themselves, where gravity is the

dominant force.

Although no-one has any idea of what dark energy may actually be, it appears to be

unsettlingly similar to the force of cosmic repulsion or “cosmological constant” discarded byEinstein back in 1929 (as mentioned in the section on The Expanding Universe and Hubble’s

Law), and this remains the most likely contender, even if its specific properties and effects

are still under intense discussion. The size of the cosmological constant needed to describe

the accelerating expansion of our current universe is very small indeed, around 10-122 in

Planck units. Indeed, the very closeness of this to zero (without it actually being zero) has

worried many scientists. But even a tiny change to this value would result in a very different

universe indeed, and one in which life, and even the stars and galaxies we take for granted,

could not have existed.

Perhaps equally worrying is the colossal mismatch between the infinitesimally small

magnitude of dark energy, and the value predicted by quantum theory, our best theory of

the the very small, as to the energy present in apparently empty space. The theoretical value

of dark energy is over 10120 times smaller than this, what some scientists have called the

worst failure of a prediction in the history of science! Some scientists have taken some

comfort about the unexpectedly small size of dark energy in the idea that ours is just one

universe in an unimaginably huge multiverse. Out of a potentially infinite number of parallel

universes, each with slightly different properties and dark energy profiles, it is not so unlikely

that ours just happens to be one with a dark energy that allows for the development of starsand even life, an example of the anthropic principle.




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(Representation of the Higgs mechanism and particle)

There has been some speculation that dark energy may be connected to the little

understood (and still entirely theoretical) Higgs field. According to the theoretical work of

the English physicist Peter Higgs and others in the 1960s, the vacuum of space is actually

permeated by what has become known as a Higgs field. It is the interactions with this fieldthat gives the other elementary particles their mass, as it stops them from flying off at the

speed of light by clustering around them and impeding their progress.

Excitations of the Higgs field form particles known as Higgs bosons, an essential component

of the current Standard Model of particle physics, but unfortunately also likewise theoretical

and unproven. It is hoped that experiments underway at the Large Hadron Collider at CERN

may be able to isolate, and prove the existence of, the Higgs boson, considered the Holy

Grail of particle physics. As of the end of 2011, tantalizing "glimpses" of just such a particle

have in fact been found, although not yet sufficient for a definitive scientific announcement.

Another possible candidate for dark energy arises from the theoretical work on

supersymmetry, which effectively doubles the number of elementary particles in the current

Standard Model with the postulation of massive unknown “super-partners” for each particle,




28

whose spin differs by ½. Yet other candidates are so-called “quintessence” and so-called

“phantom energy”, although these ideas are essentially still at the hypothesis stage.

Neither is it clear whether the effects of dark energy are constant or changing over time,

although research using data from the Hubble Space Telescope suggests that it was already

at work boosting the expansion of the universe as much as nine billion years ago.

ANTIMATTER

Another area which perhaps needs some additional explanation is the concept of antimatter,

and why our universe consists almost entirely of matter and hardly any antimatter.

According to theory, the Big Bang should have produced matter and antimatter in equalquantities. Thus, for every quark produced in the early stages of the Big Bang, there would

also have been an antiquark; for every electron, a positron (the antiparticle of the electron);

etc. The apparent asymmetry of matter and antimatter in the visible universe is one of the

greatest unsolved problems in physics.

The British physicist Paul Dirac first predicted the existence of antimatter in 1928. For each

of his theoretical equations, there appeared to exist another associated solution, with all the

properties reversed, which did not seem to physically exist in the known universe. This

antimatter, then, is the “mirror image” of matter, and the antiparticles of which it is

composed are the mirror images of normal particles, being the same size but having

opposite electrical charge.

Dirac’s equations also predicted that, if enough energy could be concentrated, an

antielectron (always accompanied by an electron in order to preserve the overall electrical

charge) could in theory be produced where none had existed before! In 1933, Carl Anderson

successfully demonstrated the appearance of this hypothetical antielectron (which he called

the positron), and definitively showed that matter could in fact be created in the laboratory

in a controlled experiment. With the development of super-high-acceleration machines after

World War II, other particles (such as protons and neutrons) and their respective

antiparticles were created, and even stored in magnetic “bottles”.

However, when matter and antimatter meet, they completely annihilate each other in a

brilliant flash of light produced by extremely high-energy gamma photons. This explosive

annihilation mirrors the huge energy required to produce the matter-antimatter pairs in the

first place.




29

(Pair production and pair annihilation of hydrogen and antihydrogen particles)

For example, the high-energy cosmic rays which regularly impact the Earth's atmosphere

produce minute quantities of antimatter in the resulting particle jets, which are immediately

annihilated by contact with nearby matter. The tiny quantities of antimatter which scientists

have managed to create in the laboratory have always been accompanied by an equal

quantity of normal matter, and the two tend to cancel each other out almost immediately.

While it is technically possible that substantial amounts of antimatter do exist somewhere in

the universe, isolated in some way from normal matter, no substantial quantities of

antimatter have actually been discovered. Which begs the question of why this hugeapparent imbalance exists, and why all matter and antimatter did not just annihilate each

other completely very early in the history of the universe (and therefore, ultimately, why we

are here at all!)




30

It is assumed that, very early in the life of the universe, in a process known as baryogenesis,

massive numbers of particles and antiparticles were created and did in fact annihilate each

other. The cosmic microwave background radiation which pervades the universe today

represents the remains of the energy produced by this wholesale annihilation of the

matched particle-antiparticle pairs. But a small imbalance remained, in the form of an excess

of matter, of the order of one extra matter particle per billion matter-antimatter particle

pairs. It has been calculated that this apparently tiny imbalance in the early universe would

be sufficient to make up the amount of matter presently observable in the universe.

In 1966, the Russian physicist Andrei Sakharov outlined three conditions necessary for a

matter-antimatter imbalance to be possible: first, protons must decay, but so slowly that for

all the protons in the Earth, fewer than a bread crumb's worth should have decayed so far;

second, there must be specific constraints on the way in which the universe has cooled after

the Big Bang; and third, there must be a measurable difference between matter and

antimatter.

James Cronin and Val Fitch won the Nobel Prize in the 1960s for their work on a particle

called the kaon, which showed that particles and their antiparticles might not in fact be

exact opposites, and it does seem possible that kaons might actually live longer than

antikaons, but it is still far from clear whether this could account for the triumph of matter

over antimatter in the universe.

THE BIG CRUNCH, THE BIG FREEZE AND THE BIG RIP

Clearly, further advances in fundamental physics are required before it will be possible to

know the ultimate fate of the universe with any level of certainty. However, scientists

generally agree that this fate will depend on three things: the universe’s overall shape or

geometry, on how much dark energy it contains, and on the so-called “equation of state”

(which essentially determines how the density of the dark energy responds to the expansionof the universe).

If the geometry of the universe is “closed” (like the surface of a sphere), then there are two

main possibilities, as has been mentioned in the section on Accelerating Universe and Dark

Energy. If the universe has a large amount of dark energy (as recent findings suggest it may

well have), then the expansion of the universe could theoretically continue forever. If,

however, the universe lacks the repulsive effect of dark energy, then gravity will eventually

stop the expansion of the universe and it will start to contract until all the matter in the

universe collapses to a final singularity, a mirror image of the Big Bang known as the "BigCrunch”, somewhere in the region of a hundred billion years from now.




31

Models of a collapsing universe of this kind suggest that, at first, the universe would shrink

more or less evenly, because, on a gross scale, matter is reasonably consistently distributed.

At first, the rate of contraction would be slow, but the pace would gradually pick up. As the

temperature begins to increase exponentially, stars would explode and vaporize, and

eventually atoms and even nuclei would break apart in a reverse performance of the early

stages after the Big Bang.

As the universe becomes compacted into a very small volume, any slight irregularities will

become ever more magnified and, in the final stages, the collapse will probably be wildly

chaotic, and gravity and the warping of space-time will vary immensely depending on the

direction the singularity is approached by an in-falling body. According to some predictions,

very close to the singularity, the warpage of space-time will become so violent and chaotic

that space and time will actually “shatter” into “droplets” and all current concepts of time,

distance and direction will become meaningless.

(The expansion and contraction of a closed universe to a Big Crunch)

This model offers intriguing possibilities of an oscillating or cyclic universe (or “Big Bounce”),

where the Big Crunch is succeeded by the Big Bang of a new universe, and so on, potentially

ad infinitum. However, in the light of recent findings in the 1990s (such as the evidence for




32

an accelerating universe described previously), this is no longer considered the most likely

outcome.

If, on the other hand, the geometry of space is “open” (negatively curved like the surface of

a saddle), or even “flat”, the possiblities are very different. Even without dark energy, a

negatively curved universe would continue expanding forever, with gravity barely slowingthe rate of expansion. With dark energy thrown into the equation, the expansion not only

continues but accelerates, and just how things develop depends on the properties of the

dark energy itself, which remain largely unknown to us.

One possibility is where the acceleration caused by dark energy increases without limit, with

the dark energy eventually becoming so strong that it completely overwhelms the effects of

the gravitational, electromagnetic and weak nuclear forces. Known as the “Big Rip”, this

would result in galaxies, stars and eventually even atoms themselves being literally torn

apart, with the universe as we know it ending dramatically in an unusual kind of gravitational

singularity within the relatively short time horizon of just 35 - 50 billion years.

Perhaps the most likely possibility, however, based on current knowledge, is a long, slow

decline known as the "Big Freeze" (or the “Big Chill” or “Heat Death”). In this scenario, the

universe continues expanding and gradually “runs down” to a state of zero thermodynamic

free energy in which it is unable to sustain motion or life. Eventually, over a time scale of

1014 (a hundred trillion) years or more, it would reach a state of maximum entropy at a

temperature of very close to absolute zero, where the universe simply becomes too cold to

sustain life, and all that would remain are burned-out stars, cold dead planets and black

holes.




33

(Possible shapes of the universe (closed, open and flat))

What happens after that is even more speculative but, eventually, even the atoms making

up the remaining matter would start to degrade and disintegrate, as protons and neutrons

decay into positrons and electrons, which over time would collide and annihilate each other.

Depending on the rate of expansion of the universe at that time, it is possible that some

electrons and positrons may form bizarre atoms billions of light years in size, known as

positronium, with the distant particles orbiting around each other so slowly it would take a

million years for them to move a single centimetre. After perhaps 10116 years, even the

positronium will have collapsed and the particles annihilated each other.

In this way, all matter would slowy evaporate away as a feeble energy, leaving only black

holes, ever more widely dispersed as the universe continues to expand. The black holes

themselves would break down eventually, slowly leaking away "Hawking radiation", until,

after 10200 years, the universe will exist as just empty space and weak radiation at atemperature infinitesimally above absolute zero. At the end of the universe, time itself will

lose all meaning as there will be no events of any kind, and therefore no frame of reference

to indicate the passage of time or even its direction.




34

Interestingly, recent analyses from the WMAP satellite and the Cosmic Background Imager,

seem to be confirming other recent observations indicating that the universe is in fact flat

(as opposed to closed or open). These experiments have revealed hot and cold spots with a

size range of approximately one degree across, which, according to current theory, would be

indicative of a flat universe.

SUPERSTRINGS AND QUANTUM GRAVITY

To fully understand questions like where the universe came from, why the Big Bang occurred

13.7 billion years ago and what, if anything, existed before it, we need to better understand

singularities like those in black holes and the singularity which marked the birth of the

universe itself.

In order to achieve that, most scientists agree that a “quantum theory of gravity” (also

known as "quantum gravity" or "unification" or the “theory of everything”) is needed, which

combines the General Theory of

Relativity (our current best theory of the very large) and quantum theory (our current best

theory of the very small). These may seem like fundamentally incompatible concepts, and

even Einstein, who devoted most of the latter part of his life to unification, came up short.But attempts are nevertheless continuing on several fronts to find just such a synthesis.

In the 1970s, the strongest candidate for a unified theory was probably “supergravity”, a

field theory combining the principles of supersymmetry and general relativity. But, although

the approach appeared promising, it soon became apparent that the calculations involved

were so long and difficult that it may never be provable. Around 1984, however, largely in

response to a ground-breaking paper by Michael Green and John Schwarz, there was a

remarkable change of opinion in the world of theoretical physics in favour of string theory

(or, more specifically, superstring theory), a paradigm shift sometimes referred to as the"First Superstring Revolution".

String theory had first been posited in the late 1960s as a result of work by Gabriele

Veneziano, Leonard Susskind and others. It views the basic building blocks of matter not as

point-like particles but as unimaginably small one-dimensional vibrating “strings” of energy,

which have length but no other dimension, like infinitely thin pieces of string or twine. A

string may be open (i.e. have ends) or closed (i.e. joined up in loops), and the history of a

string over time is represented by a two-dimensional strip (for open strings) or tube (for

closed strings).

There might seem to be an inconsistency between the idea of a universe composed of

strings and the point-like particles we actually observe in experiments. However, this is

because the strings are so tiny that we cannot resolve their shape, even with our best




35

technology, so that they just appear to us as tiny featureless points, like the difference

between a speck of dust seen with the naked eye and under a microscope. To give some

idea of the scales involved, a string is as small compared to an electron as a mouse is to the

whole Solar System (around 20 order of magnitude smaller).

(Artist's impression of the fundamental entities of superstring theory by Flavio Robles)

But the real beauty of string theory is that it looks on everything in the universe, all matter

and all forces as well, as being made up of one single ingredient. Strings are composed of

super-concentrated mass-energy which vibrate like a violin strings, with each distinct

vibration mode corresponding to a fundamental particle (such as an electron or a photon,

etc). The emission or absorption of one particle by another is represented by the dividing or

joining together of strings, and the forces acting on particles correspond to other strings

linking the particle strings in a complex “web”.

According to string theory, then, the universe is a kind of symphony and the laws of physics

are its harmonies. The vibrations of strings, however, occur in a ten-dimensional world, with

each one-dimensional point in our ordinary space actually consisting of a complicated

geometrical structure in six dimensions, all wrapped up on the scale of the Planck length (the




36

smallest distance or size about which anything can be known, approximately 1.6 × 10-35

metres). The vibratory quality of these tiny threads of energy is what replaces particles and

fields in the quantum description of the universe. The strength of the vibrations is what we

see in the world as mass, and the patterns of vibrations are the fundamental forces.

The speculation on incorporating additional dimensions into space-time goes back to theideas of the Polish physicist Theodor Kaluza in 1919 and, independently, the Swedish physicist

Oscar Klein in 1926. They asked why it was not possible that electromagnetism could be uni-

fied with gravity in a notional five-dimensional universe, or that perhaps the electromagnetic

force may relate to some curvature in a fifth dimension, just as gravity is due to curvature in

four-dimensional space-time, as demonstrated by Einstein’s General Theory of Relativity. In

to the 10 dimensional theory known as superstring theory (shorthand for "supersymmetric

string theory") after the discovery of a symmetrical mathematical object called a “Calabi-Yao

shape”.

(3-D projection of a multi-dimensional Calabi-Yao manifold)

General Relativity, which implicitly interprets gravity as curvature in four-dimensional space-

time, is built in to the basic precepts of superstring theory in a way that may be consistent




37

with quantum mechanics, and so it is hoped that the long-sought synthesis between gravity

and quantum theory will naturally emerge. In fact, over ten dimensions (in which all but the

four we are familiar with are “curled up” into tiny strings with diameters on the order of the

Planck scale), it may even be possible that all the fundamental forces in nature can be

accommodated into one “theory of everything”, known as quantum gravity.

Superstring theory may also go some way towards explaining another problem which has

dogged physicists for years: why gravity is so very weak compared to the other fundamental

forces. If strings, which are too small for us to see or measure, incorporate other

dimensions, then it has been posited that perhaps the effects of gravity can only be felt in

their entirety at the level of higher dimensions which we cannot perceive. However, the very

fact that strings are too small for us to see (and probably too small for us to EVER see) have

led some to question whether string theory is science at all, or whether it falls into the realm

of philosophy.

The validation of superstring theory, though, is all in the mathematics, and it remains

frustratingly abstract and theoretical, particularly as we are clearly not able to actually

observe such tiny objects, nor to clearly visualize the multi-dimensional aspects. Moreover,

at least five different and competing superstring theories have developed, none of which are

conclusive, however elegant. Since Ed Whitten's contribution to the field in 1995, though,

there is some evidence that the inclusion of an eleventh dimension might be able to

reconcile these competing theories, to show them as being just five different way of looking

at the same thing. It might also make superstring theory consistent with supergravity theory

(which had been largely disregarded since the early 1980s).




38

(Artist's visualization of rippling membranes)

With the additional eleventh dimension, the fundamental building block of the universe was

therefore no longer a string but a “membrane” or “brane”, leading to the theory's designa-

tion as “membrane theory” or “M-Theory”, first described by M-Theory pioneer Bert Ovrut in

2001. It soon became clear, though, that the new eleventh dimension was, if anything, evenstranger than the other spacial dimensions of superstring theory, being infinitely long but

only 10-23 metres wide, so that it theoretically exists at less than a trillionth of a millimetre

from every point in our three-dimensional world but is totally insensible to us.

M-Theory and the incorporation of an eleventh dimension is also consistent with the

existence of a multiverse, a convenient but ultimately unprovable solution to many of the

more intransigent problems in theoretical physics. For example, if the membranes move and

ripple, as it is supposed they do, then events like singularities (and the Big Bang itself) can be

visualized as the result of chance collisions between rippling, wave-like membranes, with the

initial Big Bang of our universe being just one of many in the constant encounters between

membranes in parallel universes.

This vision of the eleventh dimension suggests a much more violent and active place than the

early visualizations of membranes serenely floating in space. It also suggests that time can in

fact be followed back though the initial singularity of the Big Bang of the universe we know

to the parallel universes which gave rise to it (in what is sometimes described as the "Big

Splat"), a possible solution to an intractible problem which has dogged physicists since the

Big Bang theory was first mooted. This all conjures up the rather unsettling idea of an infinite

number of universes, potentially each with different laws of physics, of which ours is just a

single insignificant member, part of an endless multiverse where Big Bangs are taking place

all the time.

But the existence of parallel universes seems to provide plausible solutions to most of the

outstanding problems with the theory. For example, some physicists (notably Lisa Randall)

if the strings that we experience as gravity (known as gravitons) are not open-ended strings

which are tied down to our three-dimensional membrane or universe (as are the strings of

particles and other forces), but self-contained closed loops of string which are therefore freeto escape into other dimensions we are not able to experience. Or, alternatively, if we are

only experiencing small leaks of the full force from other nearby membranes (and other uni-

verses).

Superstring theory (and its off-shoot, M-Theory), though, is by no means the only candidate

for a "theory of everything" which is being pursued. Indeed, some physicists think that it has

been a disaster for science, taking many of the best brains off on a wild goose chase. Other

approaches include "loop quantum gravity" (in which space is represented by a network

lengths of space, evolving over time in discrete steps), "causal dynamical triangulation" (a

background independent approach which attempts to show how the space-




39

time fabric itself evolves), "causal sets" (an approach which assumes that space-time is

fundamentally discrete and that space-time events are related by a partial order) and even a

recent one called “An Exceptionally Simple Theory of Everything”.

CONCLUSION

The theory of the Big Bang, as modified by the inclusion of dark matter, cosmic inflation and

dark energy, is still the best explanation we have for the origin of the universe. However,

there are still gaps and inconsistencies in our knowledge, and perhaps the nagging suspicion

that the more we learn and the more questions we answer, the more there is to learn and

the more new questions arise.

Since the 1980s, steps have been taken towards a “quantum theory of gravity”, such as the

theory of superstrings mentioned in the previous section, steps which many physicist believe

are necessary before we can advance any further in our understanding of the universe.

However, the mathematics involved is hugely complicated, the tiny scale is inherently

unobservable, and it is difficult to tell just how much progress is actually being made, and

how much of the enthusiasm being shown is merely due to the elegance and the compelling

apparent “rightness” of the theory.

It is apparent, though, that the laws of physics and the fundamental forces that have led to

the creation of the universe as we know it (with all the complexity of stars and galaxies, a

complex and interactive periodic table of elements, intelligent life, etc), are extremely sensit-

ive to any change. For example, even a relatively slight difference in the ratio of the strength

of the strong force holding atoms together to the force of gravity (about 1038) would result in

a much shorter or longer life for stars and much less favourable conditions for complex

evolution, quickly leading to a featureless, sterile universe. If the very small mass difference

between neutrons and protons (about one part in a thousand) were changed by only a factor

of two, then the abundance of elements in the universe would be radically different from

that observed today.




40

(Artist's impression of parallel universes making up a multiverse)

For some, the extent of these apparent coincidences and "fine tuning" have led them toattribute it to the hand of God and so-called “intelligent design”. Others have invoked the

“anthropic principle” that this universe appears to be fine-tuned for life, specifically human

life, and therefore could not be any other way (if it were, then would not be here to observe

it).

As for the oft-posed question of what was there before the Big Bang, physics as it stands has

no answer, and such a question is considered effectively meaningless by most physicists. If

matter, space and time all came into being with the singularity we call the Big Bang, then so

did the concerns of physics, and any discussion of what came before is therefore an exercisein metaphysics and philosophy, not physics. If pressed, most scientists would probably have

to answer: “As far as we know, nothing”.




41

New work on eleven dimensional M-theory, though is suggesting plausible answers to even

this audacious question. It is hypothesized that the universe that we inhabit is just one of a

potentially infinite number of parallel universes (the “multiverse”), some of which may have

the same physical laws and fundamental forces but fine-tuned slightly differently, and some

of which may have an entirely different set of laws and forces. What we think of as the Big

Bang was just one of many collsions between rippling membranes in the eleventh

dimension, and merely the result of two parallel universes momentarily coming together.




42




43

Special and General

Relativity—

-----------------------------------------------------------------------------------------------------


- Introduction……………………………………………………………………..(44)

- Speed of Light and the Principle of Relativity……………....…..(44)

- Special Theory of Relativity……………………………………………....(47)

- Space-Time………………………………………………………………………..(50)

- E = mc2……………………………………………………………………………….(52)

- Gravity and Acceleration………………………………………………..….(55)

- Curved Space…………………………………………………………….……….(58)

- General Theory of Relativity…………………………………………..….(61)

- Conclusion…………………………………………………………………………(63)

------------------------------------------------------------------------------------------------------




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INTRODUCTION

The “theory of relativity” (or simply “relativity”) generally refers to two theories of Albert

Einstein, his Special Theory of Relativity (or simply special relativity) of 1905, and his General

Theory of Relativity (or general relativity) of 1916. Along with quantum theory, relativity is

one of the two main planks on which almost the whole of modern physics is built.

The idea of relativity had been studied almost three centuries earlier by Galileo, when he

stated the principle of relativity in 1632 (that the fundamental laws of physics are the same

for all bodies in uniform motion). Later in the 17th Century, Sir Isaac Newton also took the

principle of relativity for granted, asserting that if his famous laws of motion held in one

inertial frame, then they also held in a reference frame moving at a constant velocity relative

to the first frame.

As we will see, Einstein’s theories are somewhat more involved, even if his starting point was

in many respects the same. His ground-breaking theories take into account the speed of

light, the structure of space-time and the equivalence of acceleration and gravity. They have

led to some remarkable consequences, including the dilation of time, the contraction of

length, mass-energy equivalence and the bending of light, as well as the prediction of the

existence of black holes, wormholes and the “birth” of the universe in a Big Bang.

Einstein's theories still hold up well today, after exhaustive experimentation and testing, and

have been described as the single most important contribition by one man to science.

SPEED OF LIGHT AND THE PRINCIPLE OF RELATIVITY

Logically, one would expect the ultimate cosmic speed limit to be infinity, which after all is

defined as the biggest number imaginable. However, in our universe, the relatively modest

speed of 300,000 kilometres per second, the speed of light, is the de facto maximum speed,

and in practice one can never catch up with a beam of light. It was the 16 year old Albert

Einstein who first gave serious consideration to why this might be the case, in the final years

of the 19th Century.

The speed of light had been measured often and very accurately, going back to the Danish

astronomer Ole Rømer (or Roemer) who had shown in 1675 that light travels at a finite

(although very high) speed. Rømer’s observations of the moons of Jupiter yielded a speed of

light of about 225,000 km/s, although subsequent, more accurate, experiments have

actually shown it to be 299,792,458 metres per second (about 300,000 km/s).




45

In 1868, the equations of the Scottish mathematician and physicist James Clerk Maxwell,

building on the earlier work of Ampère, Coulomb and Faraday, noted that all

electromagnetic waves travelled at exactly the same speed as light in empty space, and that

light itself was a kind of wave rippling through the invisible magnetic and electric fields.

Maxwell concluded that light and other electromagnetic waves should travel at a certain

fixed speed relative to some unconfirmed ambient medium he called “aether”.

The famous Michelson-Morley experiments of 1887, in a failed attempt to prove that light

travels through a medium known as aether, had unexpectedly demonstrated that light

travels at the same speed regardless of whether in was measured in the direction of the

Earth’s motion or at right angles to it. At least this is the case when light travels through a

vacuum: when light moves from medium to medium (like from air to glass, for example), its

speed can of course change depending on the new medium's index of refraction, and this

“bending” of light is essentially how lenses work, as had long been understood.

(The reasoning used by Rømer in 1675 to determine the speed of light)

Thus, whether a source of light is moving towards you or away from you, the light still travelsat a steady 300,000 km/s, completely contrary to classical physics and common sense. It was

the young Einstein's genius to explain just WHY the speed of light is constant and does not

depend on the speed of its source or its observer. In 1905, Einstein (and also the French




46

mathematician Henri Poincaré, who was coming to similar conclusions at around the same

time, although from a more mathematical point of view) realized that the whole idea of

aether as a medium for light to travel in was totally unnecessary, providing, as we will see,

that one was willing to abandon the idea of absolute time.

Einstein also realized that that Maxwell’s equations led to an apparent paradox orinconsistency in the laws of physics, because it suggested that if one could catch up to a

beam of light one would see a stationary electromagnetic wave, which is an impossibility.

Einstein hypothesized, therefore, that the speed of light actually plays the role of infinite

speed in our universe, and that in fact nothing can ever travel faster than light (and certainly

that nothing in the universe could ever travel at anything like infinite speed). It should be

noted that Einstein did not actually PROVE the constancy of the speed of light in all frames of

reference. Rather, it is an axiom (an underlying assumption) from which he derived the rest

of his theory. The axiom can be experimentally verified, but it is not proven in any theoretic

sense.

The constant speed of light was to become one of the two main planks of his Special Theory

of Relativity, which we will examine in more detail in the next section. The other main plank

was the "principle of relativity" (or "principle of invariance"), an idea first stated by the great

Italian physicist Galileo Galilei as early as 1632. Galileo argued that the mechanical laws of

physics are the same for every inertial observer (those moving uniformly with constant

speed in a straight line), and therefore that, purely by observing the outcome of mechanical

experiments, one cannot distinguish a state of rest from a state of constant velocity.

(The principle of relativity says that the laws of physics are the same in all inertial systems)

Galileo used the example of a ship travelling at constant speed, without rocking, on a

smooth sea, and he noted that any observer doing experiments in a dark room below deck

would not be able to tell whether the ship was moving or stationary. As a slightly updated

example, a ball thrown in an airplane flying at 800 kilometres per hour 12,000 metres above

the Earth follows the same path, and is indistinguishable from, one thrown in the airplane at

rest on the ground.




47

When he combined the principle of relativity with the constant speed of light, it became

clear to Einstein that the speed of light was also independent of the speed of the observer

(as well as of the speed of the source of the light), and that everyone in the universe, no

matter how fast they were moving, would always measure the speed of light at exactly the

same 300,000 km/s.

SPECIAL THEORY OF RELATIVITY

In the Special Theory of Relativity, published in his so-called “miraculous year” of 1905,

Einstein had the audacity to turn the question around and ask: what must happen to our

common notions of space and time so that when the distance light travels in a given time is

measured, the answer is always 300,000 km/s? For example, if a spaceship fires a laser beamat a piece of space debris flying towards it at half the speed of light, the laser beam still

travels at exactly the speed of light, not at one-and-a-half times the speed of light. He began

to realize that either the measurement of the distance must be smaller than expected, or

the time taken must be greater than expected, or both.

In fact, Einstein realized, the answer is both: space “contracts” and time “dilates” (or slows).

Some of the motion through space can be thought of as being "diverted" into motion

through time (and vice versa), in much the same way as a car travelling north-west diverts

some of its northwards motion towards the west. Thus, the dimensions of space and timeaffect each other, and both space and time are therefore relative concepts, with only the

unvarying speed of light providing the bedrock on which the universe is built. This

revolutionary idea flew in the face of the long-held notion of simultaneity (the idea that

events that appear to happen at the same time for one person should appear to happen at

the same time for everyone in the universe) and suggested that it was impossible to say in

an absolute sense whether two events occurred at the same time if those events were

separated in space.




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( At relativistic speeds, space “contracts” and time “dilates”)

In a nutshell, the Special Theory of Relativity tells us that a moving object measures shorter

in its direction of motion as its velocity increases until, at the speed of light, it disappears. It

also tells us that moving clocks run more slowly as their velocity increases until, at the speed

of light, they stop running altogether. In fact, it also tells us (as we will see in subsequent

sections) that the mass of a moving object measures more as its velocity increases until, at

the speed of light, it becomes infinite.

Thus, one person’s interval of space is not the same as another person’s, and time runs at

different rates for different observers travelling at different speeds. To some extent, the

faster you go, the slower you age and the slimmer you are! The reason this is not obvious in

everyday situations is that the differences at everyday speeds are infinitesimally small, and

only really become apparent at speeds approaching that of light itself (“relativistic” speeds).

The closer the speed of an objects approaches to the speed of light, the more warped

lengths and time intervals become.

The amount of length contraction and time dilation is given by the Lorentz factor, named

after the Dutch physicist Hendrik Lorentz, who had been exploring such transformation

equations since as early as 1895, long before Einstein began his work (indeed some would

claim that Lorentz and Henri Poincaré between them anticipated almost everything in

Einstein's Special Theory of Relativity). The Lorentz factor, γ (gamma) is given by the

equation γ ≡ , so that the effect increases exponentially as the object's velocity v approachesthe speed of light c. Thus, the calculations show that at 25% of the speed of light, the effect

is just 1.03 (a mere 3% slowing of time or contraction of length); at 50% of the speed of light,

it is just 1.15; at 99% of the speed of light, time is slowed by a factor of about 7; and at




49

99.999, the factor is 224. So, if it were possible to travel in a spaceship at, say, 99.5% of the

speed of light, a hypothetical observer looking in would see the clock moving about 10 times

slower than normal and the astronaut inside moving in slow-motion, as though through

treacle.

A couple of real-life examples may help to make the effects of special relativity clearer.Experiments have been carried out where two identical super-accurate atomic clocks were

synchronized, and then one was flown around the world on an airplane while the other

stayed at home. The clock which travelled recorded marginally less passage of time than the

other (as predicted by the theory), although the difference was of course minimal due to the

relatively slow speeds involved. Our fastest military airplanes can only travel at about

1/300,000 of the of the speed of light, so the time dilation effect γ is onl y about a ten-

thousandth of 1%.

At very high speeds, however, the effect is much more noticeable. Experiments have

demonstrated that an ultra-short-lived muon particle, which habitually travels at 99.92% of

the speed of light, actually lives about 25 times longer and travels about 25 times further

than it theoretically should. Particles travelling at speeds up to 99.99% the speed of light in

the CERN particle accelerator in Switzerland experience the same kind of relativity-induced

time travel, experiencing a γ factor of around 5,000, allowing the artificial persistence of

even shorter-lived particles such as phi mesons.

(In the "twins effect" (or paradox), a space traveller returns to Earth younger than his twin)

So, travelling at close to the speed of light would theoretically allow time travel into the

future, as time slows down for the speeding object in order to "protect" the cosmic speed

limit of the speed of light. A corollary of all this is that, if it were possible to exceed the

speed of light, then it would also be possible to go back in time, which raises the possibility




50

of time-travel paradoxes (where a person goes back in time and interferes in their own past

or kills their own grandparents, etc), although some scientists believe that some as yet

undiscovered law of physics may intervene to prevent such paradoxes. Actually, special

relativity does not specifically forbid the existence of particles that travel faster than light,

and there is a hypothetical sub-atomic particle called a tachyon, which would indeed spend

its entire life travelling faster than the speed of light, but it is currently still hypothetical.

Another phenomenon associated with the dilation of time is the so-called “twins effect”

(sometimes referred to as the “twins paradox”), where an astronaut returns from a near-

light speed voyage in space to find his stay-at-home twin many years older than him (as

travelling at relativistic high speeds has allowed him to experience only one year of time

while ten years have elapsed on Earth). This is sometimes considered a paradox in that each

twin sees the other twin as travelling, and so, it is argued, each should see the other aging

more slowly. But in fact this is based on a misunderstanding of relativity, because in reality

only one twin experiences acceleration and deceleration, and so only one twin ages less.

An equivalent paradox concerning the related phenomenon of length contraction is often

referred to as the "tunnel paradox", whereby a hypothetical train approaching a tunnel at

near-light speed sees the tunnel as much shorter than it really is, whereas someone in the

tunnel sees the approaching train as short.

Essentially, then, the Special Theory of Relativity can be boiled down to its two main

postulates: firstly, that physical laws have the same mathematical form when expressed in

any inertial system (so that all motion, and the forces that result from it, is relative); and

secondly that the speed of light is independent of the motion of its source and of the

observer, and so it is NOT relative to anything else and will always have the same value

when measured by observers moving with constant velocity with respect to each other. Not

such a scary proposition at first glance, perhaps, but it does lead to some rather interesting

implications, which we will begin to consider in subsequent sections.

SPACE-TIME

Another corollary of special relativity is that, in effect, one person’s interval of space is

another person’s interval of both time and space, and one person’s interval of time is also

another person’s interval of both space and time. Thus, space and time are effectively

interchangeable, and fundamentally the same thing (or at least two different sides of the

same coin), an effect which becomes much more noticeable at relativistic speeds

approaching the speed of light.

Einstein’s former mathematics professor, Hermann Minkowski, was perhaps the first to notethis effect (and perhaps understood it even better than Einstein himself), and it was he who

coined the phrase “space-time” to describe the interchangeability of the four dimensions. In

1908, Minkowski offered a useful analogy to help explain how four-dimensional space-time




51

can appear differently to two observers in our normal three-dimensional space. He

described two observers viewing a three-dimensional object from different angles, and

noting that, for example, the length and width can appear different from the different view

points, due to what we call perspective, even though the object is clearly one and the same

in three dimensions.

(The path taken by an object in both space and time is known as the space-time interval)

The idea perhaps becomes even clearer when we consider that our picture of the Moon is

actually what the Moon was like 1¼ seconds ago (the time light takes to reach the Earth

from the Moon), our picture of the Sun is actually how it looked 8½ minutes ago, and by the

time we see an image of Alpha Centauri, our nearest star system, it is already 4.3 years out

of date. We can therefore never know what the universe it like at this very instant, and the

universe is clearly not a thing that extends just in space, but in space-time.

Due to the relativistic effects of time dilation, our idea of “now” is therefore something of a

fictitious concept, one which we as humans have invented for ourselves, but for which

nature itself has no real use. Physicists do not regard time as “passing” or “flowing” and time

is not a sequence of events which happen: the past and the future are simply there, laid out

as part of space-time. The "twins paradox" mentioned in the previous section can be

considered an example of this: whereas the stay-at-home twin’s progress through space-

time was wholly through time, the travelling twin’s progress was partly through space, so

that his progress through time was less than that of the stay-at-home twin (so that he aged

less).

Therefore, as Einstein remarked, “For us physicists, the distinction between past, present

and future is only an illusion, however persistent”, and these concepts really do not figure at




52

all in special relativity. Similarly, our whole conception of space becomes unreliable as the

relativistic effects of length contraction become apparent at high relative speeds.

But the malleability and blurring of space and time also has implications for other aspects of

physics. Just as Maxwell had shown that the electric and magnetic fields, once considered

completely separate entities, were both just part of a single seamless entity known as theelectromagnetic field, likewise (although perhaps more difficult to grasp and perhaps more

unexpected) energy and mass turn out to be just different faces of the same coin, a

connection encapsulated in Einstein’s justifiably famous formula, E = mc2, which we will look

at in the next section.

E = mc 2

As the Sun pumps out energy and light, it actually also loses some of its mass, although very

slowly (less than 0.1% since its birth). As a comet’s path passes near to the Sun, a tail of

glowing gases billows out away from the Sun. Both of these examples suggest that the

energy (photons) leaving the Sun actually weighs something, actually has mass, even if very

little. Although photons of sunlight have no intrinsic mass (otherwise, as we will see, they

would be unable to travel at the speed of light), they must have an “effective mass” by virtue

of their energy in order to be able to push a comet’s tail.

If a body with mass is pushed ever closer to the speed of light, the body would have to

become harder and harder to push, so that its speed never actually reached or exceeded the

speed of light, which we know to be the de facto maximum speed. In fact, by extension, if a

material body were ever to reach the speed of light it would effectively have to have

acquired an infinite mass. As a body approaches the speed of light, then, the energy put into

pushing the body clearly can not be used to increase its velocity and must therefore go

somewhere else.




53

(Mass-Energy Venn diagram)

The Law of Conservation of Energy dictates that energy can neither be created nor

destroyed, only transformed from one form to another. It follows, then, that if the energypushing the body is being converted into additional mass, then mass itself is just another

form of energy, and, vice versa, energy (energy of any sort, not just light, including sound

energy, electrical energy, energy of motion, etc) therefore has an effective mass.

This connection between energy and mass, known as mass-energy equivalence, was im-

mortalized in Einstein’s equation E = mc2, where E stands for energy, m stands for mass and

c is a constant (which happens to be equal to the speed of light). Actually, E = mc2

is just the

simplest case scenario, that for a body or mass at rest. For a body in motion, with a

velocity v, the equation becomes E = . We have already seen that the Lorentz factor γ

≡ , so we can therefore also say that E = γmOc2 (where mo is the rest mass of the object). As

can perhaps be reasonably easily deduced from these equations, as the velocity (v) ap-

proaches the speed of light (c), energy (E) approaches infinity, indicating that the body would

in fact require an infinite amount of energy to accelerate to the speed of light. We can also

see how (as mentioned in a previous section) the mass of a moving object becomes greater

and greater as its velocity increases until, at the speed of light, it becomes infinite.

Like other aspects of relativity, though, the effects are very hard to observe in the everyday

world. Your cup of coffee actually does weight more when you have added heat energy to it,

but by such an infinitesimal amount as not to be noticeable or even measurable. Likewise,




54

when a piece of coal is burned, mass-energy is converted to heat energy, and the total

products of burning (ash, gases, etc) would in fact weigh slightly less than the original coal.

To take another example, although four atoms of hydrogen can be used to produce one

atom of helium, as occurs during nuclear fusion in the heart of the Sun or in a hydrogen

bomb, the atom of helium actually weighs 0.8% less than four hydrogen atoms, the balancebeing converted into heat energy. This tiny "weight" of heat energy, however, represents

about a million times as much energy as an equivalent weight of coal could produce, partly

due to the prodigious strength of the strong nuclear force which holds the nucleus of an

atom together.

(The Large Hadron Collider at CERN, Switzerland, uses extremely high energy to create

particles of mass)

The same principle applies in reverse in particle colliders like that at CERN, the European

centre for particle physics in Switzerland. In a particle collider, sub-atomic particles are

accelerated to huge speeds and then crashed together, in the hope of creating new exotic

particles of matter out of the massive energy dischange which results. The prodigious

amounts of energy required to cause particles to literally pop out of thin air in this way is an

indication of just how much energy is encapsulated within mass.

In fact, mass is the most concentrated form of energy known. When one considers the

equation E = mc2, the term c stands for the speed of light (300,000 kilometres per second)

and this term is squared, which results in a very large number indeed. Thus, it may come as




55

no surprise that applying the equation to one kilogram of matter shows that it contains 9 x

1016

joules of energy, enough to lift the entire population of the Earth into space!

However, converting matter into energy is not easy. The nuclear processes in the Sun and in

a hydrogen bomb liberate barely 1% of the energy locked up in matter. A black hole spinning

at its maximum possible rate is much more efficient, though, and as matter swirls into ablack hole, it liberates energy (as heat and light) equivalent to 43% of the mass of the

matter. This is one reason why scientists believe that the huge energy output of quasars can

only be generated by a supermassive black hole at its heart.

In fact, the only process that converts mass into energy with 100% efficiency is the meeting

of matter and antimatter. Unfortunately, our universe appears to contain hardly any

antimatter (which remains something of a puzzle because, when antimatter is created in the

laboratory, its birth is always accompanied by an equal amount of matter), and scientists

have only succeeded in producing less than a billionth of a gram. The production of

antimatter is fraught with expense and difficulty (especially given that it tends to annihilate

as soon as it meets ordinary matter!), but if an efficient method of production could be

found one day, we would have at our command the most powerful energy source

imaginable (like the antimatter drive of fictional Star Trek).

GRAVITY AND ACCELERATION

The drawback to Einstein’s Special Theory of Relativity, however, is that it is “special” in the

respect that it only considers the effects of relativity to an observer moving at constant

speed. Motion at constant speed is clearly a very special case, and in practice bodies change

their speed with time. Einstein wanted to generalize his theory to consider how a person

sees another person who is accelerating relative to them.

At around this time (1907), he also started to wonder how Newtonian gravitation would

have to be modified to fit in with special relativity, and how the effects of gravity could be

incorporated into the formulation. His resulting General Theory of Relativity, over ten yearsin the making (it was published in 1916), has been called the greatest contribution to science

by a single human mind.

Initially, Einstein had been puzzled by the fact that Sir Isaac Newton’s Law of Universal

Gravitation, which had stood undisputed since 1687, appeared to be fundamentally

incompatible with his own Special Theory. Newton’s theory (permanently linked, at least in

the popular mind, with his observation of an apple falling from a tree) stated that every

massive body exerts an attractive force on every other massive body, a force which is

proportional to the product of the two masses and inversely proportional to the square of the distance between the bodies.




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(Newton's Law of Universal Gravitation)

Thus, according to this theory, gravity is relatively strong when objects are near each other,

but weakens with distance, and the bigger the bodies, the more their force of mutual

attraction. This “inverse-square law” is quite sophisticated enough to expla in why a

cannonball fired horizontally travels further before hitting the ground the faster it is

launched, why a certain minimum speed (about 11.2 kilometres per second) would be

required to allow objects to break out of Earth’s gravity and into orbit and why the planets

travel in an elliptical orbit around the Sun (although not quite sophisticated enough to

predict the slight anomaly in Mercury’s orbit).

Gravity is the organizing force for the cosmos, crucial in allowing structure to unfold from an

almost featureless Big Bang origin. Although it is a very weak force (feebler than the other

fundamental forces which govern the sub-atomic world by a factor of 1036

or

1,000,000,000,000,000,000,000,000,000,000,000,000), it is a cumulative and consistent

force which acts on everything and can act over large distances. So, even though gravity can

be effectively ignored by chemists studying how groups of atoms bond together, for bodies

more massive than the planet Jupiter the effects of gravity overwhelm the other forces, and

it is largely responsible for building the large-scale structures in the universe. Thus, gravity

squeezes together massive bodies like our own Sun, and it is only the explosive outward

energy in the Sun’s ultra-hot core that holds it in hydrostatic equilibrium and stops it from

collapsing into a super-dense white dwarf star.

Even before Newton, the great 17th Century Italian physicist Galileo Galilei had shown that

all bodies fall at the same rate, any perceived differences in practice being caused by

differences in air resistance and drag. Galileo’s famous (and probably apocryphal)

experiment involving the dropping of two balls of different masses from the Leaning Tower

of Pisa was repeated with even more dramatic results in 1972 when a hammer and a featherwere dropped together on the airless Moon and, just as Galileo had predicted, both hit the

ground together.




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Newton, however, had assumed that the force of gravity acts instantaneously, and Einstein

had already shown that nothing can travel at infinite speed, not even gravity, being limited

by the de facto universal speed limit of the speed of light. Furthermore, Newton had

assumed that the force of gravity was purely generated by mass, whereas Einstein had

shown that all forms of energy had effective mass and must therefore also be sources of

gravity.

Einstein’s ground-breaking realization (which he called “the happiest thought of my life”)

was that gravity is in reality not a force at all, but is indistinguishable from, and in fact the

same thing as, acceleration, an idea he called the “principle of equivalence”. He realized that

if he were to fall freely in a gravitational field (such as a skydiver before opening his

parachute, or a person in an elevator when its cable breaks), he would be unable to feel his

own weight, a rather remarkable insight in 1907, many years before the idea of freefall of

astronauts in space became commonplace.

(The principle of equivalence says that gravity is not a force at all, but is in fact the same

thing as acceleration)




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A simple thought experiment serves to clarify this: if an astronaut in the cabin of a spacecraft

accelerating upwards at 9.8 metres per second per second (the same acceleration as gravity

imparts to falling bodies near the Earth’s surface) were to drop a feather and hammer they

too would hit the floor of the cabin simultaneously (in the absence of air resistance), exactly

as would have happened if they had fallen on Earth under gravity. That, and the feeling of

his feet being glued to the ground just as they would be in Earth’s gravity, would be enough

to convince the astronaut that the acceleration of the spaceship was indistinguishable from

the pull of gravity on the Earth.

The influence of gravity also creates effects of time dilation (see the section on the Special

Theory of Relativity for a more detailed discussion of time dilation), sometimes referred to

as "gravitational time dilation". As Einstein predicted, the closer a body is to a large mass,

with a commensurately large gravitational pull, the slower time runs for it. It is almost as

though gravity is pulling on time itself, slowing its progress. Gravitational time dilation also

raises the theoretical possibility of time travel. For example, if a spaceship were to orbitclose enough (but not too close!) to a hugely massive object such as a supermassive black

hole, the gravitational effects may be significant enough to slow down time for the

occupants compared to elsewhere, effectively allowing them to travel into the future.

On a much smaller scale, because gravity is slightly stronger closer to the centre of the Earth,

then theoretically time passes more slowly for someone living on the first floor of an

apartment block than for someone living at the top. With modern atomic clocks of sufficient

accuracy, differences in the passage of time at different altitudes above sea level (and

therefore different distances from the Earth's centre of gravity) can be measured, and eventhe tiny differences due to the changing shape of the Earth as the tidal force of the Moon

pulls and stretches it. A real-life example of gravitational time dilation can be seen in GPS

systems in geo-synchronous orbits above the Earth, which need to constantly adjust their

clocks to account for time differences due to the weaker gravity they experience compared

to that on the Earth's surface (it is estimated that their accuracy would be out by as much as

10 kilometers a day without this adjustment).

So, gravity, Einstein realized, is not really a force at all, but just the result of our surroundings

accelerating relative to us. Or, perhaps a better way of looking at it, gravity is a kind of

inertial force, in the same way as the so-called centrifugal force is not a force in itself, merely

the effect of a body’s inertia when forced into a circular path. In order to rationalize this

situation, though, Einstein was to turn our whole conception of space on its head, as we will

see in the next section.

CURVED SPACE

If we imagine again the astronaut in his accelerating spaceship cabin from the previous

section, and imagine him pointing a laser horizontally across the cabin, the upward motion

of the spacecraft would result in the path of the laser appearing to curve (very) slightly




59

downwards as it crosses the cabin. Now, we known that light always takes the shortest path

between two points, which we usually think of as a straight line. However, a straight line is

only the shortest distance between two points on a flat surface. On a curved surface, the

shortest distance between two points is actually a curve, technically known as a geodesic,

which we can perhaps visualize when we think, for example, of a plane flying the shortest

route between London and New York which, as travellers will know, follows a "great circle"

path over Newfoundland rather than what appears to be a more direct straight line on a flat

map.

The only possible interpretation of the curving laser beam, then, is that the space inside the

cabin is in some way curved. If we combine this concept with Einstein’s principle of

equivalence, then it would appear that light in the presence of gravity follows a curved

trajectory, or, put in another way, gravity bends the path of light. In fact, it turns out that

gravity is nothing more than curved space, or, more specifically, the curvature or warpage of

four-dimensional space-time.

(A geodesic is the shortest path between two points in curved space)

A simple analogy might help us to understand this notoriously hard-to-visualize concept. If a

group of ants spend their entire lives on the essentially 2-dimensional surface of a

trampoline, and a heavy weight like a bowling ball is place in the middle of the trampoline,

the ants will find their paths mysteriously bent towards the bowl-like depression in the

trampoline. The ants might explain it by saying that the weight is exerting a force of

attraction on them, but, from the elevated point of view of the third dimension, it is clear

that the ants are merely following the curve of the trampoline and that no actual force is

acting on them.




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An even better visual analogy might be if a marble is rolled along the trampoline surface. It

might roll straight past the bowling ball, or it might be deflected a little (or a lot) as it follows

the dip but then “escapes” (similar to the idea of using gravity to deflect or brake or

slingshot a spaceship around a planet’s orbit). Or, if the marble comes too close, then it

might be drawn inexorably into the depression of the bowling ball, rolling in ever-decreasing

circles until it joins the ball in its hollow.

(Gravity causes space-time to curve around massive objects)

The path of the Earth as it travels though space is constantly bent towards the Sun in this

way, so much so that the planet traces out a nearly circular orbit. From the God-likeperspective of the fourth dimension, however, it can be seen that there is no actual force

being exerted on the Earth, merely that the Sun has created a valley-like depression in four-

dimensional space, and the Earth is just following the shortest path along a geodesic through

the curved space-time (just as the ants were in three-dimensional space).

The Earth, then, is actually in free fall around the Sun and so we do not feel the Sun’s gravity

on earth, just as astronauts on the International Space Station in free fall around the Earth

do not feel the Earth’s gravity. Thus, although free fall is usually defined as motion with no

acceleration other than that provided by gravity, what it is really is just a body travelling

along the straightest possible path through space-time. We only “feel” gravity on the Earth

when our natural motion of free fall towards the centre of the Earth is thwarted by the

ground, an inertial force similar to centrifugal force, as was mentioned in the previous

section.

This may at first seem counter-intuitive. We are used to the Newtonian idea that, when we

throw a ball straight up in the air, for example, a graph of its height versus time traces out a

parabola curve. Under relativity, however, we must recognize that a massive body like the

Earth actually curves the coordinate system itself, so that rather than following a curved

path in a flat (Cartesian) coordinate system, the ball actually follows a minimum-distance

path, or geodesic, in a curved coordinate system, returning to the thrower’s hand at a later

time because the geodesic leads it there.




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GENERAL THEORY OF RELATIVITY

As we have seen, matter does not simply pull on other matter across empty space, as

Newton had imagined. Rather matter distorts space-time and it is this distorted space-time

that in turn affects other matter. Objects (including planets, like the Earth, for instance) fly

freely under their own inertia through warped space-time, following curved paths because

this is the shortest possible path (or geodesic) in warped space-time.

This, in a nutshell, then, is the General Theory of Relativity, and its central premise is that the

curvature of space-time is directly determined by the distribution of matter and energy

contained within it. What complicates things, however, is that the distribution of matter and

energy is in turn governed by the curvature of space, leading to a feedback loop and a lot of very complex mathematics.

In practice, in our everyday world, Newton’s Law of Universal Gravitation is a perfectly good

approximation. The curving of light was never actually predicted by Newton but, in

combination with the idea from special relativity that all forms of energy (including light)

have an effective mass, then it seems logical that, as light passes a massive body like the

Sun, it too will feel the tug of gravity and be bent slightly from its course. Curiously,

however, Einstein’s theory predicts that the path of light will be bent by twice as much as

does Newton’s theory, due to a kind of positive feedback. The English astronomer ArthurEddington confirmed Einstein’s predictions of the deflection of light from other stars by the

Sun’s gravity using meaurements taken in West Africa during an eclipse of the Sun in 1919,

after which the General Theory of Relativity was generally accepted in the scientific

community.

The theory has been proven remarkably accurate and robust in many different tests over the

last century. The slightly elliptical orbit of planets is also explained by the theory but, even

more remarkably, it also explains with great accuracy the fact that the elliptical orbits of

planets are not exact repetitions but actually shift slightly with each revolution, tracing out a

kind of rosette-like pattern. For instance, it correctly predicts the so-called precession of the

perihelion of Mercury (that the planet Mercury traces out a complete rosette only once

every 3 million years), something which Newton’s Law of Universal Gravitation is not

sophisticated enough to cope with.




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(General relativity predicts the gravitational bending of light by massive bodies)

Gravity Probe B was launched into Earth orbit in 2004, specifically to test the space-time-

bending effects predicted by General Relativity using ultra-sensitive gyroscopes. The final

analysis of the results in 2011 confirms the predicted effects quite closely, with a tiny 0.28%

margin of error for geodetic effects and a larger 19% margin of error for the much less

pronounced frame-dragging effect.

The General Theory of Relativity can actually be described using a very simple equation: R =

GE (although Einstein's own formulation of his field equations are much more complex).Unfortunately, the variables in this simple equation are far from simple: R is a complicated

mathematical object made up of 16 separate numbers in a matrix or "tensor" that describes

the distortion of space-time; G is the gravitational constant; and E is another complicated

number, also represented by a tensor, representing the energy of the object (or more

accurately the 4-dimensional "energy momentum density"). Given that, though, what the

equation says is simple enough: that what gravity really is is not a force but a distortion of

space and time, and that the geometry of space and time depends not just on velocity (as

the Special Theory of Relativity had indicated) but on the energy of an object. This makes

sense when we consider that Newton had already shown that gravity depends on mass, and

that Einstein's Special Theory of Relativity had shown that mass is equivalent to energy.




63

Stephen Hawking and Roger Penrose’s singularity theorem of 1970 used the General Theory

of Relativity to show that, just as any collapsing star must end in a singularity, the universe

itself must have begun in a singularity like the Big Bang (providing that the universe does in

fact contain at least as much matter as it appears to). The theorem also showed, though,

that general relativity is an incomplete theory in that it cannot tell us exactly how the

universe started off because it predicts that all physical theories (including itself) necessarily

break down at a singularity like the Big Bang.

The theory has also provided endless fodder for the science fiction industry, predicting the

existence of sci-fi staples like black holes, wormholes, time travel, parallel universes, etc. Just

as an example, the notionally faster-than-light “warp” speeds of Star Trek are based firmly

on relativity: if the space-time behind a starship were in some way greatly expanded, and

the space-time in front of it simultaneously contracted, the starship would find itself

suddenly much closer to its destination, without the local space-time around the starship

being affected in any relativistic way. Unfortunately, however, such a trick would require theharvesting of vast amounts of energy, way in excess of anything imaginable today.

CONCLUSION

Almost a century later, the General Theory of Relativity remains the single most influential

theory in modern physics, and one of the few that almost everyone, from all walks of life,has heard of (even if they may be a little hazy about the details). Einstein’s General Theory

predicted the existence of black holes many years before any evidence of such phenomena,

even indirect evidence, was obtained, and was highly suggestive of an origin of the universe

beginning with a Big Bang type event, although Einstein himself was highly suspicious of

both of those possibilities.

The theory also predicts, or at least permits, the existence of "wormholes", tunnel-like short-

cuts through space-time, and even the theoretical possibility of time travel. In fact, the

Austrian-American mathematician Kurt Gödel’s elegant solution to Einstein’s field equations

(assuming a uniformly spinning universe with constant uniform energy density) specifically

predicts the possibility of travel back in time, although it should be said that his model of the

universe does not entirely accord with our own. For now at least, these ideas remain firmly

in the realm of science fiction.

The way forward for physics now rests with attempts to combine the theory of relativity (the

theory of the very large, which describes one of the fundamental forces of nature, gravity)

with quantum theory (the theory of the very small, which describes the other three

fundamental forces, electromagnetism, the weak nuclear force and the strong nuclear force)

in a unified theory of quantum gravity (or quantum theory of gravity), the so-called “theory

of everything”. Candidates like superstring theory and loop quantum gravity, however, still

need to overcome major formal and conceptual problems.




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65

BLACK HOLES AND WORMHOLES—

-------------------------------------------------------------------------------------


- Introduction…………………………………………………………(66)

- Stars, Supernovas and Neutron Stars……………..……(67)

- Creation of Black Holes………………………………………..(71)

- Black Hole Theory & Hawking Radiation…………..….(74)

- Event Horizon and Accretion Disk……………………..….(77)

- Singularities………………………………………………………….(80)

- Wormholes…………………………………………………………..(81)

- Conclusion……………………………………………………….……(83)

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INTRODUCTION

Black holes are a phenomenon predicted by Albert Einstein's General Theory of Relativity, which was

published in 1916. In fact, the idea of a black hole was proposed as early as 1783 by the amateur

British astronomer John Michell (and independently by the Frenchman Pierre-Simon Laplace in

1795).

Ironically, Einstein himself did not believe in the existence of black holes, and he strongly resisted the

idea, even though his own theory predicted them. The general scientific consensus is now that black

holes do in fact exist, and that they are actually one of the most important features of our universe.

Astronomers have detected them indirectly in enough different ways that there is little doubt of their

existence.

(Artist's impression of a star torn apart by the gravity of a black hole)

A black hole (the phrase is usually credited to the American physicist John Wheeler in 1967, and iscertainly a distinct improvement on the original label of “gravitationally completely collapsed

objects”) is a region of space in which the gravitational field is so powerful that nothing, including




67electromagnetic radiation such as visible light, can escape its pull - a kind of bottomless pit in space-

time.

At its centre lies an infinitely small, infinitely dense singularity, a place where the normal laws of

physics break down. As the comedian Steven Wright once remarked: “Black holes are where God

divided by zero”.

Einstein’s work was also at the heart of the theory of wormholes, or “bridges” as he called them. The

idea of a hypothetical topological feature of space-time that is essentially a short-cut through space

and time, potentially linking widely separated parts of the universe (or even different universes), has

been understandably much loved by science fiction writers over the years, although there is also

much theoretical work to support them.

To better understand how black holes might be formed, a little background knowledge of the life

cycle of stars is useful (which will be covered in the following section), as well as an understanding of

general relativity and curved space-time (which is a separate topic in its own right).

STARS, SUPERNOVAS AND NEUTRON STARS

A star begins its life as a cloud of dust and gas (mainly hydrogen) known as a nebula. A

protostar is formed when gravity causes the dust and gas of a nebula to clump together in a

process called accretion. As gravity continues to pull ever more matter inward towards the

core, its temperature, pressure and density increases. If a critical temperature in the core of

a protostar is reached, then nuclear fusion begins and a star is born. If the critical

temperature is not reached, however, it ends up as a brown dwarf, or dead star, and never

attains star status.

A typical star like our own Sun (technically a yellow dwarf star), then, is fuelled by nuclear

fusion, the conversion of hydrogen (the simplest atom, with a nucleus consisting of just one

proton) into helium (the second simplest, with two protons and two neutrons in its nucleus).

The nucleus of a helium atom actually weighs only 99.3% as much as the two protons and

two neutrons that go to make it up, the remaining 0.7% being released as heat and light

energy. This 0.7% coefficient, which is essentially due to the extent to which the strong

nuclear force is able to overcome the electrical repulsion in the atoms, turns out to be a

critical one in determining the life-cycle of stars and the development of the variety of atoms

we see in the universe around us.

The Sun’s own gravity traps and squeezes this ultra-hot gas into a confined space, thus

generating enough heat for the fusion reaction to take place. The process remains in

equilibrium as long as it retains enough fuel to create this heat- and light-producing outward

energy which counteracts the inward pressure of its gravity (known as hydrostatic

equilibrium). This is the period known as the main sequence of the star.




68

(The process of star formation)

Already about 4.5 - 5 billion years old, when the Sun’s hydrogen fuel starts to run out (in an

estimated further 5 billion years or so), its main sequence comes to an end, and it starts to

cool down and collapse under its own gravity. However, energy from the collapse then heats

up the core even more, until it is hot enough to start burning helium and, under the extra

heat of the helium burning, its outer layers expand briefly (for a “mere” 100 million years)

into a massive red giant star.

Eventually, the outer layers blow off completely and the core settles down into a white

dwarf star, a small cinder about the size of the Earth composed mainly of carbon and

oxygen. Over a very long stretch of time, white dwarfs will eventually fade into black dwarfs,

and this is the ultimate fate of about 97% of stars in our galaxy. The matter which makes up

white and black dwarfs is largely composed of, and supported by, electron-degenerate

matter, in which the atoms making up the star are prevented from further collapse by the

effective pressure of their electrons, due to the Pauli Exclusion Principle (which states that

no two electrons can occupy identical states, even under the pressure of a collapsing star of

several solar masses).

However, a star significantly larger than our Sun is hotter and burns up its fuel more quickly

and generally has a shorter but more dramatic life. A star of ten solar masses, for example,

would burn fuel at about a thousand times the rate of the Sun, and would exhaust its

hydrogen fuel in less than 100 million years (compared to the Sun’s 10 billion year lifetime).A star 20 times the mass of our Sun would burn its fuel 36,000 times faster than the Sun, and

might live only a few million years in total.




69

Larger stars are much hotter and the higher temperatures within such a star are sufficient to

fuse even helium. The helium then becomes the star’s raw fuel, and it goes on to release

ever higher levels of energy as the helium is fused into carbon and oxygen, while the outer

layer of hydrogen actually cools and expands significantly in the star’s red giant phase.

Even larger stars continue in further rounds of nuclear fusion, each of successively increasedviolence and shorter duration, as carbon fuses into neon, neon into magnesium and oxygen,

then to silicon and finally iron. So, although a star the size of our own Sun does not progress

very far along this path, a larger star continues though a chain of transmutations to

progressively heavier nuclei. Eventually, a star of sufficient initial mass becomes a red

supergiant, which has a core layered like an onion, with a broad shell of hydrogen on the

outside, surrounding a shell of helium, and then successively denser shells of carbon, then

neon, then oxygen, then silicon, and finally a core of white-hot iron.

The iron in the star’s core is very resistent to further fusing, however high the temperatures,

and the heat from its nuclear fusion is no longer sufficient to support it against its own

crushing gravity and it will suddenly and catastrophically collapse. The final collapse of a

massive star under its own gravity happens incredibly quickly: in a thousandth of a second it

can shrink from thousands of kilometres across to a ball of ultra-condensed matter just a few

kilometres across.

This rapid collapse results in a massive rebound when the core reaches the density of an

atomic nucleus, like a ball bouncing off a brick wall, resulting in ultra-hot shock-waves which

are imparted to the rest of the star. In this way, the star ulimately ends its life in a

cataclysmic explosion known as a supernova, and for a few short weeks it burns as brightly

as several billion suns, briefly outshining the star's entire home galaxy. For example, the

supernova whose remnants we see today as the Crab Nebula, was recorded by Chinese

astronomers in the year 1054 as visible to the naked eye for several months, even in the

daytime, and bright enough to read by at night, despite its being about 6,500 light years

away. The visible light of a supernova, though, represents only about 1% of the released

energy, the vast majority being in the form of ultraviolet light, x-rays, gamma rays and,

particularly, neutrinos.

The conditions in the blast of a supernova are even hotter and more violent than in the core

of the old star and this finally allows elements even heavier than iron to be created, such as

radioactive versions of cobalt, aluminum, titanium, etc. In the process of its explosion, a

supernova blows out into space a nebula of debris containing a mix of all of the naturally-

occurring elements, in proportions which agree closely with those calculated to exist on

earth. The variety of atoms in the dusty cloud from which our own Sun (and the Earth itself)

were formed 4.5 billion years ago were essentially the ashes of generations of earlier stars

having run through their entire life-cycles. Supernovas are therefore ultimately responsible

for providing the mix of atoms on Earth, and the building blocks for the intricate chemistry of

life. Most of these building blocks (carbon, oxygen, iron, etc) were therefore not produced in

the the Big Bang at the start of the universe - at the time the very first stars were being

formed, their composition would have been about 75% hydrogen and 25% helium with just




70

traces of the next heaviest element, lithium - but much later in the centre of stars and their

supernova explosions. It is in this respect that people talk of humans as being composed of

“stardust” (or, for the less romantically inclined, nuclear waste).

(Evolution of high and low mass stars)

When a star explodes as a supernova, most of its matter is blown away into space to form a

nebula (such as the Crab Nebula). The ultra-dense remnants of the imploding core which are

left behind are known as a neutron star, as its electrons and protons are crushed together in

the huge gravity to form neutrons. In 1935, the young Indian-American astrophysicist

Subrahmanyan Chandrasekhar established that there is in fact a limit, known as the

Chandrasekhar limit, of about 1.4 solar masses above which a star must continue to collapse

under its own gravity into a neutron star rather than settling down into a white dwarf (a

similar discovery was made around the same time by the Russian scientist Lev Davidovich

Landau).

A neutron star is typically between 1.4 and 4 times as massive as our own Sun, but is

squeezed into a volume only about twenty kilometres in diameter, and so has an extremely

high density. Given that, as Sir Isaac Newton pointed out as long ago as the 17th Century,

gravity is subject to an inverse-square law (so that as the distance from the source

decreases, gravity increases by the square of that amount), the gravitational pull of a small,

dense neutron star is much greater than that around a normal star of many times its size. In

fact, the gravitational force on a massively dense neutron star is about a million million times




71

fiercer than on the Earth, and a projectile would need to attain almost half the speed of light

in order to escape its gravity. Under conditions of such powerful gravity, Sir Isaac Newton’s

Law of Universal Gravitation (which generally works well enough in our own Solar System)

becomes redundant, and the more sophisticated model of Albert Einstein’s General Theory

of Relativity is needed. Thus, clocks on a neutron star would run 10 - 20% slower than those

on Earth, and any light from its surface would be so strongly curved that, viewed from afar,

part of the back of the neutron star would be visible as well.

Because neutron stars retain the angular momentum of the original much larger star, they

usually rotate at very high speed (as fast as several hundred times per second in a newly

formed neutron star), in the same way as an ice skater spins faster as she tucks in her arms.

In some cases, their intense magnetic fields sweep regular pulses of radio waves across the

universe, for which they are known as pulsars. We know of about 2,000 neutron stars in our

own Milky Way galaxy, the majority of which were detected as radio pulsars.

A particular type of large neutron star known as a magnetar has a particularly powerful

magnetic field (up to a hundred trillion times the strength of the Earth's magnetic field),

which powers the emission of copious amounts of high-energy electromagnetic radiation,

particularly X-rays and gamma rays, as it decays over a period of around 10,000 years.

Perhaps 1 in 10 neutron stars develop as magnetars.

CREATION OF BLACK HOLES

A slightly different kind of supernova explosion occurs when even larger, hotter stars (blue

giants and blue supergiants) reach the end of their short, dramatic lives. These stars are hot

enough to burn not just hydrogen and helium as fuel, but also carbon, oxygen and silicon.

Eventually, the fusion in these stars forms the element iron (which is the most stable of all

nuclei, and will not easily fuse into heavier elements), which effectively ends the nuclear

fusion process within the star. Lacking fuel for fusion, the temperature of the star decreases

and the rate of collapse due to gravity increases, until it collapse completely on itself,

blowing out material in a massive supernova explosion.

If the mass of the compressed remnant of the star exceeds about 3 - 4 solar masses, then

even the degeneracy pressure of neutrons is insufficient to halt the collapse and, instead of

forming a neutron star, the core collapses completely into a gravitational singularity, a single

point containing all the mass of the entire original star. The gravity in such a phenomenon is

so strong that it overwhelms all other forces, to the extent that even light can not escape

from it, hence the name black hole. Thus, the gravity of a body just a few times denser than

a neutron star would result in its inevitable further collapse into a black hole.

Although singularity at the the centre of a black hole is infinitely dense, the black hole itself

is not necessarily huge, as is sometimes assumed. A black hole with the mass of our Sun, for

example, would have a radius of just three kilometres (roughly two hundred million times

smaller than the Sun), while one with the mass of the Earth would fit in the palm of your




72

hand! Having said that, black holes can grow to great size over time as they assimilate more

and more matter and even other black holes, and some do become extremely massive.

Contrary to popular belief, a black hole does not just "suck up" everything around it in an

uncontrolled orgy of destruction: it actually exerts no more gravitational pull on the objects

around it than the original star from which it was formed, and any objects orbiting theoriginal star (and which survived the supernova blast) would now orbit a black hole instead

(an object would need to approach quite close to a black hole before being sucked in). The

very largest blue stars may skip even the supernova stage, so that even their outer shells

become incorporated into the singularity.

( Simulated black hole in front of the Milky Way)

By definition, we cannot observe black holes directly, but they can be detected by the

gravitational effect they exert on other bodies or on light rays. This is especially easy to spot

in the case of binary star systems where an ordinary star is orbiting around a black hole. In

the early 1990s, Reinhard Genzel pioneered this work, using the then new technique of

adaptive optics to plot and track the motions of stars near the centre of our own Milky Way

galaxy, to show that they must be orbiting a very massive, but invisible, object. From theimmense speed with which the stars closest to the centre of the galaxy are orbiting - millions

of kilometeres per hour - we know that there is a "supermassive black hole" (known as

Sagittarius A) at the centre of the Milky Way, with a mass of around 2 - 4 million times that




73

of our Sun. In addition, in the Milky Way galaxy alone, there are many millions of black holes

of at least ten solar masses each.

Supermassive black holes lurk in the centres of most galaxies, forming the hubs around

which the galaxies rotate. In fact, from observations of the intense radiation of gases swirling

around them at close to the speed of light, we can infer that there are much largersupermassive black holes in the centres of other galaxies, some of them weighing as much as

several billion suns. The black hole at the centre of a galaxy known as M87 has a mass

estimated at around 20 billion solar masses, and may be as large as our entire Solar System.

It seems likely that the early universe, in which very large, short-lived stars were the norm,

was scattered with many, many black holes, which gradually merged together over time,

creating larger and larger black holes. Observations have shown that is not uncommon for

two black holes to swirl around each other in a kind of cosmic dance as their gravitational

fields interact. The ripples in space-time caused by two black holes orbiting around each

other - typically in a three-leaved clover shape or more complex multi-pass configuration,

rather than the simple orbit of an electron within an atom, and ever-smaller and faster as

the two objects inevitably approach each other - can be recorded visually and even audibly.

In the case of the largest events, moments after the creation of a black hole, the heat and

the hugely amplified magnetic field of the collapsing star combine to focus a pair of tight

beams or jets of radiation, perpendicular to the spinning plane of the accretion disk. These

beams focus vast amounts of particles and energy (of the order of a billion billion times the

energy output of our Sun) away from the black hole at close to the speed of light. The shock

waves of this massively enegetic beam cause gamma rays to be emitted in a phenomenon

known as a "gamma ray burst" or "hypernova" event (so named because its energy and

brightness dwarfs even that of a supernova, by a factor of upto a hundred million times).

Gamma ray bursts are by far the brightest electromagnetic events occurring in the universe,

and can last from mere milliseconds to nearly an hour - a typical burst lasts a few seconds -

usually followed by a longer-lived “afterglow” emitting at longer wavelengths (x-ray,

ultraviolet, visible, infrared and radio waves). It is likely that collisions between neutron

stars, or between a neutron star and a black hole, can also cause gamma ray bursts.




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(Long and short gamma ray bursts)

Interestingly, it appears to be easier for stars with fewer heavy elements to turn hypernova

and generate gamma ray bursts. That, and the fact that larger, more short-lived stars were

more common earlier in the life of the universe, mean that the phenomenon of gamma ray

bursts is actually rarer today than it was. Having said that, NASA's Swift Probe, launched in

2004 with a mission specifically to locate gamma ray bursts throughout the universe, is

recording at least one such event each day, so these are not rare incidents. (It should be

remembered that any supernovas or gamma ray bursts we observe today in galaxies, say, 9

billion light years away, actually occurred 9 billion years ago.)

BLACK HOLE THEORY & HAWKING RADIATION

The simplest type of black hole, in which the core does not rotate and just has a singularity

and an event horizon, is known as a Schwarzschild black hole after the German physicist Karl

Schwarzschild who pioneered much of the very early theory behind black holes in the 1910s,

along with Albert Einstein. In 1958, David Finkelstein published a paper, based on Einstein

and Schwarzschild’s work, describing the idea of a “one-way membrane” which triggered a

renewed interest in black hole theory (although the phrase itself was not coined until a

lecture by John Wheeler in 1967).




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In 1963, the New Zealander Roy Kerr discovered a solution to Einstein’s field equations of

general relativity which described a spinning object, and suggested that anything which

collapsed would eventually settle down into a spinning black hole. It spins because the star

from which it formed was spinning, and it is now thought that this is actually likely to be the

most common form in nature. A rotating black hole would bulge outward near its equator

due to its rotation (the faster the spin, the more the bulge).

(Spinning and non-spinning black holes)

In the mid-1960s, the young English mathematician Roger Penrose devoted himself to the

study of black holes and, in 1965, he proved an important theorem which showed that agravitational collapse of a large dying star must result in a singularity, where space-time

cannot be continued and classical general relativity breaks down. Penrose and Wheeler went

on to prove that any non-rotating star, however complicated its initial shape and internal

structure, would end up after gravitational collapse as a perfectly spherical black hole,

whose size would depend solely on its mass.

In the late 1960s, Penrose collaborated with his Cambridge friend and colleague, Stephen

Hawking, in more investigations into the subject. They applied a new, complex mathematical

model derived from Einstein's theory of general relativity, which led, in 1970, to Hawking's

proof of the first of several singularity theorems. Such theorems provided a set of sufficient

conditions for the existence of a gravitational singularity in space-time, and showed that, far




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from being mathematical curiosities which appear only in special cases, singularities are

actually a fairly generic feature of general relativity.

Although it may seem a very complex, peculiar and perhaps counter-intuitive object, a black

hole can essentially be described by just three quantities: how much mass went into it, how

fast it is spinning (its angular momentum) and its electrical charge. This came to be known asthe “No Hair Theorem”, after John Wheeler’s comment that “black holes have no hair”, by

which he meant that any other information about the matter which formed a black hole (for

which "hair" is a metaphor) remains permanently inaccessible to external observers within

its event horizon, and is all but irrelevant.

Brandon Carter and Stephen Hawking proved the No-Hair Theorem mathematically in the

early 1970s, showing that the size and shape of a rotating black hole would depend only on

its mass and rate of rotation, and not on the nature of the body that collapsed to form it.

They also proposed four laws of black hole mechanics, analogous to the laws of

thermodynamics, by relating mass to energy, area to entropy, and surface gravity to

temperature.

(Hawking radiation as particle pairs are created near a black hole)

In 1974, Hawking shocked the physics world by showing that black holes should in fact

thermally create and emit sub-atomic particles, known today as Hawking radiation, until

they exhaust their energy and evaporate completely. According to this theory, black holesare not completely black, and neither do they last forever.

Hawking showed how the strong gravitational field around a black hole can affect the

production of matching pairs of particles and anti-particles, as is happening all the time in




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apparently empty space according to quantum theory. If the particles are created just

outside the event horizon of a black hole, then it is possible that the positive member of the

pair (say, an electron) may escape - observed as thermal radiation emitting from the black

hole - while the negative particle (say, a positron, with its negative energy and negative

mass) may fall back into the black hole, and in this way the black hole would gradually lose

mass. This was perhaps one of the first ever examples of a theory which synthesized, at least

to some extent, quantum mechanics and general relativity.

A corollary of this, though, is the so-called “Information Paradox” or “Hawking Paradox”,

whereby physical information (which roughly means the distinct identity and properties of

particles going into a black hole) appears to be completely lost to the universe, in

contravention of the accepted laws of physics (sometimes referred to as the "law of

conservation of information"). Hawking vigorously defended this paradox against the

arguments of Leonard Susskind and others for almost thirty years, until he famously

retracted his claim in 2004, effectively conceding defeat to Susskind in what had becomeknown as the "black hole war". Hawking's latest line of reasoning is that the information is in

fact conserved, although perhaps not in our observable universe but in other parallel

universes in the multiverse as a whole.

Unfortunately, Susskind's proposed solution is even more difficult, and almost impossible to

envisage or explain in an understandable way. He suggests that, as an object falls into a

black hole, a copy of the information that makes it up is sort of scrambled and smeared in

two dimensions around the edge of the black hole. Furthermore, Susskind believes that a

similar process occurs in the universe as a whole, which raises the rather alarming idea thatwhat we think of as three-dimensional reality is in fact something like a holographic

representation of a "real" reality, which is actually contained in two dimensions around the

edge of the universe.

It is also theoretically possible that "primordial" or "mini" black holes could have been

created in the conditions during the early moments after the Big Bang, possibly in huge

numbers. No such mini black holes have ever been observed, however - indeed, they would

be extremely difficult to spot - and they remain largely speculative. It is anyway likely that all

but the largest of them would have already evaporated by now as they leak away Hawking

radiation. According to Hawking's theory, the amount of mass lost is greater for small black

holes, and so quantum-sized black holes would evaporate over very short time-scales. But it

is hoped that such mini black holes might be experimentally re-created in the extreme

conditions of the Large Hadron Collider at CERN, which, among other things, would lend

much-needed credence to some of the current theoretical predictions of superstring theory

regarding gravity.

EVENT HORIZON AND ACCRETION DISK

A black hole’s mass is concentrated at a single point deep in its heart, and clearly cannot be

seen. The “hole” that can, in principle, be seen (although no -one has ever actually seen a




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black hole directly) is the region of space around the singularity where gravity is so strong

that nothing, not even light, the fastest thing in the universe, can escape, and where the

time dilation becomes almost infinite.

A black hole is therefore bounded by a well-defined surface or edge known as the “event

horizon”, within which nothing can be seen and nothing can escape, because the necessaryescape velocity would equal or exceed the speed of light (a physical impossibility). The event

horizon acts like a kind of one-way membrane, similar to the "point-of-no-return" a boat

experiences when approaching a whirlpool and reaching the point where it is no longer

possible to navigate against the flow. Or, to look at it in a different way, within the event

horizon, space itself is falling into the black hole at a notional speed greater than the speed

of light.

The event horizon of a black hole from an exploding star with a mass of several times that of

our own Sun, would be perhaps a few kilometres across. However, it could then grow over

time as it swallowed dust, planets, stars, even other black holes. The black hole at the centre

of the Milky Way, for example, is estimated to have a mass equal to about 2,500,000 suns

and have an event horizon many millions of kilometres across.

Material, such as gas, dust and other stellar debris that has come close to a black hole but

not quite fallen into it, forms a flattened band of spinning matter around the event horizon

called the accretion disk (or disc). Although no-one has ever actually seen a black hole or

even its event horizon, this accretion disk can be seen, because the spinning particles are

accelerated to tremendous speeds by the huge gravity of the black hole, releasing heat and

powerful x-rays and gamma rays out into the universe as they smash into each other.




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(Event horizon, accretion disk and gamma ray jets of a black hole)

These accretion disks are also known as quasars (quasi-stellar radio sources). Quasars are

the oldest known bodies in the universe and (with the exception of gamma ray bursts) the

most distant objects we can actually see, as well as being the brightest and most massive,

outshining trillions of stars. A quasar is, then, a bright halo of matter surrounding, and being

drawn into, a rotating black hole, effectively feeding it with matter. A quasar dims into a

normal black hole when there is no matter around it left to eat.

A non-rotating black hole would be precisely spherical. However, a rotating black hole

(created from the collapse of a rotating star) bulges out at its equator due to centripetal

force. A rotating black hole is also surrounded by a region of space-time in which it is

impossible to stand still, called the ergosphere. This is due to a process known as frame-

dragging, whereby any rotating mass will tend to slightly "drag" along the space-time

immediately surrounding it. In fact, space-time in the ergosphere is technically dragged

around faster than the speed of light (relative, that is, to other regions of space-time

surrounding it). It may be possible for objects in the ergosphere to escape from orbit around

the black hole but, once within the ergosphere, they cannot remain stationary.

Also due to the extreme gravity around a black hole, an object in its gravitational field

experiences a slowing down of time, known as gravitational time dilation, relative to




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observers outside the field. From the viewpoint of a distant observer an object falling into a

black hole appears to slow down and fade, approaching but never quite reaching the event

horizon. Finally, at a point just before it reaches the event horizon, it becomes so dim that it

can no longer be seen (all due to the time dilation effect).

SINGULARITIES

In the centre of a black hole is a gravitational singularity, a one-dimensional point which

contains infinite mass in an infinitely small space, where gravity become infinite and space-

time curves infinitely, and where the laws of physics as we know them cease to operate. As

the eminent American physicist Kip Thorne describes it, it is "the point where all laws of

physics break down".

Current theory suggests that, as an object falls into a black hole and approaches thesingularity at the centre, it will become stretched out or “spaghettified” due to the

increasing differential in gravitational attraction on different parts of it, before presumably

losing dimensionality completely and disappearing irrevocably into the singularity. An

observer watching from a safe distance outside, though, would have a different view of the

event. According to relativity theory, they would see the object moving slower and slower as

it approaches the black hole until it comes to a complete halt at the event horizon, never

actually falling into the black hole.

(A gravitational singularity is hidden within a black hole)




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The existence of a singularity is often taken as proof that the theory of general relativity has

broken down, which is perhaps not unexpected as it occurs in conditions where quantum

effects should become important. It is conceivable that some future combined theory of

quantum gravity (such as current research into superstrings) may be able to describe black

holes without the need for singularities, but such a theory is still many years away.

According to the "cosmic censorship" hypothesis, a black hole's singularity remains hidden

behind its event horizon, in that it is always surrounded by an area which does not allow

light to escape, and therefore cannot be directly observed. The only exception the

hypothesis allows (known as a “naked” singularity) is the initial Big Bang itself.

It seems likely, then, that, by its very nature, we will never be able to fully describe or even

understand the singularity at the centre of a black hole. Although an observer can send

signals into a black hole, nothing inside the black hole can ever communicate with anything

outside it, so its secrets would seem to be safe forever.

WORMHOLES

Like black holes, wormholes arise as valid solutions to the equations of Albert Einstein's

General Theory of Relativity, and, like black holes, the phrase was coined (in 1957) by the

American physicist John Wheeler. Also like black holes, they have never been observed

directly, but they crop up so readily in theory that some physicists are encouraged to think

that real counterparts may eventually be found or fabricated.

In 1916, the Austrian physicist Ludwig Flamm, while looking over Karl Schwarzschild's

solution to Einstein's field equations, which describes a particular form of black hole known

as a Schwarzschild black hole, noticed that another solution was also possible, which

described a phenomenon which later came to be known as a “white hole”. A white hole is

the theoretical time reversal of a black hole and, while a black hole acts as a vacuum,

drawing in any matter that crosses the event horizon, a white hole acts as a source that

ejects matter from its event horizon. Some have even speculated that there is a white hole

on the "other side" of all black holes, where all the matter the black hole sucks up is blown

out in some alternative universe, and even that what we think of as the Big Bang might in

fact have been the result of just such a phenomenon.

Flamm also noticed that the two solutions, describing two different regions of space-time

could be mathematically connected by a kind of space-time conduit, and that, in theory at

least, the black hole "entrance" and white hole "exit" could be in totally different parts of

the same universe or even in different universes! Einstein himself explored these ideas

further in 1935, along with Nathan Rosen, and the two achieved a solution known as anEinstein-Rosen bridge (also known as a Lorentzian wormhole or a Schwarzschild wormhole).




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To better visualize a wormhole, consider the analogy of a piece of paper with two pencil

marks drawn on it (to represent two points in space-time), the line between them showing

the distance from one point to the other in normal space-time. If the paper is now bent and

folded over almost double (the equivalent of drastically warping space-time), then poking

the pencil through the paper provides a much shorter way of linking the two points, a short-

cut through space-time much like a wormhole.

Some theorists are encouraged to think that real counterparts may eventually be found or

fabricated and, perhaps, used as a tunnel or short-cut for high-speed space travel between

distant points or even for time travel (with all the potential paradoxes that might entail).

However, a generally accepted property of wormholes is that they are inherently highly

unstable and would probably collapse in a much shorter time than it would take to get

through to the other side. At any rate, it is predicted that they would collapse instantly if

even the tiniest amount of matter (even a single photon) attempted to pass through them.

(A wormhole is a theoretical "short-cut" between distant regions of space-time)

Although some possible theoretical ways around this problem have been suggested (for

example, using “cosmic strings” or “negative matter” or some other exotic matter with

“negative energy”) to prevent the wormhole from pinching closed, the idea remains largely

in the realm of science fiction for the time being. It has, however, still not been

mathematically proven beyond all doubt that some kind of exotic matter with negative

energy density is an absolute requirement for wormholes, nor has it been established that

such exotic matter cannot exist, so the possibility of a practical application of the theory still

remains.




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Because a wormholes is a conduit through 4-dimensional space-time, and not just through

space, Stephen Hawking and others have also posited that wormholes might theoretically be

utilized for travel through time as well as through space, although it is widely believed that

time travel into the past will never be possible due to the potential for paradoxes and self-

destructive feedback loops.

CONCLUSION

Neither wormholes or black holes have actually ever been seen directly, even with the

sophisticated equipment in use today, but both follow inevitably from Albert Einstein’s

General Theory of Relativity, and plenty of indirect evidence has been obtained (at least for

black holes). The ideas have certainly been more than readily accepted by the science fiction

community, for whom they suggest intriguing possibilities.

One of the most famous black hole theorists, the British physicist Stephen Hawking,

proposed the four laws of black hole mechanics back in the 1960s, and calculated in 1974

that black holes should thermally create and emit sub-atomic particles, known today as

Hawking radiation, until they eventually exhaust their energy and evaporate. Yet, as recently

as 2004, he admitted to losing a bet he made with the Caltech physicists Kip Thorne and

John Preskill, and overturned his long-held belief that any “information” crossing the event

horizon of a black hole is lost to our universe, and is now convinced that black holes will

eventually transmit, albeit in a garbled form as we perceive it in our observable universe,

information about all matter they swallow (“information” in this sense may be loosely

defined as “that which can distinguish one thing from another”, and essentially refers the

identity of a thing and all of its properties).

This is a good indication that the theory is far from cut and dried, and research (both theory

and observation) into this challenging area proceeds unabated. As an example of the

complexity of the subject matter, a short quote from Professor Hawking’s 2004 presentation

may suffice: “The Euclidean path integral over all topologically trivial metrics can be done by

time slicing and so is unitary when analytically continued to the Lorentzian”. Wow!




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85

Quantum Theory and the

Uncertainty Principle—

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- Introduction……………………………………………………………………..(86) -

Early Developments in Atomic Theory…………………………………(87) -

Quanta and Wave-Particle Duality………………………………………(90) -

Probability Waves and Complementarity………………………….…(94) -

Superposition, Interference and Decoherence………………….…(98) -

Quantum Tunnelling and the Uncertainty Principle……….…...(102) -

Nonlocality and Entanglement………………………………………….…(108) -

Spin and the Pauli Exclusion Principle…………………………….…...(110) -

Conclusion…………………………………………………………………………..(114)

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INTRODUCTION

Quantum theory (otherwise known as quantum physics or quantum mechanics) is one of the

two main planks of modern physics, along with general relativity, and between them the two

theories claim to explain virtually everything about the universe. General relativity gives us

our picture of the very big (space-time and gravity), while quantum theory gives us our

picture of the very small (atoms and their constituents).

Technically, quantum theory is actually the theory of any objects isolated from their

surroundings but, because it is very difficult to isolate large objects from their environments,

it essentially becomes a theory of the microscopic world of atoms and sub-atomic particles.

This is especially true for those parts of the theory which rely on the absolute

indistinguishability of fundamental particles, an indistinguishability which is impossible to

find in the everyday world of composite, large-scale objects.

Quantum theory is used in a huge variety of applications in everyday life, including lasers,

CDs, DVDs, solar cells, fibre-optics, digital cameras, photocopiers, bar-code readers,

fluorescent lights, LED lights, computer screens, transistors, semi-conductors, super-

conductors, spectroscopy, MRI scanners, etc, etc. By some estimates, over 25% of the GDP

of developed countries is directly based on quantum physics. It even explains the nuclear

fusion processes taking place inside stars.




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(A small part of the derivation for Schrödinger's equation of quantum mechanics)

However, some of its findings and principles are distinctly counter-intuitive and fiendishly

difficult to explain in simple language, without resorting to complex mathematics way

beyond the comfort level of most people (myself included!). This situation is not helped by

the fact that the “theory” is largely a patchwork of fragments accrued over the last centuryor so, that some elements of it are still not well understood by the scientists themselves, and

that some of the bizarre behaviour it predicts appears to fly in face of what we have come to

think of as common sense.

Richard Feynman, winner of the 1965 Nobel Prize for Physics and arguably one of the

greatest physicists of the post-war era, is unapologetically frank: “I think I can safely say that

nobody understands quantum mechanics”. Niels Bohr, one the main pioneers of quantum

theory, claimed that: “Anyone who is not shocked by quantum theory has not understood

it”.

In the 1920s and 1930s, Bohr, Schrödinger, Heisenberg and others discovered that the

atomic world is in fact full of murkiness and chaos, and not the precision clockwork

suggested by classical theory. Classical physics can be considered as a good approximation to

quantum physics, typically in circumstances with large numbers of particles. Indeed, classical

physics has served us well up until the 20th Century, and for most everyday purposes it still

does. But modern physics, which includes quantum physics and general relativity, is more

all-encompassing, more fundamental and altogether more accurate - physics taken to a

different level. Momentum and position, for instance, are approximations of the world of

larger-sized things that we call the classical world, but the underlying reality of the quantum

world is quite different.

EARLY DEVELOPMENTS IN ATOMIC THEORY

The development of quantum theory was arguably many centuries in the making. As early as

the 5th Century B.C., the Greek philosophers Democritus and Leucippus first put forward the

idea that everything around us was made of tiny indivisible pieces called atoms scattered in

an infinite void.

However, finding (even indirect) evidence of such atoms had to wait over two millennia until

the work on gases and pressure by the 17th Century English scientist Robert Boyle and, later,

the 18th Century Swiss mathematician Daniel Bernoulli, which bolstered the case for the

existence of tiny grain-like atoms flying around in empty space.

In 1789, the French aristocrat and scientist Antoine Lavoisier identified (albeit slightly

incorrectly) 23 elements which he claimed could not be broken down into simpler

substances. In the very early 19th Century, the English chemist and physicist John Daltonidentified the atomic weights of certain atoms, and first posited the idea that chemical

combinations were due to the interaction of atoms of definite and characteristic weight.

More direct evidence of atoms came with the erratic jittery motion, known as “Brownian




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motion”, of pollen grains suspended in water noted by the botanist Robert Brown in 1827

(even if Brown himself never quite solved the problem of their mysterious behaviour.

But, in order to identify exactly what makes one atom different from another, it was

necessary to look inside an atom. In 1895, the British physicist J. J. Thomson had discovered

the first sub-atomic particle, the electron, a tiny particle 2,000 smaller than even a hydrogenatom, although he had actually envisaged an atom as a multitude of these tiny negatively-

charged electrons embedded in a diffuse ball of positive charge, like raisins in a pudding.

Experiments in 1907 by the New Zealander Ernest Rutherford, however, showed that the

vast majority (more than 99.9%) of the mass of an atom was packed into a tiny central

nucleus (about 100,000 times smaller than the overall atom, depending on the particular

element, sometimes compared to the head of a pin in a football stadium) orbited by the

even tinier electrons, like planets round a sun. Coming across the nucleus inside an atom has

been likened to finding a pea suspended in the space of a cathedral. So, for the first time,

the familiar solid world around us was revealed to be composed of mainly empty space.

Later, in 1913, Rutherford’s model of the atom was fleshed out and refined by the Danish

physicist Niels Bohr. It was not until as late as 1980, though, with the invention of the

Scanning Tunnelling Microscope of Gerd Binnig and Heinrich Rohrer, that the first atoms

were actually "seen", showing materials to be composed of spherical atoms stacked row on

row, much as Democritus had envisaged.

(Thomson's plum pudding model and Rutherford's nuclear (planetary) model of the atom)

As technology improved, the nucleus of an atom was shown to consist of a combination of

positively-charged protons and zero-charged neutrons. By the late 1960s, it was known that

protons and neutrons themselves have size and extension and so could not be considered

fundamental particles, but were made up of yet smaller matter. Each proton and neutron

was revealed to be comprised of three quarks, elementary particles which come in six

different flavours (up, down, top, bottom, strangeness and charm) and which are several

orders of magnitude smaller than the protons and neutrons they make up.




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The very names of these properties tell of their arbitrariness, and it is impossible to say what

they might look like in the physical world. The name quark was taken from a particularly

difficult James Joyce novel by Murray Gell-Man (who was instrumental in their discovery),

and was used in preference to Richard Feynman’s more light -hearted suggestion of the

“parton” (after Dolly Parton). A hypothetical sub-component of quarks and electrons,

dubbed the preon, was postulated in the 1970s and 1980s, but it has since been largely

abandoned as superfluous and experimentally unproven. Quarks and electrons do in fact

appear to be elementary particles.

The current Standard Model of particle physics consists of 16 elementary particles: up,

down, top, bottom, charm and stange quarks; electrons; muon and tau leptons; three types

of neutrinos; Z and W bosons; photons; and gluons. These particles are acted up on by three

fundamental forces: the electromagnetic force, the strong nuclear force and the weak

nuclear force. In addition to these particles, the Higgs boson (dubbed the "God particle" by

the popular media) is an integral part of the model, as it explains why the other elementaryparticles (with the exception of gluons and photons) have mass, and therefore why matter in

general has mass. It has proved to be a remarkably robust and successful model, and several

particles which were merely theoretical when it was first formulated in the mid-1970s have

since been observed experimentally (although the Higgs boson remains tantalizingly elusive).

(The Standard Model of Particle Physics)




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But, to return to Rutherford’s findings in the early 20th Century, though, it quickly became

abundantly clear that his findings were completely at odds with the known physics of the

time. According to the classical theory of electromagnetism developed in the second half of

the 19th Century by James Clerk Maxwell (who demonstrated that electricity, magnetism

and even light are all manifestations of the same phenomenon, the electromagnetic field),

any charged particle which accelerates or changes speed or direction gives out

electromagnetic waves. In that case, then, an electron circling a nucleus should in theory be

continually sapped of its energy, and should therefore spiral into, and collide with, the

nucleus of the atom within micro-seconds.So, even before 1910, it was becoming apparent

that atoms should not even be able to exist according to classical theory!

QUANTA AND WAVE-PARTICLE DUALITY

The earliest steps in the development of quantum physics arose from the investigation into

something as mundane as why metal glows red when hot. The great German physicist Max

Planck had been studying the problem of black body radiation in the late 1890s. The

“problem” Planck was dealing with was the observation that the greatest amount of energy

being radiated from a black body (or any perfect absorber) actually falls near the middle of

the electromagnetic spectrum, rather than in the ultraviolet region as classical theory

suggested.

While Planck’s initial black body radiation law described the experimentally observed black

body spectrum quite well, it was not perfect, and it was Planck’s genius to realize that the

only way the law could work perfectly was to incorporate the supposition that

electromagnetic energy could be emitted only in “quantized” form (i.e. restricted to discrete

values rather than to a continuous set of values). In 1900, he proposed that light and other

electromagnetic waves were emitted in discrete packets of energy, which he called

"quanta", which can only take on certain discrete values (multiples of a certain constant,

which now bears the name the “Planck constant”). He concluded that the energy radiated

from a black body could only be a multiple of an elementary unit, E, where E = hv (where h is

the Planck constant, and v is the frequency of the radiation).

In effect, Planck showed that the very structure of nature is discontinuous, in the same way

as the population of a city, for example, can only change in discrete increments (i.e. whole

number of people). Although, quantization was a purely formal assumption in Planck’s work

at this time, and he never fully understood its radical implications (that had to await Albert

Einstein’s interpretations in 1905), it has come to be regarded as the first essential stepping

stone in the development of quantum theory, and the greatest intellectual accomplishment

of Planck's career, for which he was awarded the Nobel Prize in Physics in 1918.

Building on this earlier research by Planck and by Philippe Lenard, Einstein became, in 1905,the first person to clearly realize that light was made up of photons. He saw it as the only

way to make sense of the so-called "photoelectric effect" (the phenomenon whereby certain

metals, when exposed to light, eject electrons).




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(Emission of electrons from a metal plate (photoelectric effect))

Einstein found that, no matter how bright the light shone on the metal, only light above a

certain frequency caused electrons to be given off. Above that point, as the frequency of the

light is increased, the energy of the electrons given off also increased. Furthermore, he

noted that all the electrons were emitted instantaneously, with no delay whatsoever, which

could not happen if the light was a wave sweeping over the metal, but only if the electron

emissions were caused by individual particles of light.

Einstein, therefore, extended Planck’s discovery by theorizing that energy itself (not just the

process of energy absorption and emission) is quantized. Light, he concluded, must consist

of tiny bullet-like particles, now known as photons. In fact, it was for this work on the

photoelectric effect in 1905 that Einstein was awarded the 1921 Nobel Prize in Physics, not

for his better known work (in the same year) on the Special Theory of Relativity.

In 1913, the Danish physicist Niels Bohr further built on Planck’s insights and on the recent

discoveries of J. J. Thomson and Ernest Rutherford about the structure of atoms. Bohr

introduced the idea that electrons can only orbit an atom's nucleus at certain discretedistances. This happens because electrons are also waves of specific frequencies, and the

waves only fit (without interfering with themselves or cancelling each other out) on orbits of

certain sizes. Electrons closer to the nucleus have lower energy than those further away

(even though they are travelling faster).

However, although an electron can only exist in certain discrete energy levels (or "quantum

states"), it can move from one energy level to another. For example, if an atom is heated or

forced to collide, the energy imparted can cause an electron to move to a higher energy

level (we say that the electron is "excited"). Bohr noted that it did not gradually pass through

a continuum of energy levels in between, but rather there was a "quantum leap" or

“quantum jump”, and the electron instantly leaped from one energy level to the next. A




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useful analogy is that of climbing a set of stairs, where it is possible to stand on any given

step, but not somewhere in between two steps.

(Quantum jump of an electron from one energy level to another)

He also discovered that when an electron drops from a higher energy orbit to a lower one

(which it will do whenever there is a lower energy state available for it to occupy), it emits in

the process a photon (an individual quantum, or packet, of electromagnetic radiation) with

energy exactly equal to the difference between the energy levels of the two orbits. This is

essentially why a heated object glows: the heat causes electrons to jump into excited states;

then, when they drop back down to the "ground" state, the atom gives off photons of light.

Likewise, if light with the right energy strikes an atom, then its electrons will be excited and

rise to a higher energy state, and the light will be absorbed.

It had been observed for some time that, as atoms were heated, light was emitted not as a

diffuse, blurred smear but in distinct and separate bands of colour, with each element

producing its own unique spectral pattern, its own distinct "spectral fingerprint", but it had

always been beyond the explanatory powers of classical physics. Bohr's revelation, that an

electron jumps from one distinct state to another, neatly explained why the light was

emitted in distinct bands of colour, as electrons with specific energy levels within different

elements changed their quantum states.

This arrangement of the electrons within atoms also has some very useful practical

applications. Because of the very structured and regular arrangement of atoms in solids, the




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energy levels of electrons within constituent atoms combine to form continuous energy

bands (known as valence bands) separated by band gaps. The band structure of a material

determines several characteristics, in particular the material's electronic and optical

properties (e.g. some materials have very close, or even overlapping, bands so that electrons

can easily move between them, which makes them good conductors of electricity; other

materials have very large band gaps which makes them good insulators; etc).

So, the early stepping stones towards a fundamentally new type of physics (which was to

become known as quantum theory or quantum mechanics) were gradually falling into place,

and it was becoming clear that an essential element of it was the conception of light (and

indeed all radiation and all matter) as composed of discrete quanta or particles.

(Wave interference in Thomas Young's double-slit experiment)

However, it had already been demonstrated beyond doubt that light was in fact a wave.Thomas Young’s experiments with his “double-slit” apparatus at the beginning of the 19th

Century had shown that light caused interference, a characteristic property of waves, and he

was even able to determine its wavelength, which he established was less than a thousandth

of a millimetre. This had seemed at the time to settle forever the dispute which had been

raging since the 17th Century between those (such as Christiaan Huygens) who favoured a

wave theory of light and others (such as Sir Isaac Newton) who favoured a corpuscular or

particle theory of light.

The developments by Einstein and Bohr in the early decades of the 20th Century, therefore,

meant that physicists had to come to terms with the idea that light was both a wave AND a

particle, and that sometimes it behaved like a wave and sometimes it behaved like a particle,

an idea which became known as wave-particle duality. In an absolute sense, then, light is




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actually neither a particle nor a wave, but only exhibits wave or particle properties,

depending on the experiment being performed.

Such an idea, however, was totally incompatible with all physics that had gone before, and

particularly with the whole edifice of Maxwell’s theory of electomagnetic waves which had

become by that time the orthodoxy of classical physics. It was also impossible to visualizeand totally counter-intuitive at first glance. Perhaps wave-particle duality may be most easily

understood by analogy: consider, for example, that a novel is both a story and a collection of

individual words; or that the mind consists simultaneously of both thoughts and a series of

electrical impulses.

But there was more to come. In 1923, Arthur Compton’s famous “Compton scattering”

experiment showed how x-rays (generally understood as waves of electromagnetic

radiation) can be observed to bounce off electrons, thus exhibiting particle-like properties,

just like billiard balls impacting with other billiard balls. He also showed how this particle-like

characteristic of electromagnetic radiation could be measured by its frequencies, previously

considered a characteristic property only of waves.

Furthermore, in 1924, the French physicist Louis de Broglie would show that wave-particle

duality was not merely an aberrant behaviour of light, but rather was a fundamental

principle exhibited by both radiation and ALL particles of matter. According to de Broglie’s

findings, then, at least in theory, everything (a baseball, a car, even a person) has a

wavelength, although their wavelengths are so small as to be not noticeable. Just as Planck

and Einstein had shown that waves can have particle-like characteristics, de Broglie showed

that particles can have wave-like characteristics. In a strange twist of fate, George Thompson

received the 1937 Nobel Prize in Physics for definitively proving the wave properties of he

electron, just as his father had won the 1906 Prize for his discovery of the electron as a

particle.

However, it should be noted that this is not to say that an atom can spread itself out in a

broad beam of some sort: the wave we are talking about is a wave of information, of what

can be known about the atom, a probability wave (which we will discuss in more detail in the

next section). Essentially, the wave is not the particle itself but a measure of the probability

attached to its particle nature.

PROBABILITY WAVES AND COMPLEMENTARITY

The acceptance of light as composed of particles (or photons) led to another shocking

realization. For example, if light shines on an imperfectly transparent sheet of glass, it may

happen that 95% of the light transmits through the glass while 5% is reflected back. This

makes perfect sense if light is a wave (the wave simply splits and a smaller wave is reflected

back). But if light is considered as a stream of identical particles, then all we can say is that

each and every photon arriving at the glass has a 95% chance of being transmitted and a 5%

chance of being reflected.




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The actual behaviour of any individual photon is therefore totally random and unpredictable,

not just in practice but even in principle. Although the tossing of a coin, for example, is

random in practice, if we knew precisely everything about the force, angle, shape, air

currents, etc, we could, in principle, predict the outcome accurately. The behaviour of a sub-

atomic particle, however, is random on a whole different level, and can never be predicted.

Thus, it is not possible to predict a single definite result for an obervation, only a number of

different possible outcomes, each with a particular likelihood or probability. Physics had

therefore changed overnight from a study of absolute certainty, to one of merely predicting

the odds!

The reason we do not see the effects of this on a more macro scale is that everyday objects

are composed of billions or trillions of sub-atomic particles. Although the position of each

individual particle may be highly uncertain, because there are so many of them acting in

unison in an everyday object, the combined probabilities add up to what is, to all intents and

purposes, a certainty.

In order to reconcile the wave-like and particle-like behaviour of light, its wave-like aspect

needs to be able to “inform” its particle-like aspect about how to behave, and vice versa. It

was the Austrian physicist Erwin Schrödinger, along with the German Max Born, who first

realized this and worked out the mechanism for this information transference in the 1920s,

by imagining an abstract mathematical wave called a probability wave (or wave function)

which could inform a particle of what to do in different situations. Erwin Schrödinger

proposed a ground-breaking wave equation, analogous to the known equations for other

wave motions in nature, to describe such a wave. Born further demonstrated that the

probability of finding a particle at any point (its "probability density") was related to the

square of the height of the probability wave at that point.




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(Probability density plots of some hydrogen atomic orbitals)

Schrödinger worked out the exact solutions of the wave equation for the hydrogen atom,

and the results perfectly agreed with the known energy levels of these atoms. It was soon

found that the equation could also be applied to more complicated atoms, and even to

particles not bound in atoms at all. In fact, in theory it applies to ALL matter, althoughmassive objects exhibit very small wavelengths, so small that it is rather pointless to think of

them in a wave fashion. But for small objects like elementary particles, the wavelength can

be observable and significant.

Like light, then, particles are also subject to wave-particle duality: a particle is also a wave,

and a wave is also a particle. Using Schrödinger's wave equation, therefore, it became

possible to determine the probability of finding a particle at any location in space at any

time. This ability to describe reality in the form of waves is at the heart of quantum

mechanics. In 1926, Schrödinger published a proof showing that Heisenberg’s matrix

mechanics and his own wave mechanics were in fact equivalent, and merely represented

different versions of the same theory.

The Danish physicist Niels Bohr, who, along with Heisenberg and Schrödinger, was integrally

involved in the early development of quantum mechanics, tried to come to grips with some

of the philosophical implications of quantum theory in the early 1920s. He felt that the

classical and quantum mechanical models were two complementary ways of dealing with

physics, both of which were necessary, an idea he called “complementarity”. This idea of

complementarity formed the basis of what became known as the “Copenhagen

interpretation” of quantum physics, a deeply divisive idea in the world of physics at the time.

Bohr felt that an experimental observation “collapsed” or “ruptured” the wave function to

make its future evolution consistent with what we observe experimentally (an idea that will

become very important in our subsequent explanations of quantum effects such as

decoherence, entanglement and the uncertainty principle). As soon as a photon, for

example, is observed or detected in a particular place, then the probability of its being

detected in any other place suddenly becomes zero. Up until that point, the particle's

position is inherently uncertain and unpredictable, an uncertainty that only disappears when

it is observed and measured. This immediate transition from a multi-facted potentiality to a

single actuality (or, alternatively, from a multi-dimensional reality to a 3-dimensional reality

compatible with our own everyday experience) is sometimes referred to as a quantum jump.




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(The collapse of a probability wave function)

However, Bohr also believed that there was no precise way to define the exact point at

which such a collapse occurred, and it was therefore necessary to discard the laws governing

individual events in favour of a direct statement of the laws governing aggregations.

According to this model, there is no deep quantum reality, no actual world of electrons and

photons, only a description of the world in these terms, and quantum mechanics merely

affords us a formalism that we can use to predict and manipulate events and the properties

of matter.

The Copenhagen interpretation, then, is essentially a pragmatic view, effectively saying that

it really does not matter exactly what quantum mechanics is all about, the important thing

being that it “works” (in the sense that it correlates with reality) in all possible experimental

situations, and that no other theory can explain sub-atomic particles in any more detail.

Albert Einstein, whose work had been instrumental in much of the early development of

quantum theory, had grave philosophical difficulties with the Copenhagen interpretation,

and carried on an extensive correspondence with both Bohr and Heisenberg on the matter,arguing that the physical world must have real properties whether or not one measures

them, famously claiming in 1926 that “I, at any rate, am convinced that He *God+ does not

throw dice". He took particular exception to Bohr’s claim that a complete understanding of

reality lies forever beyond the capabilities of rational thought. Both Einstein and Erwin

Schrödinger published a number of thought experiments designed to show the limitations of

the Copenhagen interpretation and to show that things can exist beyond what is described

by quantum mechanics.

Einstein's position was not so much that quantum theory was wrong as that it must beincomplete. He insisted to his dying day that the idea that a particle's position before

observation was inherently unknowable (and, particularly, the existence of quantum effects

such as entanglement as a result of this) was nonsense and made a mockery of the whole of




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physics. He was convinced that the positions and quantum states of particles (even

supposedly entangled particles) must already have been established before observation.

However, the practical impossiblity of experimentally proving this argument one way or

another made it essentially a matter of philosophy rather than physics.

And so it remained until the experimental work of the American physicist John Clauser andothers in the early 1970s, as we will see in the later section on Nonlocality and

Entanglement.

SUPERPOSITION, INTERFERENCE AND DECOHERENCE

In the same way as it is possible in the everyday world to get big rolling waves in the sea with

tiny ripples superimposed on them, it is also possible in the sub-atomic world for a

combination or superposition of waves to exist.

Schrödinger’s theory of probability waves permits the existence of two or more waves. In

the example from the previous section of light shining on an imperfectly transparent sheet

of glass, one wave would correspond to a photon passing through the glass and another

wave would correspond to the photon bouncing back. But it is also possible for both waves

to have superposed waves, which leads to the possibility of the photon being both

transmitted AND reflected, and therefore being on both sides of the glass simultaneously!

In fact, this leads to the possibility of a potentially unlimited number of superposed waves,

which means that microscopic particles can theoretically be located in a potentially

unlimited number of places at once, and to behave in a potentially unlimited number of

different ways. Just one of the intriguing possibilities this idea suggests is in the realm of

computing, for example, where a "quantum computer" (still largely hypothetical at this time)

could take advantage of an atom’s ability to be in a superposition of states to produce

prodigiously increased power and speed of calculations.

Interestingly, a modern incarnation of Thomas Young’s double-slit experiment using a very

feeble light source that spits out one photon at a time, leads to the same evidence of

interference, even though there are no waves as such to interfere with each other. The onlyway the single photons can experience interference, then, is if each photon somehow goes

though both slits simultaneously (due to superposition) and interferes with itself! The entire

experiment can in fact be performed with electrons, atoms or other sub-atomic particles

instead of light, demonstrating that ALL particles manifest both wave and particle aspects

(this interference is the mechanism by which a hypothetical quantum computer would

combine its multiple calculations into one answer).




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(Double-slit experiment with a single photon)

In 1978, John Wheeler proposed a variation of Young’s double-slit experiment (and Richard

Feynman’s later refinements of it), often referred to as the "delayed choice” experient. He

posited that the detection of a photon even AFTER passing through a double slit would be

sufficient to change the outcome of the experiment and the behaviour of the photon.Therefore, if the experimenters know which slit it goes through, the photon will behave as a

particle, rather than as a wave with its associated interference behaviour. This somewhat

counter-intuitive hypothesis was finally verified in a practical experiment in 2007.

It should be noted that, in reality, superpositions can never actually be observed - all we can

see is the consequences of their existence, after individual waves of a superposition

interfere with each other. Thus, we can never observe an atom in its indeterminate state, or

being in two places at once, only the resulting consequences, and physical reality is not

determined until the act of measurement takes place and “solidifies” the situation into one

state or another.

Part of the problem of observing and measuring superpositions is known as decoherence.

Any attempt to measure or obtain knowledge of quantum superpositions by the outside

world (or indeed any kind of interaction with their environment, even with just a single

photon) causes them to decohere, effectively destroying the superposition and reducing it to

a single location or state, and also destroying the ability of its individual states to interfere

with each other. Decoherence, then, results in the collapse of the quantum wave function

and the settling of a particle into its observed state under classical physics, its transition

from quantum to classical behaviour.

Decoherence is also the main reason that quantum theory really only applies in practice to

the sub-atomic world: in the large-scale world in which we live, it is all but impossible to




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isolate anything from interaction with its environment, especially given the countless

trillions of photons bouncing off every object all the time. Even an object made of just 60

atoms requires extreme cold to to prevent it from becoming “classical” rather than

"quantum". It is the interaction of quantum objects with the environment that produces

what we understand as classical objects, such as cats and tables. Thus, in practice we never

observe a quantum system directly; we only observe its effect on its environment.

(Artist's impression of Schrödinger's Cat thought experiment)

In 1935, the Austrian physicist Erwin Schrödinger devised his famous thought experiment or

paradox, known as “Schrödinger’s cat”, to graphically illustrate the problem of decoherence

(and to illustrate the general bizarreness of quantum mechanics). He proposed a scenario

with a cat in a sealed box, where the cat's life or death was dependent on the state of a

particular sub-atomic particle. According to the Copenhagen interpretation, the cat is in a

kind of limbo represented by a wave function which contains the possibility that the cat is

dead but also the possibility that it is alive. The cat, then, remains both alive AND dead until

the box is opened, i.e. a superposition. This is usually taken as a demonstration of the way

that quantum physics breaks down when dealing with large objects.

Although the best known and most popular view, the Copenhagen interpretation is,

however, not the only interpretation of the quantum world. Some scientists, going back to

Hugh Everett III in 1957 and subsequently championed by Bryce DeWitt, have hypothesized

that each possible state of a superposition actually exists in a totally separate parallel reality,

which are all part of a potentially infinite multiverse, the so-called “many worlds” idea. Thus,

every indeterminism in a quantum system generates a multifoliate reality in which the

universe is continually branching into myriads of physically disconnected (but equally real)

parallel universes.

Unlike the Copenhagen interpretation (in which consciousness actually influences reality),

the many worlds intepretation requires no observers to shape reality, since it is not




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necessary to collapse a wave function to actualize the universe, and every reality that CAN

conceivably exist is automatically created. On the down side, however, the many worlds

hypothesis is about as extravagant as it is possible to get, quite the opposite of the principle

of parsimony usually known as Occam’s Razor, which dictates that theories should be

constructed using the least possible number of principles and assumptions.

In 1986, John Cramer proposed another alternative interpretation of quantum physics,

which many people feel overcomes the drawbacks of both the standard Copenhagen

interpretation and the many worlds interpretation, and resolves many quantum paradoxes.

Known as the "transactional interpretation", Cramer described quantum interactions as

involving both a wave going forwards in time and a wave gong backwards in time.

(Illustration of the transactional interpretation of quantum mechanics)

According to the transactional interpretation, in any quantum transation (such as the

propagation of an electromagnetic wave between two particles, for example), the receiving

particle’s backward wave reinforces the emitting particle’s forward wave so long as it is

between the particles, but it cancels out the emitting particle’s own backward wave (which

is why a backward wave is not seen before the emission); and the receiv ing particle’s

forward wave also cancels out the emitting particle’s forward wave (which is why a forwardwave is not preceived after the receiving particle has absorbed it). Thus, the particles

interact in the way called for by quantum physics (including the “collapse of the wave

packet”, etc) without requiring an act of measurement by an external observer, allowing for




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the existence of a "real world" out there independent of us. The interpretation has it critics,

but has gained much traction since its initial proposal.

There are also many other competing interpretations of quantum theory, including the

consistent histories interpretation, the ensemble interpretation, relational quantum

mechanics, stochastic mechanics, objective collapse theories, the many mindsinterpretation, the modal interpretation, and several others. It remains a highly active (and

controversial) area of modern physics.

QUANTUM TUNNELLING AND THE UNCERTAINTY PRINCIPLE

One of the consequences of light having a wave-like aspect is exemplified by its apparent

ability to jump gaps. For instance, light penetrating through a block of glass at a shallow

angle is effectively trapped within the glass by the barrier of air at the far side, unless a

second glass block is placed close to it (but not touching). Because of the spread-out nature

of the wave, some of it penetrates the air barrier and if encounters more glass beyond it can

continue, thus apparently jumping the air gap and escaping its prison.

A similar thing happens at the sub-atomic scale, when alpha particles try to escape from

unstable nuclei during radioactive decay. The particles are effectively held in the nucleus by

the nuclear forces and, in principle, should not be able to escape. However, escape they do,

using a process known as quantum tunnelling, which makes use of the wave-like aspect of

the particles, but also of a more general phenomenon known as "uncertainty" (which we will

look at in more detail below).

Due to the wave-like aspect of particles, and the ability to describe an object by means of a

probability wave, as we have seen, quantum physics predicts that there is a finite probability

that an object trapped behind a barrier (without the energy to overcome the barrier) may at

times appear on the other side of the barrier, without actually overcoming it or breaking it

down. For instance, if an electron approaches the nucleus of an atom, there is some

probability, however small, that it will find itself on the other side of the electromagneticfield which repels it.

This possibility of being detected on the other side of a barrier has become known as

tunnelling, although there is certainly no actual physical digging going on. It can perhaps be

best visualized by imagining a broad wave approaching, and then slightly overlapping, a

barrier. Although the main part of the wave may never penetrate the barrier, a small part of

it does, allowing for the possibility of the particle which is generating the wave suddenly

being located on the other side of the barrier.

The uncertainty principle was first recognized by the German physicist Werner Heisenberg in

1926 as a corollary of the wave-particle duality of nature. He realized that it was impossible

to observe a sub-atomic particle like an electron with a standard optical microscope, no




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matter how powerful, because an electron is smaller than the wavelength of visible light. He

conceived of an imaginary microscope which used gamma rays (which have a wavelength

much smaller than an electron) rather than visible light. But, because gamma rays are so

much more energetic than visible light, they would have the effect of changing the speed

and direction of the electron in an unpredictable and uncontrollable way. So, in solving one

part of the problem, another problem is necessarily created.

(Quantum tunnelling through a barrier)

In fact, through his famous “microscope” thought experiment, he realized that a similar

thing was happening to some extent even within a standard optical microscope. To measure

the position and velocity of a particle, a light can be shone on it, and then the reflection




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detected. On a macroscopic scale this method works fine, but on sub-atomic scales the

photons of light that hit the sub-atomic particle will cause it to move significantly. So,

although the position may have been measured accurately, the velocity of the particle will

have been altered, and, by learning the position, any information previously known about

the velocity has been rendered useless. In other words, the very act of observation affects

the observed.

Heisenberg realized, then, that the values of certain pairs of variables cannot BOTH be

known exactly, so that the more precisely one variable is known, the less precisely the other

can be known. If the speed (or, more strictly, the momentum) of a particle is known exactly,

then its location must be uncertain; conversely, the more certainly its location is known, the

less certain is the particle’s speed (or momentum). Likewise, if the energy state of a particle

is known with certainty, then it can not be determined how long it will remain in that state

(and vice versa). In slightly more mathematical terms, he showed that the uncertainty in the

position of a particle times the uncertainty in its velocity times its mass can never be smallerthan a certain quantity, known as the Planck constant.




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(Heisenberg's microscope thought experiment to illustrate the effects of the uncertainty

principle)

With the advent of the uncertainty principle, then, particles could no longer be said to have

separate, well-defined positions and velocities, but only a “quantum state”, a combination of

position and velocity. If it is not possible to know the values of all of the properties of thesystem at the same time, then those properties that are not known with precision must be

described by probabilities. The principle effectively overturned in one fell swoop the whole

doctrine of scientific determinism which had been implicitly assumed since Newton and

Laplace in the 17th Century, and redefined the task of physics as the discovery of laws that

will allow us to predict events UP TO THE LIMITS set by the uncertainty principle.

In a way, the uncertainty principle exists to protect quantum theory, in that if the properties

of atoms and particles could be known with certainty, then they would decohere and their

wave behaviour and their ability to interfere would thereby be destroyed. There is therefore

a built-in limit to our knowledge of the microscopic world, and nature does not permit us to

measure precisely all we would like to measure. However, it should be noted that this is not

due to imprecise measurements in practice (technology is advanced enough to

hypothetically yield correct measurements); rather, the blurring of the measurable

quantities of a particle (mass, velocity and position) is a fundamental property of nature

itself, and does not depend on the type of particle or the method of measurement.

Returning, then, to the earlier question (introduced back in the section on the Early

Developments in Atomic Theory) of why orbiting electrons do not lose energy and spiral into

the nucleus of an atom, it is the uncertainty principle that prevents electrons from

approaching the nucleus too closely. If an electron gets too close then its location in space

would be very precisely known, and its velocity would therefore be very uncertain and it

could acquire enormous speed, enough to ensure that it did not stay confined in the

nucleus.




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(An energy-losing electron should fall into the nucleus)

In the same way, the uncertainty principle explains how an alpha particle is able to escape

the nucleus of a radioactive atom. Trapped in the nucleus, the alpha particle is very localized

in space, and its position is pinned down with great accuracy. In that case, according to the

uncertainty principle, its velocity must be very uncertain, possibly much greater than we

would have expected, and possibly enough to escape the pull of the nucleus.

A similar situation in reverse explains how nuclear fusion is possible in the Sun, when the

temperatures in the Sun are actually about a thousand times cooler than the massive

temperatures which are theoretically needed to provide the incoming protons with enough

energy and speed to overcome the strong repulsive electromagnetic force of the receiving

hydrogen atoms. By virtue of quantum tunnelling and Heisenberg’s uncertainty principle, the

protons can “tunnel” through the barrier even given the apparently insuf ficient temperature

and energy.

The uncertainty principle also explains why a typical atom is over 100,000 bigger than the

nucleus at its centre. Strictly speaking, the uncertainty principle holds that it is a particle’s

position and its momentum (its mass times its velocity, as opposed to just its velocity) which

cannot be simultaneously be known with certainty. Because an electron is about 2,000 times

less massive than the protons in a nucleus, and because the repulsive electromagnetic force

it is subject to is about 50 times weaker than the strong nuclear force in the nucleus, these

two factors together result in the need for about 100,000 times as much space for the

electron to move around in.




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(Virtual electron-positron pairs appearing at random near an electron)

In fact, the uncertainty principle can be reformulated in yet another way to say that it is

impossible to simultaneously measure the energy of a particle and the interval of time for

which it has been in existence. Over a very tiny interval of time, there can therefore be a

large uncertainty in the energy content of a particular location, and energy or even pairs of

fundamental particles (known as "virtual particles", because they exist for such a short time

that they are not considered part of everyday reality) could even appear out of nothing in

the apparently empty vacuum of space and exist for a split-second before disappearing

again. The more energy that is put into a vacuum, the more particles causelessly pop out of

it. It appears, then, that there is no such thing as empty space.

Such a phenomenon (particles constantly popping in and out of existence), unlikely though it

may sound, is well documented and has actually been indirectly observed through

observations of the changing energy of existing electrons which are buffeted by such

appearances and disappearances. In effect, the energy needed to create these virtual

particles can be "borrowed" from the vacuum for a period of time, but the net energy from

the reaction is still zero. Because overall they cancel each other out, they cannot be said to

even exist in the classical world, nor to break any of the laws of classical physics.




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NONLOCALITY AND ENTANGLEMENT

Another of the remarkable features of the microscopic world prescribed by quantum theory

is the idea of nonlocality, what Albert Einstein rather dismissively called “spooky actions at a

distance”. This was first described in the “EPR papers” of Einstein, Boris Podolsky and Nathan

Rosen in 1935, and it is sometimes referred to as the EPR (Einstein-Podolsky-Rosen) paradox.It was even more starkly illustrated by Bell’s Theorem, published by John Bell in 1964, and

the subsequent practical experiments by John Clauser and Stuart Freedman in 1972 and by

Alain Aspect in 1982.

Nonlocality describes the apparent ability of objects to instantaneously know about each

other’s state, even when separated by large distances (potentially even billions of light

years), almost as if the universe at large instantaneously arranges its particles in anticipation

of future events.

Thus, in the quantum world, despite what Einstein had established about the speed of light

being the maximum speed for anything in the universe, instantaneous action or transfer of

information does appear to be possible. This is in direct contravention of the "principle of

locality" (or what Einstein called the "principle of local action"), the idea that distant objects

cannot have direct influence on one another, and that an object is directly influenced only by

its immediate surroundings, an idea on which almost all of physics is predicated.

(An entangled pair of particles can be seen to have complementary properties when

measured)




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Nonlocality suggests that universe is in fact profoundly different from our habitual

understanding of it, and that the "separate" parts of the universe are actually potentially

connected in an intimate and immediate way. In fact, Einstein was so upset by the

conclusions on nonlocality at one point that he declared that the whole of quantum theory

must be wrong, and he never accepted the idea of nonlocality up till his dying day.

Nonlocality occurs due to the phenomenon of entanglement, whereby particles that interact

with each other become permanently correlated, or dependent on each other’s states and

properties, to the extent that they effectively lose their individuality and in many ways

behave as a single entity. The two concepts of nonlocality and entanglement go very much

hand in hand, and, peculiar though they may be, they are facts of quantum systems which

have been repeatedly demonstrated in laboratory experiments.

For example, if a pair of electrons are created together, one will have clockwise spin and the

other will have anticlockwise spin (spin is a particular property of particles whose details

need not concern us here, the salient point being that there are two possible states and that

the total spin of a quantum system must always cancel out to zero). However, under

quantum theory, a superposition is also possible, so that the two electrons can be

considered to simultaneously have spins of clockwise-anticlockwise and anticlockwise-

clockwise respectively. If the pair are then separated by any distance (without observing and

thereby decohering them) and then later checked, the second particle can be seen to

instantaneously take the opposite spin to the first, so that the pair maintains its zero total

spin, no matter how far apart they may be, and in total violation of the speed of light law.

Despite Einstein's misgivings about entanglement and nonlocality and the practical

difficulties of obtaining proof one way or the other, Irish physicist John Bell attempted to

force the issue by making it experimental rather than just theoretical. Bell’s Theorem,

published in 1964, and referred to by some as one of the most profound discoveries in all of

physics, effectively showed that the results predicted by quantum mechanics (for example,

in an experiment like that described by Einstein, Podolsky and Rosen) could not be explained

by any theory which preserved locality. The subsequent practical experiments by John

Clauser and Stuart Freedman in 1972 seem (despite Clauser's initial espousal of Einstein's

position) to definitively show that the effects of nonlocality are real, and that "spooky

actions at a distance" are indeed possible.

In theory, the concepts of entanglement and nonlocality may have applications in

communications and even teleportation, although these ideas are still largely hypothetical at

this stage. Due to the effects of the uncertainty principle, the mere act of observing the

properties of particles at a quantum level (spin, charge, etc), disturbs the quantum system

irrevocably, and this would appear to prevent us from using this system as a means of

instantaneous communication. However, Anton Zeilinger's work at two observatories in the

Canary Islands has shown promising indications that entangled particles can indeed be

reconstituted in a different place (although the leap from this to a teleportation device of

the kind envisaged in Star Trek is a profound one).




110

SPIN AND THE PAULI EXCLUSION PRINCIPLE

To return to the property of spin of fundamental particles, mentioned briefly in the previous

section, spin can perhaps be most easily thought of as a rotation of particles around their

own axis, although this is in fact something of a simplification, and in reality it is impossible

to tell whether something as small as an electron is spinning at all. In general, though, spin

obeys the same mathematical laws of angular momentum as do spinning objects in classical

physics (such as the Earth, for instance), and there are really only two important aspects to

consider: the speed of rotation and the direction of the axis it rotates about (referred to as

“up” and “down”).

When spin was first discovered in 1922 by Otto Stern and Walther Gerlach, however, their

experiments indicated that the intrinsic angular momentum, or spin, of a particle such as an

electron was quantized i.e. it could only take certain discrete values. The spin of composite

particles (such as protons, neutrons and atomic nuclei) is just the sum of the spins and

orbital angular momentum of the constituent particles, and is therefore subject to the same

quantization conditions. Spin is therefore a completely quantum mechanical property of a

particle, and cannot be explained in any way by classical physics.

(Artist's representation of the spin and charge of an electron)

Now, it turns out that there are two sub-categories of particles: those with “integer” spin,

which are known as bosons, and which include photons, gluons, W and Z bosons andhypothetical gravitons; and those with “half -integer spin”, which are known as fermions, and

which include electrons, neutrinos, muons, and the quarks which make up composite

particles like protons and neutrons. Another way of describing the difference between




111

bosons and fermions is that bosons have symmetric wave functions while fermions have

antisymmetric wave functions. The concept of a particle with half-integer spin is just another

example of the apparently counter-intuitive nature of sub-atomic particles: crudely speaking,

a fermion such as an electron has to spin around TWICE before it presents the same face as

before.

The significance of this distinction for quantum theory is that the probability waves of

bosons “flip” or invert before they interfere with each other, which effectively leads to their

more “gregarious” nature, which in turn can lead to collective behaviour like that of lasers,

superfluids and superconductors. Fermions, however, do not flip their probability waves,

which, among other implications, leads to their “unsociable” nature. Thus, the spins of

particles have to be added together very carefully using special rules for addition of angular

momentum in quantum mechanics.

This discussion of the property of spin leads us to one of the most important principles in

quantum physics, the Pauli exclusion principle (formulated by Wolfgang Pauli in 1925), which

states that no two identical fermions may occupy the same quantum state simultaneously

(although two electrons, for example, may acquire opposite spin in order to differentiate

their quantum states). Another way of stating the principle is that no two fermions in a

quantum system can have the same values of all four quantum numbers at any given time.

This principle effectively explains the continued existence of very high density white dwarf

stars, but also the very existence of different types of atoms in the universe and the large-

scale stability and bulk of matter.




112

(Electron shell diagram for the element uranium (using the Bohr model of the atom))

To understand why, it is necessary to know that, according to the Bohr model of the atom,

electrons in an atom (which exist in the same quantity as the number of protons in the

nucleus of the particular atom, so that the overall electric charge is zero) are constrained to

occupy certain discrete orbital positions or “shells” around the nucleus. The closer electrons

are to the nucleus, the more strongly the electric force is pulling them in and the more

energy would be required to free it from the clutches of the nucleus (or, looked at another

way, the more energy of its own an electron has, the less additional energy it needs in orderto escape). The innermost shell can accommodate just two electrons, one with spin “up” and

one with spin “down” in order to differentiate their quantum states. The next shell out, in a

higher energy level, can accommodate a further eight, the next a further eighteen and the

next thirty-two.

Actually, more resent research has yielded a more accurate “refined” Bohr model of the

atom with each energy level composed of a certain number of sub-shells (named s, p, d and

f) which can each hold only a certain number of electrons. For instance, the s sub-shell can

only hold 2 electrons, the p sub-shell can hold 6, the d sub-shell can hold 10 and the f sub-shell can hold 14. The number of available sub-shells increases as the energy level increases,

so that successive shells can hold a total of 2, 8, 18 and 32 electrons.




113

(Depiction of an atom of nitrogen (using the refined Bohr model))

It is the Pauli exclusion principle which dictates this arrangement and effectively forces

electrons to “take up space” in the atom through this arrangement of shells. By recognizing

that no two electrons may occupy the same quantum state simultaneously, it effectively

stops electrons from "piling up" on top of each other, thus explaining why matter occupies

space exclusively for itself and does not allow other material objects to pass through it, while

at the same time allowing light and radiation to pass.

It also explains the existence of the different atoms in the periodic table of elements and the

sheer variety of the universe around us. For example, when an atom gains a new electron, it

always goes into the lowest energy state available (i.e. the outermost shell). Two atoms with“closed” shells find they cannot form a chemical bond with each other because the electrons

in one atom find no available quantum states in the other which it can occupy. So, the

arrangement of electrons, particularly of the electrons in the outermost shell, also affects

the chemical properties of an element and how atoms are able to bond and combine with

other atoms (the basis of chemistry), and therefore the way in which molecules interact to

form gases, liquids or solids, and how they aggregate themselves in living organisms.

Another effect of the Pauli exclusion principle is that, if two identical particles are forced (for

instance, by an extremely strong gravitational force) to try to have the same quantumnumbers, they will respond with a repelling outward force, known as "degeneracy pressure"

or "Pauli repulsion". A type of star called a degenerate white dwarf is held up entirely by this

force.




114

The Pauli exclusion principle is one of the most important principles in quantum physics,

largely because the three types of particles from which ordinary matter is made (electrons,

protons and neutrons) are all subject to it, so that all material particles exhibit space-

occupying behaviour. Interestingly, though, the principle is not enforced by any physical

force understood by mainstream science. When an electron enters an ion, it somehow

mysteriously seems to "know" the quantum numbers of the electrons which are already

there, and therefore which atomic orbitals it may enter, and which it may not.

CONCLUSION

This has been a necessarily abbreviated and condensed foray into the wonderful, and

sometimes bizarre, world of quantum mechanics. If a couple of fundamental principles of

quantum physics were to be singled out from all of the above, they would probably be the

dual wave-like and particle-like behaviour of matter and radiation, and the prediction of

probabilities in situations where classical physics predicts certainties.

For many, even those in the scientific community, these are difficult concepts to come to

terms with, and even Albert Einstein had serious philosophical problems with a universe

which behaves in an apparently totally random manner at the sub-atomic level, repeatedly

claiming that “God does not play dice” (although he is widely considered to have “lost” the

extensive public debates he carried on with Niels Bohr). For a better or more comprehensiveunderstanding of this complex and confusing subject, there is a copious amount of literature

on the subject, both for the beginner and the expert alike, a few of which are mentioned on

the Sources page.

Despite its difficulties, however, quantum theory remains an essential part of the bedrock of

modern physics. It is arguably one of the most successful theories in all of science, and,

despite its seemingly esoteric nature, it is primarily a practical branch of physics, paving the

way for applications such as the laser, the electron microscope, the transistor, the

superconductor and nuclear power, as well as explaining at a stroke important physicalphenomena such as chemical bonding, the structure of the atom, the conduction of

electricity, the mechanical and thermal properties of solids and the density of collapsed

stars.

However, successful as it is in predicting and describing the world around us, quantum

theory only successfully explains three of the four fundamental forces: electromagnetism,

the strong nuclear force and the weak nuclear force. It does not explain the workings of

gravity.

As has been mentioned in other sections (see here, for example), the way forward forphysics seems now to rest with attempts to combine quantum theory with the General

Theory of Relativity in a unified theory of quantum gravity (or quantum theory of gravity),

the so-called “theory of everything”, which it is hoped will make sense of the entire universe.




115

Candidates like superstring theory and loop quantum gravity, however, still need to

overcome major formal and conceptual problems before such a claim can be made.

In parting, let me just mention one other interesting field of speculation related to quantum

theory. It has been suggested that, if the whole universe (space, time, energy and everything

else) is quantized and consists of indivisible fundamamental particles, then it has a finitenumber of components and a finite number of states, like the bits and pixels of a computer

program. In theory, this makes the universe "computable", and has led some to hypothesize

that perhaps all of reality as we perceive it might actually be part of a huge Matrix-like

computer simulation, individual parts of which only assume definite form when observed.

Speculation only, perhaps, but an intriguing one nonetheless, and one which seems

increasingly difficult to disprove as the details of quantum theory are ironed out.




116




117

The Beginnings of Life—

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- Introduction………………………………………………………………(118) -

The Early Earth and the Building Blocks of Life…………….(119) -

Early Theories………………………………………………………………(123) -

The Primeval Soup Theory…………………………………………..(125) -

Other Terrestrial Theories…………………………………………...(127) -

Exogenesis……………………………………………………………………(129) -

Conclusion…………………………………………………………………...(134)

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118

INTRODUCTION

What are the origins of life on Earth? How did things go from non-living to living, from something

that could not reproduce to something that could? How can a collection of inanimate atoms become

animate? How did organic molecules achieve a high enough level of complexity to be considered as

“living”? The short answer is: we do not really know how life originated on this planet. The longer

answer, however, is much more interesting.

The study of the origin of life on Earth or, more specifically, how life on Earth began from inanimate

matter, is technically known as abiogenesis (as opposed to biogenesis, which is the process of

lifeforms producing other lifeforms, and as opposed to evolution, which is the study of how living

things have changed over time since life first arose).




119(Illustration and comparison of of RNA and DNA molecules)

The modern definition of abiogenesis, however, is concerned with the formation of the simplest

forms of life from primordial chemicals, rather than the old Aristotelian concept of abiogenesis,

which postulated the formation of fully-formed complex organisms by spontaneous generation. It

becomes, then, the search for some kind of molecule (along the lines of RNA or DNA) that is simple

enough that it can be made by physical processes on the young Earth, yet complicated enough that it

can take charge of making more of itself, which is probably what most people would recognize as

constituting “life”.

The first living things on Earth, single-celled micro-organisms or microbes lacking a cell nucleus or cell

membrane known as prokaryotes, seem to have first appeared on Earth almost four billion years ago,

just a few hundred million years after the formation of the Earth itself. By far the longest portion of

the history of life on Earth, therefore, has involved the biochemical evolution of these single-celled

micro-organisms, bacteria and archaea: we can find individual fossilized microbes in rocks 3.4 billion

years old, yet we can only conclusively identify multi-celled fossils in rocks younger than 1 billion

years.

It is presumed that, over a few hundred million years of evolution, pre-biotic molecules evolved into

self-replicating molecules by natural selction. While some aspects of the subject are well understood,

others remain clouded in mystery and are the source of much contention among scientists. Although

much progress has been made, there is still no single definitive theory.

Life, for all its complexity, is woven out of just 30 or so different molecules, constructed from some of

the most abundant elements in the universe: oxygen, hydrogen, carbon, nitrogen, sulphur and

phosphorus. However, no one has yet succeeded in synthesizing a “protocell” using basic

components which would have the necessary properties of life (something which has been mademuch of by religious creationists and anti-evolutionists), although recent work work, such as that of

Jack Szostak at Massachusetts General Hospital and the Howard Hughes Medical Institute, may be

about to change that.

The beginnings of life is, strictly speaking, a matter of biology not physics, and essentially unrelated

to most of the rest of the content of this website. I have included a brief discussion of it here,

however, partly because it is another aspect of modern science which many people find confusing

and puzzling, partly just because I find it really interesting (for which I made no apologies!), and

partly because we are, after all, made of “stardust”.

THE EARLY EARTH AND THE BUILDING BLOCKS OF LIFE

The Earth was formed about 4.5 billion years ago from the swirling dust and gas remnants of an old

star’s supernova explosion. As the molten mass settled and cooled, a solid crust soon formed,

probably within as little as about 150 million years, along with a rudimentary atmosphere composed

largely of carbon dioxide, water vapour and nitrogen.

After a near-catastrophic collision with another planet soon after the Earth's formation (which

created the Moon in the process), it is thought that warm oceans gradually formed, from steam




120escaping from the crust and from volcanic activity and icy meteorites, relatively soon after the

Earth’s formation, perhaps within 750 million years of Earth's formation (about 3.8 billion years ago).

Although the environment at that time (including the constant bombardment by asteroids and

prodigious volcanic activity) would have been highly hazardous to life, the necessary ingredients

were all present in some form or another: liquid water, chemical building blocks (usually taken to be

the six elements: oxygen, hydrogen, carbon, nitrogen, sulphur and phosphorus) and some kind of

energy source.

Liquid water is considered essential to the initial development of life because many chemicals

dissolve easily in water allowing them to mix together and react, because liquid water is the right

temperature for chemical reactions to happen, and also because many chemicals have parts which

are attracted to water and parts which are repelled by it (which also helps reactions happen). Carbon

is important because of its ability to form long chain-like molecules (carbon chains form the

backbone of organic molecules). Hydrogen and oxygen (the two elements that make up water

molecules) as well as nitrogen can all bond with carbon in many different ways, and large molecules

made from carbon, hydrogen, oxygen and nitrogen also tend to be very stable. All chemical reactions

need an energy source to drive them, whether it be ultraviolet light from the Sun or electrical energy

from lightning or chemical energy from deep-sea vents, all of which would have been available on

the early Earth.

(Artist's impression the Earth's early atmosphere and oceans)




121Cyanobacteria (also known as blue-green algae), one of the earliest types of prokaryotic bacteria,

formed into colonies or mats called stromatolites, and fossilized remains of these have been found in

Australia dating back to between 3.4 and 2.8 billion years ago. Ancient as their origins are, these

bacteria (which are still around today) were already biologically complex, with cell walls protecting

their protein-producing DNA, so scientists think it likely that life actually began much earlier, perhaps

as early as 3.8 billion years ago.

These early cyanobacteria were the first oxygen-producing, evolving, phototropic organisms, and

they were responsible for the initial oxygenation of the Earth's atmosphere, as they produced oxygen

while sequestering carbon dioxide in organic molecules during the period from 2.7 to 2.2 billion years

ago. Photosynthesizing plants evolved later and continued this process, leading to the build up of

increasing levels of oxygen in the atmosphere, as well as the release of nitrogen into the atmosphere

as the oxygen reacted with ammonia. Eventually, a layer of ozone (an allotrope of oxygen) formed in

the atmosphere, which better protected early lifeforms from ultraviolet radiation. While oxygen was

apparently not needed for the origination of life on Earth (indeed it is thought by many that the

absence of oxygen was a necessary condition), the rapid explosion of life began only after oxygenbecame abundant.

The first eukaryotic cells (organisms with one or more complex cells, each of which contains a

nucleus and is surrounded by a membrane that holds the cell’s genetic material) evolved sometime

between 2.5 and 1.7 billion years ago, perhaps coincident with the rise in atmospheric oxygen to a

level able to support this more complex life. The nucleus in these cells was able to hold and protect

complex molecules such as RNA and DNA.




122

(Diagrams of a eukaryotic and a prokaryotic cell)

As recently as the 1970s, a whole new group of single-celled organisms known as archaea was

discovered, which is now recognized as a third domain of life, completely separate from both

prokaryotes and eukaryotes. Many scientists believe archaea to be the common ancestor of both

prokaryotes and eukaryotes, and as such may represent the oldest form of life on earth.

Throughout the Proterozoic era, from about 2.3 billion years ago until around 600 million years ago,

life on Earth was mostly single-celled and small, consisting of bacteria, archaea and eukaryotic algae.

The first multi-cellular life probably arise around 1.2 billion years ago, in geological terms almost

overnight, while the landmass of the Earth was still a single continent called Rodinia. It presumably

started out as a sort of symbiosis, a loose cooperation between single cells that became more and

more complex. Evolution appeared to speed up again about 550 million years ago with the sudden

appearance of the first hard-bodied animals in the fossil record.

As recently as the 1970s, a whole new group of single-celled organisms known as archaea was

discovered, which is now recognized as a third domain of life, completely separate from both

prokaryotes and eukaryotes. Many scientists believe archaea to be the common ancestor of both

prokaryotes and eukaryotes, and as such may represent the oldest form of life on earth.




123

EARLY THEORIES

Back at least to the time of Aristotle in ancient Greece, men of science used to believe in the

spontaneous generation of life from non-living matter. Aristotle laid it down as an observed fact that

some animals spring from putrid matter, that lice arise from the dew which falls on plants, that fleas

developed out of decayed material, that mice come from sweat or dirty hay, crocodiles from rotting

logs, etc.

Even Robert Hooke and Anthony van Leeuwenhoek’s discoveries of micro-organisms in the mid-17th

Century were taken as further proof for spontaneous generation, and it was not until 1861 that Louis

Pasteur’s experiments showed that organisms such as bacteria and fungi do not appear of their own

accord in sterile nutrient-rich media, and support for biogenesis and rudimentary cell theory began

to grow. However, this still did not address the question of how primitive life had arisen in the first

place.

(Carbon is the basis of all organic molecules)

“Organic” is a label which refers to chemical compounds whose molecules contain carbon, but it is

traditionally used to designate that part of chemistry which deals with compounds made by living

organisms. The synthesis of urea (a compound considered organic by virtue of its known occurrence

only in the urine of living organisms) from potassium cyanate and ammonium sulfate by Friedrich

Wöhler in 1828 is often hailed as the first proof that organic synthesis does not require some

mysterious special “vis vitalis” (or “vital force”) and sounded the death knell for the ancient doctrineof vitalism.

In 1871, Charles Darwin was bold enough to suggest, in a letter to Joseph Hooker, that the original

spark of life could be conceived to have begun in (in his words) a “warm little pond, with all sorts of




124ammonia and phosphoric salts, light, heat, electricity, etc. present, so that a protein compound was

chemically formed ready to undergo still more complex changes”. He added that “at the present day

such matter would be instantly absorbed, which would not have been the case before living

creatures were found”. This is the only place in all of Darwin's public and private writings where he

even speculated about the initial stage of life, and his main purpose in suggesting this hypothetical

scenario was mainly to refute the argument put forward by some that, if life could indeed formspontaneously, then we should see it happening today too.

(Examples of organic molecules)

The Russian biochemist Alexander Oparin was one of the pioneers, in the 1920s and 1930s, of biochemical theories of how life originated on Earth. He hypothesized that life began in the oceans of

the early Earth, between 3.9 and 3.5 billion years ago, and that the first, simple organic molecules

containing carbon formed as energy from the Sun, lightning and Earth's internal heat triggered

chemical reactions to produce small organic molecules from substances present in the atmosphere.

These molecules were organized by chance into the complex organic molecules (such as proteins,

carbohydrates and nucleic acids) that are the basis of life.

Oparin stressed that there was no fundamental difference between a living organism and lifeless

matter, and that a “primeval soup” of organic molecules could be created in an oxygen-less

atmosphere through the action of sunlight. He proposed that, although the spontaneous generationof life had in fact occurred once, it would be impossible now because the conditions found in the

early Earth had changed (notably the existence of atmospheric oxygen, which prevents the synthesis

of the organic molecules that are the necessary building blocks for the evolution of life).




125Around the same time as Oparin, the British biologist J. B. S. Haldane suggested that the earth's pre-

biotic oceans (very different from their modern counterparts) would have formed a “hot dilute soup”

in which organic compounds, the building blocks of life, could have formed. Haldane was responsible

for the well-known quote (often attributed to Arthur Eddington, who made several very similar

observations in later years) that "the universe is not only queerer than we suppose, but queerer than

we can suppose". Most currently accepted models draw at least some elements from the frameworklaid out by Oparin and Haldane.

THE PRIMEVAL SOUP THEORY

Alexander Oparin speculated that the atmosphere of the early Earth may have been “chemically

reducing” in nature, composed primarily of methane, ammonia, water, hydrogen sulfide , carbon

dioxide or monoxide, and phosphate, with molecular oxygen and ozone either rare or completelyabsent. In such a reducing atmosphere, electrical activity like lightning (or possibly impact shocks or

ultraviolet light) could catalyze the creation of certain basic small molecules or monomers such as

amino acids and other simple organic compounds.

These compounds then accumulated in a “primeval soup” and could become chemically bonded to

other monomers to form more complex organic polymers, such as the long, chain-like molecules

(such as proteins and nucleic acids) which are essential for building living creatures. In fact, the

proteins essential to life are made up of just 20 basic amino acids. In order for cells to accomplish

self-replication, the cooperative action of both proteins and nucleic acids is required, and the

complex information detailing the specific structure of the proteins inside living things is stored innucleic acids like RNA and DNA.




126

(Schematic of the Miller-Urey experiment of 1953)

In an attempt to prove Oparin's hypothesis, the famous Miller-Urey experiments, by the young

Stanley Miller and his professor Harold Urey at the University of Chicago in 1953, demonstrated the

feasibility of producing basic organic monomers such as amino acids in conditions which attemptedto simulate the conditions believed to have prevailed on the primeval Earth. The experiments

involved the simulation of an atmosphere consisting of a highly reduced mixture of gases (methane,

ammonia and hydrogen), the presence of pools of liquid water and sporadic sources of energy (with

electricity simulating lightning storms). Miller's simple but audacious experiments produced, over a

very short time period, five of the amino acids used in living cells. One of his less publicized

experiments, using gases found in volcanic explosions (also a common feature of the early Earth), has

recently been uncovered and re-analyzed and has been shown to have produced over 25 amino

acids.

In 1961, John Oró showed that the nucleic acid purine base, adenine (a chemical component of DNAand RNA), could be formed by heating aqueous ammonium cyanide solutions under conditions which

may have been similar to those of primitive Earth. In the 1950s and 1960s, the American biochemist

Sidney Fox demonstrated that amino acids could spontaneously form small peptide structures under




127conditions that might plausibly have existed early in Earth's history, and that these amino acids and

small peptides could also be encouraged to form closed spherical membranes called "microspheres",

which are similar to primitive cells. Other more recent research by Jack Szostak has shown how

simple hydrocarbon fatty acids can spontaneously form into tiny bubbles, sufficient to protect and

compartmentalize developing genetic material, like early cell membranes. He has also demonstrated

how these membranes may grow and even spontaneously self-replicate.

Most scientists now agree that RNA was the crux molecule for primitive life, effectively guiding life

through its nascent stages, and only beginning to take a backseat when DNA and proteins (which

perform their jobs much more efficiently than RNA) developed. RNA is a complex self-replicating

nucleic acid molecule, similar to DNA, made of repeating units of thousands of smaller molecules

called nucleotides that link together in very specific, patterned ways.

This “RNA world” hypothesis, however, does not explain how RNA itself first arose, only how it

helped more complex life develop later. Scientists like John Sutherland at the University of

Manchester, though, undeterred by years of failure in this field and using imaginative ways of

recreating early Earth conditions, have recently succeeded in synthesizing two of the four essential

buildng blocks of RNA molecules from simple chemicals that would have existed on Earth four billion

years ago, and are already working on the others. They have even worked out ways in which these

parts could spontaneously combine to form something similar to RNA.

Some commentators have argued that the spontaneous development of life runs counter to the

Second Law of Thermodynamics, which rules that entropy and disorder inexorably increases over

time and that order and organization always declines. The work of the Russian-born Belgian chemist

Ilya Prigogine in the 1960s and 1970s, however, showed that many systems spontaneously organize

themselves if they are forced away from thermodynamic equilibrium (such as by radiation or other

unknown influences). Besides, the law specifically applies to isolated systems, whereas living systems

are necessarily open and interactive systems. There are many other examples of apparent

spontaneous increases in order (such as the growth of crystals from featureless liquids, the

development of large scale structures in the universe, etc), but the growth of order in one place is

always at the price of entropy generated elsewhere (such as the production of heat or other types of

radiation).

Encouraging though these early experiments may perhaps be, critics point out that the Miller-Urey

experiments were far from conclusive, especially as they also resulted in several other substances

that might well cross-react with the amino acids (and potentially terminate the peptide chain).

Neither did it explain how the relatively simple organic building blocks went on to polymerize and

form more complex structures, interacting in consistent ways to form a protocell.

OTHER TERRESTRIAL THEORIES

The deep sea (or hydrothermal) vent theory for the origin of life on Earth posits that life may have

begun at submarine hydrothermal vents, where hydrogen-rich fluids emerged from below the seafloor and merged with carbon dioxide-rich ocean water. Recent research has found prolific life

(known as “thermofiles” or “extremophiles”) developing around “black smoker chimneys” at

submerged openings in the Earth's crust, in mineral-rich water as hot as 400°C.




128Geologists have also discovered 1.43 billion year old fossils of deep-sea microbes around very similar

fossilized chimneys, providing evidence that life may have originated on the bottom of the ocean,

protected from harmful ultra-violet light, and not in shallow seas as the Australia stromatolite fossils

suggest.

(Deep-sea hydrothermal vents)

According to another recent theory, Zachary Adam of the University of Washington has hypothesized

that stronger tidal processes from a much closer Moon may have concentrated grains of uranium

and other radioactive elements at the high water mark on primordial beaches. These elements are

capable of self-sustaining nuclear reactions and may have provided sufficient energy to generate

organic molecules, such as amino acids and sugars, from acetonitrile in the water, as well as releasing

soluble phosphate (with which they could react and possibly generate the building blocks of life). The

active system of plate tectonics on the early Earth would also have helped to bring radioactive

minerals to the surface.

In the 1970s, the Austrian astrophysicist Thomas Gold proposed the theory that life first developed,

not on the surface of the Earth, but several kilometres below the surface. The discovery in the late

1990s of “nanobes” (filamental structures that are smaller than bacteria, but that may contain DNA)

in deep rocks might be seen as lending support to Gold's theory. Although these deep microbes

would have needed a source of food, it is possible that this could have been provided by the out-

gassing of primordial methane from the Earth's mantle, or by hydrogen released by an interaction

between water and iron compounds in rocks.




129However, some bacteria have been found in rocks a kilometre underground, slowly digesting organic

material without the aid of oxygen and dividing only once every thousand years or so. Other bacteria

have been found that can live in sulphuric acid or even in nuclear waste, suggesting that our

assumptions about the necessary conditions for life may be much too narrow. Recent research in

California by evolutionary geobiochemist Felisa Wolfe-Simon has identified bacteria that thrives on

arsenic, an element hitherto considered entirely poisonous to anything we think of as life, and thateven incorporates arsenic into its DNA in place of phosphorus.

These discoveries raise the tantalizing possibility that Earth may in fact harbour more than one "tree

of life" (what Paul Davies calls "shadow biospheres"), and that life on Earth may have developed not

once but perhaps many times. It also increases the likelihood that life has developed elsewhere, and

that our assumptions about the necessary pre-conditions for the establishment of life may be too

narrow.

There are several other alternative terrestrial theories, with more or less support in the scientific

community, including the “iron-sulphur world” theory, the “lipid world” theory, the “PAH world”

theory, the "clay" theory, the “autocatalysis” theory, etc.

EXOGENESIS

An alternative to Earthly abiogenesis is “exogenesis”, the hypothesis that primitive life may have

originally formed extraterrestrially, either in space or on a nearby planet such as Mars. Such ideas

have had many eminent supporters over the years, including Francis Crick, the co-discoverer of thestructure of the DNA molecule, and the astrophysicist Sir Fred Hoyle among others. These theories

may go some way to explaining the presence of life on Earth so soon after the planet had cooled

down, with apparently very little time for prebiotic evolution.

In the 1980s, Hoyle developed and promoted, along with fellow astronomer Chandra

Wickramasinghe, the theory of “panspermia”. This is the idea that the origin of life on Earth must

have involved cells which arrived from space, and that evolution on earth is driven by a steady influx

of viruses arriving from space via comets. Hoyle calculated the chances of the simplest living cell

forming out of some primordial soup as infinitesimally small, and described that theory as “evidently

nonsense of a high order” (although others have argued that Hoyle's own line of reasoningincorporates a number of logical mistakes and omissions).

In the aftermath of supernova explosions and the process of star formation, many hydrocarbon

molecules (those made entirely or mostly out of hydrogen and carbon), including formaldehyde,

hydrocyanic acid and other so-called pre-biotic molecules, are spontaneously formed in nebulae.

Such pre-biotic molecules appear to exist across the universe, and existed before the creation of the

Solar System, although it is thought that about 9 billion years or so of star-making were required to

produce the right conditions.

Some complex compounds found in outer space, such as glycoaldehyde for example, have been

made to react in laboratories to make a sugar called ribose, a key ingredient of ribonucleic acid (RNA)

which, with the removal of an oxygen atom, becomes DNA (deoxyribonucleic acid). It is generally




130agreed that these compounds are not themselves products of life, but form spontaneously by banal

chemical reactions. They are, however, able to interact to form more stable, typical organic

compounds, many of them similar to substances found in living organisms.

Spectroscopic analysis of radiation, meteorites and comets has revealed the presence of amino acids

and other biologically significant compounds on celestial bodies, including, but not limited to, the

famous Murchison meteorite (which fell on Australia in 1969), Halley’s Comet (a recurring visitor to

the neighbourhood of Earth) and Saturn's moon Titan (the seas of which are believed to be made of

hydrocarbons).

(Fossilized bacteria in the Murchison meteorite)

The Murchison meteorite, a remnant from the very early days of the Solar System, has been found to

contain 411 different organic compounds, including 74 amino acids, 8 of which are found in the

proteins of living organisms. In particular, recent analysis has shown that it contains some quite

complex organic chemicals, called uracil and xanthine, which can form the self-replicating moleculesthat are the essential genetic ingredient of all known lifeforms, RNA and DNA.

Water, usually considered a prerequisite for life as we know it, has been identified as perhaps the

most common triatomic molecule in the universe, and most comets are substantially composed of

water (and other) ices. It is thought that much of the water on Earth may have been borne here by

comet collisions in the early years of the Earth.

But, additionally, we now know that many comets are encrusted by outer layers of dark material,

thought to be a tar-like substance composed of complex organic material and formed from simple

carbon compounds after reactions initiated by ultraviolet light irradiation. Studies have shown a

strong correlation between the abundances of hydrogen, carbon, oxygen, nitrogen and sulphur in

living organisms and in material found in comets. Indeed, there seems to be ample evidence that

organic matter is spread throughout the galaxy and the universe, in asteroids, comets, meteorites

and even in the gas and dust in interstellar space. In addition,




131Simulations have shown that a glancing approach from a comet would allow for the melting and

depositing of water into the Earth's atmosphere, rather than the complete vaporization a direct hit

would cause. Furthermore, it has been experimentally demonstrated that simple amino acids could

survive a glancing impact, and in fact the extreme conditions and temperatures of such an impact

may even help them combine into more complex molecules. A rain of material from comets and

meteorites could therefore have brought significant quantities of complex organic molecules to theprimordial Earth.

An alternative source of first life which has been suggested is our neighbouring planet, Mars. Being

smaller in size, Mars cooled before the Earth (by several hundreds of millions of years), allowing

prebiotic processes to develop there while the Earth was still too hot. Life, it is argued, could later

have been transported to the cooled Earth when material was blasted off the crust of Mars by

asteroid and comet impacts. Because Mars continued to cool faster (and eventually all but lost its

atmosphere), the hypothesis continues, it eventually became hostile to the continued evolution, or

even the existence, of life. Although the Earth may well be following the same fate as Mars, it is

doing so at a much slower rate, which has allowed the establishment of much more complexlifeforms.

(Artist's depiction of life falling to Earth on a meteorite, according to the panspermia hypothesis)

A meteorite found in Antarctica in 1984 (dubbed ALH84001) has been shown to have a chemical

composition very similar to that on the surface of Mars, including bubbles of trapped gas consistent

with an early Martian atmsophere, and seems to have been blasted into space (and thence to Earth)

by an asteroid or comet collision with Mars. Interestingly, the rock also shows evidence of what

appear to be microscopic wormlike formations, suggesting fossilized Martian bacteria. Although it

now seems unlikely that this represents the first evidence of life from outside the Earth, research on




132the meteorite has shown that Mars almost certainly had a magnetic field over 4 billion years ago, and

so may well have had an atmosphere more conducive to the origination and development of life.

Smaller and more distant from the Sun, life may have been able to develop on Mars much earlier

than on Earth, and may have seeded the Earth, possibly multiple times, during the early period of

bombardment.

While it seems that life is unlikely to have originated in any the other planets in the Solar System, due

to the extreme and inhospitable conditions they manifest, conditions on some of the moons of those

planets may be more conducive to life. The surfaces of three of Saturn's moons, Enceladus, Europa

and Titan, as well as Neptune's moon, Triton, and several other smaller moons, asteoids and comets

in the Solar System, appear to be rich in a substance called tholin, a reddish-brown, non-biological

organic material which contains the chemical building blocks for life to begin. Zapping the kinds of

gases which are found on these moons (methane, nitrogen, etc) with electricity or ultraviolet

radiation readily produces tholin in laboratories. A wide variety of soil bacteria are able to use tholins

as their sole source of carbon, and it can also act as an effective screen for ultraviolet radiation.

It has even been postulated that the organic molecules which made up early life on Earth originated

in other star systems. In 2004, a research team detected traces of polycyclic aromatic hydrocarbons

(or PAHs) in a distant nebula, the most complex molecules so far found in space, and a very large

range of molecules, including cyanide compounds, hydrocarbons and carbon monoxide, have been

detected in the formation of a Sun-like star. Such research is still at a very early stage, though.

A recent experiment, led by Jason Dworkin, subjected a frozen mixture of water, methanol, ammonia

and carbon monoxide to ultraviolet radiation, attempting to mimic conditions found in an

extraterrestrial environment. This combination yielded large amounts of organic material that

appeared to self-organize, and to form cell membrane-like bubbles when immersed in water. The

bubbles were also found to glow or fluoresce when exposed to ultraviolet light (perhaps a precursor

to primitive photosynthesis?), as well as providing a protective layer to diffuse any damage that

might otherwise be inflicted by the ultraviolet radiation, which would have been vital in an early

world without an ozone layer.




133

(SETI's unexplained "Wow!" signal, recorded in 1977)

The SETI Institute (Search for Extra-Terrestrial Intelligence) has been monitoring the universe for over

50 years, looking for signals being deliberately beamed towards the Earth. Results have been perhaps

underwhelming, although in 1977 what has become known as the "Wow!" signal was recorded from

the constellation Sagittarius. It lasted 72 seconds (the full duration the radio telescope was pointed

at it), has never been repeated since and is still unexplained. In 1997, an apparently engineered

signal was recorded at Greenbank Observatory in West Virginia, but it actually turned out to be from

an orbiting satellite which had been overlooked.

Given the time that such signals would require to reach us, many scientists (even within SETI) argue

that this apparent lack of results is perhaps not unexpected, and there is always the possibility that

alien races and technologies may well have come ... and gone.

But the overall likelihood of the existence of extra-terrestrial life nevertheless remains high. TheDrake Equation, devised by Frank Drake in 1961, and often quoted in the field of exobiology,

combines the factors that constrain the number, N, of civilizations in our galaxy with which

communication might be possible:

N = R* X f p X ne X f l X f l X f c X L, where

R* = the average number of stars formed per year in our galaxy;

f p = the fraction of those stars that have planets;

ne = the fraction of these that are able to potentially support life;

f l = the fraction of these that actually go on to develop life at some point;

f l = the fraction of these that actually go on to develop intelligent life;




134f c = the fraction of these that develop a technology that releases detectable signals into space;

L = the length of time such civilizations release detectable signals into space.

However small some of these fractions may be, the number of stars R* and the duration of

civilizations L may be so huge as to make the probability of communication from other intelligent life

in our galaxy (and certainly beyond) all but a sure thing. However, there is still considerabledisagreement on the values of most of these parameters.

We are beginning to find evidence that our solar system may not be untypical of other planetary

systems throughout the universe, opening up the possibility of other li fe “out there”. The search for

other planets, though, is a daunting prospect, especially because the brightness of stars tends to hide

any small nearby planets. In recent years, though, techniques have been developed allowing the

easier identification of potential life-supporting planets, including star wobble, redshift measurement

and brightness dimming as a result of planetary transits.

It was as recently as 1992 that astronomers identified the first planetary system outside our own, butsince then one discovery has followed another in an accelerating trend: in 1995, a Jupiter-like planet

was found orbiting a sun-like star; the first terrestrial planet outside our own solar system was

discovered in 2005, followed by several others; in 2007, the first Earth-sized “Goldilocks” planet (a

planet just the right distance from its star to allow liquid water to exist) was discovered; etc. To date,

over 450 (and counting) have been identified.

The hunt continues, and indeed is intensifying. The innovative Allen Telescope Array is being built at

Hat Creek, California to expand SETI's monitoring capability, and the Kepler Space Telescope was

launched in 2009 with a remit specifically to look for other possible Earth-type planets, but so far

there is still no real sign of extra-terrestrial intelligent life.

CONCLUSION

The evidence seems to suggest that all life on Earth has developed from a single organism back in the

mists of time, and perhaps even from one single common ancestral cell. Current thought suggest that

the “last universal common ancestor” (the hypothetical latest living organism from which all

organisms now living on Earth descend, or, in other words, the most recent common ancestor of all

current life on Earth) is estimated to have lived some 3.5 to 3.8 billion years ago. However, the actual

mechanism for its origination is still far from clear.

While the circumstances that led to life on Earth are no doubt special, there is no reason to suspect

that they are peculiar to Earth. As Richard Dawkins points out, if the universe contains a billion billion

planets (which some scientists consider a conservative estimate), then the chances that life will arise

on one of them is not really so remarkable. If, as some physicists claim, our universe is just one of

many in a multiverse, each of which contains a billion billion planets, then the chances that life will

arise on at least one of them is almost a certainty.

Indeed, many scientists believe that it is entirely possible that different forms of life may have

appeared quasi-simultaneously in the early history of Earth. Some of these may now be extinct, or

they may survive as extremophiles (an organism that thrives in extreme conditions that are




135detrimental to the majority of life on Earth), or they may simply have gone unnoticed. It is only in

quite recent years that living things have been discovered in conditions as unlikely as hot volcanic

vents deep beneath the sea and in totally dark and dry lava tubes in the desert.

As science progresses, there is even the possibility - or the spectre, depending on your outlook - of

one day creating man-made life. Harvard scientist George Church for example is hot on the trail to

building a completely man-made living cell. He has indentified a total of 151 essential components

which he believes represent the minimum for the creation of life - a sort of blueprint for life itself -

and has been making rapid progress in synthesizing them in the laboratory.

This remains an area of intense debate and speculation in both scientific and religious circles and,

while new discoveries are made almost every year (which may, or may not, throw some light on the

subject), no definitive solutions have yet been yielded.




136




137

Important Dates and

Discoveries-- A brief chronological listing of some of the most important discoveries in cosmology, astronomy and

physics, from ancient Babylon, India and Greece, right up to the 20th Century. Learn how some of the

essential concepts and laws of modern physics which are mentioned in this website (and the earlier

ideas out of which they grew) developed in a historical context. For a slightly different perspective,

also see the section on Cosmological Theories Through History.

For convenience sections are splitted into:

Ancient World (20th Century B.C. - 4th Century A.D.)

Medieval and Renaissance World (5th Century A.D. - 16th Century)

Early Modern World (17th Century - 19th Century)

Modern World (20th Century)

ANCIENT WORLD

20th -16th Century B.C. - Ancient Babylonian tablets show knowledge of the distinction

between the moving planets and the “fixed” stars, and the recognition that the movement of

planets are regular and periodic.

15th - 12th Century B.C. - The Hindu Rigveda of ancient India describes the origin of the

universe in which a “cosmic egg” or Brahmanda, containing the Sun, Moon, planets and the

whole universe, expands out of a single concentrated point before subsequently collapsing

again, reminiscent of the much later Big Bang and oscillating universe theories.

5th Century B.C. - The Greek philosopher Anaxagoras becomes arguably the first to

formulate a kind of molecular theory of matter, and to regard the physical universe as

subject to the rule of rationality or reason.

5th Century B.C. - The Greek philosophers Leucippus and Democritus found the school of

Atomism, which holds that the universe is composed of very small, indivisible and

indestructible building blocks known as atoms, which then form different combinations and

shapes in an infinite void.

4th Century B.C. - The Greek philosopher Aristotle describes a geocentric universe in whichthe fixed, spherical Earth is at the centre, surrounded by concentric celestial spheres of

planets and stars. Although he portrays the universe as finite in size, he stresses that it exists

unchanged and static throughout eternity.

4th Century B.C. - The Greek philosopher Heraclides proposes that the apparent daily motion

of the stars is created by the rotation of the Earth on its axis once a day, and that the Sun

annually circles a central Earth, while the other planets orbit the Sun (a geocentric model

with heliocentric aspects).

3rd Century B.C. - The Stoic philosophers of ancient Greece assert a kind of “island universe”

in which a finite cosmos is surrounded by an infinite void (similar in principle to a galaxy).

3rd Century B.C. - The Greek mathematician and geographer Eratosthenes proved that the

Earth was round, and made a remarkably accurate calculation of its circumference and its tilt

(as well as devising a system of latitude and longitude, and, possibly, estimating the distance

of the Earth from the Sun).




138

3rd Century B.C. - The Greek astronomer and mathematician Aristarchus of Samos is the first

to present an explicit argument for a heliocentric model of the Solar System, placing the Sun,

not the Earth, at the center of the known universe. He describes the Earth as rotating daily

on its axis and revolving annually about the Sun in a circular orbit, along with a sphere of

fixed stars.

2nd Century B.C. - The Greek astronomer Hipparchus of Nicea makes the first measurementof the precession of the equinoxes, and compiles the first star catalogue (in which he

proposes our modern system of apparent magnitudes). He also improves on the Solar System

model of Apollonius of Perga, in which an eccentric circle carries around a smaller circle (an

epicycle), which in turn carries around a planet.

2nd Century B.C. - The Hellenistic astronomer and philosopher Seleucus of Seleucia supports

Aristarchus’ heliocentric theory, and links the tides to the influence of the Moon.

2nd Century A.D. - The Roman-Egyptian mathematician and astronomer Ptolemy (Claudius

Ptolemaeus) describes a geocentric model, largely based on Aristotelian ideas, in which the

planets and the rest of the universe orbit about a stationary Earth in circular epicycles, which

becomes the scientific orthodoxy for nearly two millennia (essentially until Copernicus in the

16th Century). He also details the complex motions of the stars and planetary paths using

equants, allowing astronomers to predict the positions of the planets.

MEDIEVAL AND RENAISSANCE WORLD

5th Century A.D. - The Indian astonomer and mathematician Aryabhata proposes that the

Earth turns on its own axis, and describes elliptical orbits around the Sun, which some have

interpreted as heliocentrism.

6th Century A.D. - The Christian philosopher John Philoponus of Alexandria argues against

the ancient Greek notion of an infinite past, and is perhaps the first commentator to argue

that the universe is finite in time and therefore had a beginning.

7th Century - The Indian astronomer Brahmagupta, a follower of the heliocentric theory of

the Solar System earlier developed by Aryabhata, recognizes gravity as a force of attraction

in his "The Opening of the Universe" of 628, in which he describes a force of attraction

between the Sun and the Earth.

9th Century - The Muslim astronomer Ja'far ibn Muhammad Abu Ma'shar al-Balkhi

developes a planetary model which some have interpreted as heliocentric model.

9th - 11th Century - Early Muslim and Jewish theologians such as Al-Kindi, Saadia Gaon and

Al-Ghazali offer logical arguments supporting a finite universe. 11th Century - The Arab polymath Alhazen (also known as Ibn al-Haytham) becomes the first

to apply the scientific method to astronomy.

11th Century - The Persian astronomer Abu al-Rayhan al-Biruni describes the Earth's

gravitation as the attraction of all things towards the centre of the Earth, and hypothesizes

that the Earth turns daily on its axis and annually around the Sun.

11th Century - The Persian polymath Omar Khayyam demonstrates that the universe is not

moving around Earth, but that the Earth revolves on its axis, bringing into view different star

constellations throughout the night and day. He also calculated the solar year as

365.24219858156 days (correct to six decimal places).

14th Century - The Arab astronomer and engineer Ibn al-Shatir (of the Iranian Maragha

school of astronomy) refines and improves the accuracy of the geocentric Ptolemaic model

and develops the first accurate model of lunar motion.




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15th Century - The Persian astronomer and mathematician Ali Qushji rejects the Aristotelian

notion of a stationary Earth in favour of a rotating Earth.

15th Century - Somayaji Nilakantha of the Kerala school of astronomy and mathematics in

southern India develops a computational system for a partially heliocentric planetary model

in which Mercury, Venus, Mars, Jupiter and Saturn orbit the Sun, which in turn orbits the

Earth. 1543 - The Polish astronomer and polymath Nicolaus Copernicus (adapting the geocentric

Maragha model of Ibn al-Shatir to meet the requirements of the ancient heliocentric

universe of Aristarchus), proposes that the Earth rotates on its axis once daily and travels

around the Sun once a year, and demonstrates that the motions of celestial objects can be

explained without putting the Earth at rest in the centre of the universe. His Copernican

Principle (that the Earth is not in a central, specially favoured position) and its implications

(that celestial bodies obey physical laws identical to those on Earth) first establishes

cosmology as a science rather than a branch of metaphysics, and marks a shift away from

anthropocentrism.

1576 - The English astronomer Thomas Digges popularizes Copernicus’ ideas and also

extends them by positing the existence of a multitude of stars extending to infinity, rather

than Copernicus’ narrow band of fixed stars.

1584 - The Italian philosopher Giordano Bruno takes the Copernican Principle a stage further

by suggesting that even the Solar System is not the centre of the universe, but rather a

relatively insignificant star system among an infinite multitude of others.

1587 - The Danish nobleman and astronomer Tycho Brahe develops a kind of hybrid of the

Ptolemaic and Copernican models, a geo-heliocentric system similar to that of Somayaji

Nilakantha, now known as the Tychonic system. This involves a static Earth at the centre of

the universe, around which revolve the Sun and the Moon, with the other five planetsrevolving around the Sun.

EARLY MODERN WORLD

1605 - The German mathematician and astronomer Johannes Kepler establishes his three

Laws of Planetary Motion, mathematical laws that describe the motion of planets in the Solar

System, including the ground-breaking idea that the planets follow elliptical, not circular,

paths around the Sun. Newton later used them to deduce his own Laws of Motion and his

Law of Universal Gravitation.

1610 - The Italian mathematician and physicist Galileo Galilei develops an astronomicaltelescope powerful enough to indentify moons orbiting Jupiter, sunspots on the Sun and the

different phases of Mercury, all of which are instrumental in convincing the scientific

community of the day that the heliocentric Copernican model of the Solar System is superior

to the geocentric Ptolemiac model.

1632 - Galileo Galilei first describes the Principle of Relativity, the idea that the fundamental

laws of physics are the same in all inertial frames and that, purely by observing the outcome

of mechanical experiments, one cannot distinguish a state of rest from a state of constant

velocity.

1633 - The French philosopher René Descartes outlines a model of a static, infinite universe

made up of tiny “corpuscles” of matter, a viewpoint not dissimilar to ancient Greek atomism.

Descartes’ universe shares many elements of Sir Isaac Newton’s later model, although

Descartes’ vacuum of space is not empty but composed of huge swirling whirlpools of

ethereal or fine matter, producing what would later be called gravitational effects.




140

1638 - Galileo Galilei demonstrates that unequal weights would fall with the same finite

speed in a vacuum, and that their time of descent is independent of their mass. Thus, freely

falling bodies, heavy or light, have the same constant acceleration, due to the force of

gravity.

1675 - The English physicist Sir Isaac Newton argues that light is composed of particles, which

are refracted by acceleration toward a denser medium, and posits the existence of “aether”to transmit forces between the particles.

1687 –

Sir Isaac Newton publishes his “Principia”, which describes an infinite, steady state, static,

universe, in which matter on the large scale is uniformly distributed. In the work, he

establishes the three Laws of Motion (“a body persists its state of rest or of uniform motion

unless acted upon by an external unbalanced force”; “force equals mass times acceleration”;

and “to every action there is an equal and opposite reaction”) and the Law of Universal

Gravitation (that every particle in the universe attracts every other particle according to an

inverse-square formula) that were not to be improved upon for more than two hundred

years. He is credited with introducing the idea that the motion of objects in the heavens

(such as planets, the Sun and the Moon) can be described by the same set of physical laws as

the motion of objects on the ground (like cannon balls and falling apples).

1734 - The Swedish scientist and philosopher Emanuel Swedenborg proposes a hierarchical

universe, still generally based on a Newtonian static universe, but with matter clustered on

ever larger scales of hierarchy, endlessly being recycled. This idea of a hierarchical universe

and the “nebular hypothesis” were developed further (independently) by Thomas Wright(1750) and Immanuel Kant (1775).

1761 - The Swiss physicist Johann Heinrich Lambert supports Wright and Kant’s hierarchical

universe and nebular hypothesis, and also hypothesizes that the stars near the Sun are part

of a group which travel together through the Milky Way, and that there are many such

groupings or star systems throughout the galaxy.

1783 - The amateur British astronomer John Michell proposes the theoretical idea of an

object massive enough that its gravity would prevent even light from escaping (which has

since become known as a black hole). He realizes that such an object would not be directly

visible, but could be identified by the motions of a companion star if it was part of a binary

system. A similar idea was independently proposed by the Frenchman Pierre-Simon Laplace

in 1795.

1789 - The French chemist Antoine-Laurent de Lavoisier definitively states the Law of

Conservation of Mass (although others had previously expressed similar ideas, including the




141ancient Greek Epicurus, the medieval Persian Nasir al-Din al-Tusi and the 18th Century

scientists Mikhail Lomonosov, Joseph Black, Henry Cavendish and Jean Rey), and identifies

(albeit slightly incorrectly) 23 elements which he claims can not be broken down into simpler

substances.

1803 –

(Wave interference in Thomas Young's double-slit experiment)

The English scientist Thomas Young demonstrates, in his famous double-slit experiment, the

interference of light and concludes that light is a wave, not a particle as Sir Isaac Newton had

ruled.

1805 - The English chemist John Dalton develops his atomic theory, proposing that each

chemical element is composed of atoms of a single unique type, and that, though they are

both immutable and indestructible, they can combine to form more complex structures.

1839 - The English scientist Michael Faraday concludes from his work on electromagnetism

that, contrary to scientific opinion of the time, the divisions between the various kinds of

electricity are illusory. He also establishes that magnetism can affect rays of light, and that

there is an underlying relationship between the two phenomena.

1861 - The French scientist Louis Pasteur’s experiments show that organisms such as bacteria

and fungi do not appear of their own accord in sterile nutrient-rich media, suggesting that

the long-held acceptance of the spontaneous generation of life from non-living matter may

be incorrect.

1864 - The Scottish physicist James Clerk Maxwell demonstrates that electric and magneticfields travel through space in the form of waves at the constant speed of light and that

electricity, magnetism and even light are all manifestations of electromagnetism. He

collected together laws originally derived by Carl Friedrich Gauss, Michael Faraday and




142André-Marie Ampère into a unified and consistent theory (often known as Maxwell’s

Equations).

1896 - The French physicist Henri Becquerel discovers that certain kinds of matter emit

radiation of their own accord (radioactivity).

1897 - The British physicist J. J. Thomson discovers the electron, the first known sub-atomic

particle.

MODERN WORLD

1900 - The German physicist Max Planck suggests, while describing his law of black body

radiation, that light may be emitted in discrete frequencies or “quanta”, and establishes the

value of the Planck constant to describe the sizes of these quanta. This is often regarded as

marking the birth of quantum physics.

1905 - The German physicist Albert Einstein shows how the photoelectric effect is caused by

absorption of quanta of light (or photons), an important step in understanding the quantum

nature of light and electrons, and a strong influence on the formation of the concept of wave-particle duality in quantum theory.

1905 - Albert Einstein publishes his Special Theory of Relativity, in which he generalizes

Galileo's Principle of Relativity (that all uniform motion is relative, and that there is no

absolute and well-defined state of rest) from mechanics to all the laws of physics, and

incorporates the principle that the speed of light is the same for all inertial observers

regardless of the state of motion of the source.

1905 - In a separate paper, Albert Einstein derives the concept of mass-energy equivalence

(that any mass has an associated energy) and his famous E = mc2 equation.

1907 - The German mathematician Hermann Minkowski realizes that Einstein’s Special

Theory of Relativity can be best understood in a four-dimensional space, which he calls

“space-time” and in which time and space are not separate entities but intermingled in a

four-dimensional space.

1911 - The New Zealand chemist Ernest Rutherford interprets the 1909 experiments of Hans

Geiger and Ernest Marsden, establishing for the first time the “planetary model” of the atom,

where a central nucleus is circled by a number of tiny electrons like planets around a sun.

1915 - The German physicist Karl Schwarzschild provides the first exact solution to Einstein’s

field equations of general relativity (even before Einstein publishes the theory) for the

limited case of a single spherical non-rotating mass, which leads to the “Schwarzschild

radius” which defines the size of the event horizon of a non-rotating black hole. 1916 - Albert Einstein publishes his General Theory of Relativity, in which he unifies special

relativity and Newton's Law of Universal Gravitation, and describes gravity as a property of

the curvature of four-dimensional space-time. Objects (including planets, like the Earth, for

instance) fly freely under their own inertia through warped space-time, following curved

paths because this is the shortest possible path in warped space.




143

(General relativity predicts the gravitational bending of light by massive bodies)

1916 - The Austrian physicist Ludwig Flamm examines Schwarzschild’s solution to Einstein’s

field equations and points out that the equations theoretically allow for some kind of

invisible connection between two distinct regions of space-time (later to become known as a

“wormhole”).

1917 - Albert Einstein publishes a paper introducing the “cosmological constant” into the

General Theory of Relativity in an attempt to model the behaviour of the entire universe, anidea he later called his “greatest blunder” but which, in the light of recent discoveries, is

beginning to look remarkably prescient.

1919 - Ernest Rutherford is credited with the discovery of the proton when he notices the

signatures of hydrogen nuclei when alpha particles are shot into nitrogen gas. In these

experiments, he also became the first person to transmute one element into another

(nitrogen into oxygen) through a deliberate man-made nuclear reaction.

1919 - The English astrophysicist Arthur Eddington uses his measurements of an eclipse to

confirm the deflection of starlight by the gravity of the Sun as predicted in Albert Einstein’s

General Theory of Relativity.

1919 - The German mathematician Theodor Kaluza proposes the addition of a fifth

dimension to the General Theory of Relativity, a precursor to much later superstring theory

attempts to combine general relativity and quantum theory. The Swedish physicist Oskar

Klein independently proposes a similar idea in 1926.




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1922 - The Russian biochemist Alexander Oparin hypothesizes that life on Earth began in a

“primeval soup” of matter and water between 3.9 and 3.5 billion years ago, as chemical

reactions produced small organic molecules from substances present in the atmosphere,

which were then organized by chance into the more complex organic molecules that are the

basis of life.

1922 - The Russian cosmologist and mathematician Alexander Friedmann discovers theexpanding universe solution to Einstein’s general relativity field equations. The solution for a

universe with positive curvature (spherical space) results in the universe expanding for a time

and then contracting due to the pull of its gravity, in a perpetual cycle of Big Bang followed

by Big Crunch now known as the oscillating universe theory.

1925 - The American astronomer Edwin Hubble proves conclusively that nebulae such as the

Andromeda Nebula are much too distant to be part of the Milky Way and are in fact entire

galaxies outside our own, thus settling the “Great Debate” about the nature of spiral nebulae

and the size of the universe.

1925 - The Austrian theoretical physicist Wolfgang Pauli establishes an important quantum

mechanical principle known as the Pauli exclusion principle, which states that no two

identical fermions (such as electrons) may occupy the same quantum state simultaneously.

1926 –

(Heisenberg's microscope thought experiment to illustrate the effects of the uncertainty

principle)




145The German physicist Werner Heisenberg formulates his uncertainty principle, that the

values of certain pairs of variables cannot both be known exactly (i.e. the more precisely one

variable is known, the less precisely the other can be known), a central concept in quantum

physics.

1926 - The Austrian physicist Erwin Schrödinger publishes what is now known as the

Schrödinger Equation, a central and revolutionary achievement in quantum physics. Later, in1935, he proposes the famous “Schrödinger's Cat” thought experiment or paradox

concerning quantum superposition, decoherence and entanglement.

1927 - The Belgian Roman Catholic priest and physicist Georges Lemaître proposes (even

before Hubble’s corroborating evidence) that the universe is expanding, followed in 1931 by

the first definitive version of what has become known as the Big Bang theory of the origin of

the universe.

1928 - The British physicist Paul Dirac provides a description of the “spin” of elementary

particles such as electrons which is consistent with both the principles of quantum

mechanics and the Special Theory of Relativity, and predicts the existence of antimatter.

1929 - Edwin Hubble definitively shows that all the galaxies in the universe are moving away

from us, according to a formula which has become known as Hubble’s Law, showing that the

universe is not in fact static, but expanding.

1932 - The English physicist James Chadwick discovers the neutron; the American physicist

Carl Anderson identifies the positron (the anti-electron which had been predicted by Paul

Dirac a few years earlier); and the British physicist John Cockcroft and the Irish physicist

Ernest Walton succeed in transmuting lithium into helium and other chemical elements using

high energy protons, popularly referred to as “splitting the atom”.

1934 - The Swiss-American astronomer Fritz Zwicky and the German-American Walter Baade

coin the term “supernova” and hypothesize (correctly) that they are the transition of normalstars into neutron stars, as well as the origin of cosmic rays. Zwicky also uses the virial

theorem to deduce the existence of unseen matter (what is now called dark matter) in the

universe, as well as the effect of gravitational lensing.

1935 - Albert Einstein and the Israeli physicist Nathan Rosen achieve a solution to Einstein’s

field equations known as an Einstein-Rosen bridge (also known as a Lorentzian wormhole or

a Schwarzschild wormhole).

1935 - The Indian-American astrophysicist Subrahmanyan Chandrasekhar establishes the

“Chandrasekhar limit” of about 1.4 solar masses, above which a star must continue to

collapse into a neutron star rather than settling down into a white dwarf.

1939 - The discovery of nuclear fission results from the Berlin experiments of Otto Hahn, Lise

Meitner, Fritz Strassmann and Otto Frisch.

1948 –




146

(In a steady state universe, overall density remains constant)

The English astronomer Fred Hoyle and the Austrians Thomas Gold and Hermann Bondi

propose a non-standard cosmology (i.e. one opposed to the standard Big Bang model) known

as the steady state universe. This theory describes a universe that has no beginning and no

end, and that expands continuously, but in which new matter is constantly created and

inserted as it expands in order to maintain a constant density, so that its overall look does

not change over time.

1953 - The experiments of the American biochemists Stanley Miller and Harold Urey (known

as the Miller-Urey experiments) demonstrate the feasibility of producing basic organic

monomers such as amino acids from a highly reduced mixture of gases, in an attempt to back

up Alexander Oparin’s hypotheses on the origins of life on Earth.

1963 - The New Zealand mathematician Roy Kerr discovers a solution to Einstein’s general

relativity field equations which describes a spinning black hole, and argues that these are

likely to be common objects throughout the universe.

1965 - The American astronomers Arno Penzias and Robert Wilson discover the existence of

cosmic microwave background radiation, considered by most to be the best evidence for the

Big Bang model of the universe (and effectively disproving Hoyle et al’s steady state unive rse

theory).

1966 - The Russian physicist Andrei Sakharov outlines the three conditions necessary for the

observed matter-antimatter imbalance in the universe to be possible, and hypothesizes

about singularities linking parallel universes.

1969 - The Murchison meteorite falls on Australia, revealing significant quantities of organic

compounds and amino acids (the basis of early life on Earth) which originated in outer space.

1970 - The English physicist Stephen Hawking provides, along with Roger Penrose, theorems

regarding singularities in space-time, indicating that singularities and black holes are actually




147a fairly generic feature of general relativity. He also predicts that black holes should in theory

emit radiation (known today as Hawking radiation) until they eventually exhaust their energy

and evaporate.

1980 - The American physicist Alan Guth proposes a model of the universe based on the Big

Bang, but incorporating a short, early period of exponential cosmic inflation in order to solve

the horizon and flatness problems of the standard Big Bang model. 1980 - The invention of the Scanning Tunnelling Microscope, by the German Gerd Binnig and

the Swiss Heinrich Rohrer, shows visually for the first time that matter is composed of

spherical atoms stacked row on

1983 –

(Artist's impression of parallel universes making up the multiverse)

The Russian-American physicist Andrei Linde develops Guth’s cosmic inflation idea further

with his chaotic inflation (or eternal inflation) theory, which sees our universe as just one of

many “bubble universes” that have developed as part of a multiverse.

1984-6 - A series of important discoveries in string theory leads to the “first superstring

revolution”, and it is first realized that string theory might be capable of d escribing all

elementary particles as well as the interactions between them.

1995 - The American theoretical physicist Edward Witten and others develop M-theory, and

spark a flurry of new research in string theory, sometimes called the “second superstring

revolution”.

1998 - Observations of distant Type 1a supernovas, both by the American astrophysicist Saul

Perlmutter and by the Australians Nick Suntzeff and Brian Schmidt, indicate that they are

actually further away from the Earth than expected, suggesting an accelerating expansion of

the universe.

2002 - The American physicist Paul Steinhardt and South African-British physicist Neil Turok

propose another variation of the inflating universe known as the cyclic model, developedusing state-of-the-art M-theory, superstring theory and brane cosmology, which involves an

inflating universe expanding and contracting in cycles.




148




149

Cosmological Theories

Through History--"Cosmos" is just another word for universe, and "cosmology" is the study of the origin, evolution and

fate of the universe. Some of the best minds in history - both philosophers and scientists - have

applied themselves to an understanding of just what the universe is and where it came from,

suggesting in the process a bewildering variety of theories and ideas, from the Cosmic Egg to the Big

Bang and beyond. Here are some of the main ones, in approximate chronological order:

Brahmanda (Cosmic Egg) Universe - The Hindu Rigveda, written in India around the 15th -

12th Century B.C., describes a cyclical or oscillating universe in which a “cosmic egg”, or

Brahmanda, containing the whole universe (including the Sun, Moon, planets and all of

space) expands out of a single concentrated point called a Bindu before subsequentlycollapsing again. The universe cycles infinitely between expansion and total collapse.

Anaxagorian Universe - The 5th Century B.C. Greek philosopher Anaxagoras believed that

the original state of the cosmos was a primordial mixture of all its ingredients which existed

in infinitesimally small fragments of themselves. This mixture was not entirely uniform, and

some ingredients were present in higher concentrations than others, as well as varying from

place to place. At some point in time, this mixture was set in motion by the action of “nous”

(mind), and the whirling motion shifted and separated out the ingredients, ultimately

producing the cosmos of separate material objects, all with different properties, that we see

today.

Atomist Universe - Later in the 5th Century B.C., the Greek philosophers Leucippus and

Democritus founded the school of Atomism, which held that the universe was composed of

very small, indivisible and indestructible building blocks known as atoms (from the Greek

“atomos”, meaning “uncuttable”). All of reality and all the objects in the universe are

composed of different arrangements of these eternal atoms and an infinite void, in which

they form different combinations and shapes.

Aristotelian Universe –




150

The Greek philosopher Aristotle, in the 4th Century B.C., established a geocentric universe in

which the fixed, spherical Earth is at the centre, surrounded by concentric celestial spheres

of planets and stars. Although he believed the universe to be finite in size, he stressed that it

exists unchanged and static throughout eternity. Aristotle definitively established the four

classical elements of fire, air, earth and water, which were acted on by two forces, gravity

(the tendency of earth and water to sink) and levity (the tendency of air and fire to rise). He

later added a fifth element, aether, to describe the void that fills the universe above the

terrestrial sphere.

Stoic Universe - The Stoic philosophers of ancient Greece (3rd Century B.C. and after)

believed in a kind of island universe in which a finite cosmos is surrounded by an infinite void

(not dissimilar in principle to a galaxy). They held that the cosmos is in a constant state of

flux, and pulsates in size and periodically passes through upheavals and conflagrations. In the

Stoic view, the universe is like a giant living body, with its leading part being the stars and the

Sun, but in which all parts are interconnected, so that what happens in one place affects

what happens elsewhere. They also held a cyclical view of history, in which the world was

once pure fire and would become fire again (an idea borrowed from Heraclitus).

Heliocentric Universe - The 3rd Century B.C. Greek astronomer and mathematicianAristarchus of Samos was the first to present an explicit argument for a heliocentric model of

the Solar System, placing the Sun, not the Earth, at the center of the known universe. He

described the Earth as rotating daily on its axis and revolving annually about the Sun in a

circular orbit, along with a sphere of fixed stars. His ideas were generally rejected in favour of

the geocentric theories of Aristotle and Ptolemy until they were successfully revived nearly

1800 years later by Copernicus. However, there were exceptions: Seleucus of Seleucia, who

lived about a century after Aristarchus, supported his theories and used the tides to explain

heliocentricity and the influence of the Moon; the Indian astonomer and mathematician

Aryabhata described elliptical orbits around the Sun at the end of of the 5th Century A.D.; as

did the Muslim astronomer Ja'far ibn Muhammad Abu Ma'shar al-Balkhi in the 9th Century.

Ptolemaic Universe - The 2nd Century A.D. Roman-Egyptian mathematician and astronomer

Ptolemy (Claudius Ptolemaeus) described a geocentric model largely based on Aristotelian

ideas, in which the planets and the rest of the universe orbit about a stationary Earth in




151circular epicycles. In terms of longevity, it was perhaps the most successful cosmological

model of all time. Modifications to the basic Ptolemaic system were suggested by the Islamic

Maragha School in the 13th, 14th and 15th Centuries including the first accurate lunar model

by Ibn al-Shatir, and the rejection of a stationery Earth in favour of a rotating Earth by Ali

Qushji.

Abrahamic Universe - Several medieval Christian, Muslim and Jewish scholars put forwardthe idea of a universe which was finite in time. In the 6th Century A.D., the Christian

philospher John Philoponus of Alexandria argued against the ancient Greek notion of an

infinite past, and was perhaps the first commentator to argue that the universe is finite in

time and therefore had a beginning. Early Muslim theologians such as Al-Kindi (9th Century)

and Al-Ghazali (11th Century) offered logical arguments supporting a finite universe, as did

the 10th Century Jewish philosopher Saadia Gaon.

Partially Heliocentric Universe - In the 15th and early 16th Century, Somayaji Nilakantha of

the Kerala school of astronomy and mathematics in southern India developed a

computational system for a partially heliocentric planetary model in which Mercury, Venus,

Mars, Jupiter and Saturn orbited the Sun, which in turn orbited the Earth. This was very

similar to the Tychonic system proposed by the Danish nobleman Tycho Brahe later in the

16th Century as a kind of hybrid of the Ptolemaic and Copernican models.

Copernican Universe - In 1543, the Polish astronomer and polymath Nicolaus Copernicus

adapted the geocentric Maragha model of Ibn al-Shatir to meet the requirements of the

ancient heliocentric universe of Aristarchus. His publication of a scientific theory of

heliocentrism, demonstrating that the motions of celestial objects can be explained without

putting the Earth at rest in the centre of the universe, stimulated further scientific

investigations and became a landmark in the history of modern science, sometimes known as

the Copernican Revolution. His Copernican Principle (that the Earth is not in a central,

specially favoured position) and its implication that celestial bodies obey physical laws

identical to those on Earth, first established cosmology as a science rather than a branch of

metaphysics. In 1576, the English astronomer Thomas Digges popularized Copernicus’ ideas

and also extended them by positing the existence of a multitude of stars extending to

infinity, rather than just Copernicus’ narrow band of fixed stars. The Italian philosopher

Giordano Bruno took the Copernican Principle a stage further in 1584 by suggesting that

even the Solar System is not the centre of the universe, but rather a relatively insignificant

star system among an infinite multitude of others. In 1605, Johannes Kepler made further

refinements by finally abandoning the classical assumption of circular orbits in favour of

elliptical orbits which could explain the strange apparent movements of the planets. Galileo'scontroversial support of Copernicus' heliocentric model in the early 17th Century was

denounced by the Inquisition but nevertheless helped to popularize the idea.




152

Cartesian Vortex Universe - In the mid-17th Century, the French philosopher René Descartes

outlined a model of the universe with many of the characteristics of Newton’s later static,

infinite universe. But, according to Descartes, the vacuum of space was not empty at all, but

was filled with matter that swirled around in large and small vortices. His model involved asystem of huge swirling whirlpools of ethereal or fine matter, producing what would later be

called gravitational effects.

Static (or Newtonian) Universe - In 1687, Sir Isaac Newton published his “Principia”, which

described, among other things, a static, steady state, infinite universe which even Einstein, in

the early 20th Century, took as a given (at least until events proved otherwise). In Newton’s

universe, matter on the large scale is uniformly distributed, and the universe is

gravitationally balanced but essentially unstable.

Hierarchical Universe and the Nebular Hypothesis - Although still generally based on a

Newtonian static universe, the matter in a hierarchical universe is clustered on ever larger

scales of hierarchy, and is endlessly being recycled. It was first proposed in 1734 by the

Swedish scientist and philosopher Emanuel Swedenborg, and developed further

(independently) by Thomas Wright (1750), Immanuel Kant (1755) and Johann Heinrich

Lambert (1761), and a similar model was proposed in 1796 by the Frenchman Pierre-Simon

Laplace.

Einsteinian Universe - The model of the universe assumed by Albert Einstein in his

groundbreaking theory of gravity in the early 20th Century was not dissimilar to Newton’s in

that it was a static, dynamically stable universe which was neither expanding or contracting.

However, he had to add in a “cosmological constant” to his general relativity equations to

counteract the dynamical effects of gravity which would otherwise have caused the universeto collapse in on itself (although he later abandoned that part of his theory when Edwin

Hubble definitively showed in 1929 that the universe was not in fact static).




153

Big Bang Model of the Universe - After Hubble’s demonstration of the continuously

expanding universe in 1929 (and especially after the discovery of cosmic microwave

background radiation by Arno Penzias and Robert Wilson in 1965), some version of the Big

Bang theory has generally been the mainsteam scientific view. The theory describes the

universe as originating in an infinitely tiny, infinitely dense point (or singularity) between 13

and 14 billion years ago, from where it has been expanding ever since. The essentialstatement of the theory is usually attributed to the Belgian Roman Catholic priest and

physicist Georges Lemaître in 1927 (even before Hubble’s corroborating evidence), although

a similar theory had been proposed, although not pursued, 1922 by the Russian Alexander

Friedmann in 1922. Friedmann actually developed two models of an expanding universe

based on Einstein’s general relativity equations, one with positive curvature or spherical

space, and one with negative curvature or hyperbolic space.

(Timeline of the Big Bang)

Oscillating Universe - This was Einstein’s favoured model after he rejected his own original

model in the 1930s. The oscillating universe followed from Alexander Friedmann’s model of

an expanding universe based on the general relativity equations for a universe with positive

curvature (spherical space), which results in the universe expanding for a time and thencontracting due to the pull of its gravity, in a perpetual cycle of Big Bang followed by Big

Crunch. Time is thus endless and beginningless, and the beginning-of-time paradox is

avoided.

Steady State Universe - This non-standard cosmology (i.e. opposed to the standard Big Bang

model) has occurred in various versions since the Big Bang theory was generally adopted by

the scientific community. A popular variant of the steady state universe was proposed in

1948 by the English astronomer Fred Hoyle and the and Austrians Thomas Gold and

Hermann Bondi. It predicted a universe that expanded but did not change its density, with

matter being inserted into the universe as it expanded in order to maintain a constant

density. Despite its drawbacks, this was quite a popular idea until the discovery of the cosmicmicrowave background radiation in 1965 which supported the

Inflationary (or Inflating) Universe - In 1980, the American physicist Alan Guth proposed a

model of the universe based on the Big Bang, but incorporating a short, early period of




154exponential cosmic inflation in order to solve the horizon and flatness problems of the

standard Big Bang model. Another variation of the inflationary universe is the cyclic model

developed by Paul Steinhardt and Neil Turok in 2002 using state-of-the-art M-theory,

superstring theory and brane cosmology, which involves an inflationary universe expanding

and contracting in cycles.

Multiverse - The Russian-American physicist Andrei Linde developed the inflationary

universe idea further in 1983 with his chaotic inflation theory (or eternal inflation), which

sees our universe as just one of many “bubbles” that grew as part of a multiverse owing to a

vacuum that had not decayed to its ground state. The American physicists Hugh Everett III

and Bryce DeWitt had initially developed and popularized their “many worlds” formulation of

the multiverse in the 1960s and 1970s. Alternative versions have also been developed where

our observable universe is just one tiny organized part of an infinitely big cosmos which islargely in a state of chaos, or where our organized universe is just one temporary episode in

an infinite sequence of largely chaotic and unorganized arrangements.




155




156

THE UNIVERSE BY NUMBERS--Some of the numbers, both small and large, that are bandied around in modern physics are very

difficult to grasp. Below is a table listing - from the infinitesimally small to the incomprehensibly large

- some of the numbers which are relevant to the subject matter.

Obviously, SI units are used for every value: metres for distance, seconds for time, metres/second for

speed or velocity, degrees Kelvin for temperature, kilograms for mass, kilograms/metre3 for density,

Joules for energy, Coulombs for electrical charge, kilopascals for pressure.

The different types of measurements are also colour-coded for convenience:




157




158




159




160




161




162




163

A FEW RANDOM FACTS--

------------------------------------------------------------------------------------------

Index of Random Facts:

- Where in the universe is the Earth?

- How fast are we travelling through space?

- How fast does light travel?

- How far is it to space, the Moon, the Sun, the stars, etc?

- How many stars are there?

- How does the Sun shine?

- What different types of stars are there?

- What is the human body (and the Earth, the Sun, the Universe) made of?

- How many molecules/atoms are there in each cubic metre?

- What if the history of the universe were squeezed into the period of one year?

- What are the coldest and the hottest objects in the universe?

- What is the electromagnetic spectrum?

- What is a planet? What is a dwarf planet?

- Why do the planets orbit the Sun?

--------------------------------------------------------------------------------------------

Here are some intresting questions.

WHERE IN THE UNIVERSE IS THE EARTH?

A precocious child might write his or her full address as Main Street, Toronto, Canada, the Earth, the

Solar System, Orion Arm, the Milky Way, the Local Group, the Virgo Supercluster, the Universe.

The Solar System consists of the Sun and those objects bound to it by gravity (the terrestrial planets,

Mercury, Venus, Earth and Mars; the gas giants, Jupiter, Saturn, Neptune and Uranus; and various

dwarf planets, proto-planets and asteroids). However measured, it is less than a light year across.

The Milky Way galaxy is a barred spiral galaxy with a diameter of about 100,000 light-years and

containing about 200 billion stars. Our Solar System is located towards the edge of one of the MilkyWay's outer spiral arms, known as the Orion Arm or Local Spur, about 25,000 to 28,000 light years

from the galactic centre.




164The Local Group is a small group or cluster of gravitationally-bound galaxies, which includes the Milky

Way, the Andromeda galaxy and the much smaller Triangulum galaxy (which has a diameter of

around 10 million light-years), along with smaller satellite and dwarf galaxies such as the Large

Magellanic Cloud, the Sagittarius Dwarf Galaxy and Canis Major Dwarf Galaxy.

The Virgo Supercluster is an irregular group of clusters of galaxies, between 100 and 200 million light

years in diameter, which incorporates our Local Group of galaxies and about 100 other clusters. The

Local Group is located in a small filament on the outskirts of the supercluster. It is thought that

superclusters may also be arranged in even larger structures called walls (such as the Sloan Great

Wall, which is about 1.5 billion light years long), although these may not be true structures as their

parts are not gravitationally bound together.

The universe is what we usually think of as the totality of known or supposed objects and

phenomena throughout space. The observable part alone contains over ten billion trillion stars

arranged in about 100 billion galaxies, and is estimated to be around 156 billion light years in

diameter. By definition, we are at the centre of our observable universe, but it is totally unknown

where we are in the universe as a whole.

HOW FAST ARE WE TRAVELLING THROUGH SPACE?

A person on the equator is rotating around the Earth at about 1,660 kilometres per hour. A person at

the north or south pole actually has a rotational speed of zero, and is effectively turning on the spot.

Somewhere in between, a person’s rotational speed decreases as they move from the equator

towards the pole: for example, a person in Toronto, at around 45°N, is travelling about 1,230

kilometres per hour.

Actually, rotational speed around the Earth is also dependent on altitude above sea level, and a

person at the top of a mountain on the Equator is actually travelling faster than 1,660 kilometres per

hour (as he has further to go with each revolution). Taking this to an extreme, an object in

geostationary orbit around the Earth at an altitude of about 36,000 kilometres above the ground has

to travel at about 11,000 kilometres per hour.

But that is not all. The Earth circles around the Sun at about 107,000 kilometres per hour. Our Solar

System is rotating around the Milky Way galaxy at about 700,000 kilometres per hour. The galaxy is

also travelling at huge speed away from every other galaxy as the universe continues to expand,

although with vastly differing relative speeds depending on the distances of the galaxies from us. To

give some indication, scientists have calculated that our galaxy is travelling at about 2.2 million

kilometres per hour relative to the cosmic background radiation which pervades the universe.

HOW FAST DOES LIGHT TRAVEL?

Light travels at exactly 299,792,458 metres per second in a vacuum (about 300,000 kilometres per

second or just over 1 billion kilometres per hour). As a comparison, sound waves travel at a paltry

343.14 metres per second (about 1,235 kilometres per hour), almost a million times slower than lightwaves, and the fastest military airplane, the SR-71 Blackbird, can fly at about 980 metres per second

(about 3,500 kilometres per hour).

At that speed light takes:




165

0.0000033 seconds to travel 1 kilometre;

1.3 seconds to reach us from the Moon;

8.32 minutes to reach us from the Sun;

4.37 years to reach us from Alpha Centauri, the nearest star system to the Solar System

(Alpha Centauri is therefore said to be 4.37 light years away, but now according to latest

observations Proxima Centauri is the nearest star to the sun. It is only 4.2 lightyears away ); 26,000 years to reach us from the centre of our Milky Way galaxy;

2,500,000 years to reach us from the Andromeda Galaxy, our next nearest galaxy (and the

most distant object visible to the naked eye, although only as a barely perceptible smudge);

59 million years to reach us from the Virgo Cluster, the nearest large galaxy cluster; and,

theoretically, about 78 billion years to reach us from the edge of the observable universe

(this is actually longer ago than the 13.7 billion year age of the universe, because the

continued expansion of space has significantly increased the distance the light from these

early objects has had to travel).

HOW FAR IS IT TO SPACE, THE MOON, THE SUN, THE STARS, ETC?

Earth’s atmosphere is divided up into several layers: the troposphere from about 6 - 20 kilometres

up; the stratosphere from 20 - 50 kilometres; the mesosphere from 50 - 85 kilometres; the

thermosphere from 85 - 690 kilometres; and the exosphere out to about 10,000 kilometres. “Space”

is often considered to start at about 100 kilometres up, known as the Kármán line, where the Earth's

atmosphere becomes too thin for aeronautical purposes. The International Space Station orbits the

Earth about 350 kilometres up (in the thermosphere).

The Moon is about 360,000 kilometres away from the Earth (and it is receding from us at a rate of

about 4 centimetres a year as its orbit gradually speeds up). The nearest planet to the Earth is either

Venus, which varies between 42 million kilometres and 258 million kilometres away (its orbit is highly

irregular), or Mars which varies between 56 million kilometres and 100 million kilometres away. The

Sun is about 150 million kilometres away from the Earth (sometimes referred to as 1 astronomical

unit, or 1 AU).

The next nearest star to us (other than the Sun) is Proxima Centauri, in the Alpha Centauri star

system (still part of our Milky Way galaxy), which is about 40 trillion (40,000,000,000,000) miles away

or, using the more convenient unit based on the distance light travels in a year (which is about 9.46

trillion kilometres), 4.24 light years. Sirius A and B, in the Sirius star system, are about 81 trillion

(81,000,000,000,000) kilometres or 8.58 light years away.

The centre of the Milky Way galaxy is about 26,000 light years, or roughly 245 quadrillion

(245,000,000,000,000,000) kilometres. Our next closest galaxy is the Andromeda Galaxy, which is

about 2.5 million light years away, or roughly 26,000,000,000,000,000,000 kilometres. The best

estimate of the size of the observable universe (given that it has been expanding for 13.7 billion

years), is about 156 billion light years (1,560,000,000,000,000,000,000,000 kilometres) across.

HOW MANY STARS ARE THERE?

About 3,000 stars are visible to the unaided eye on a clear moonless night. About 100,000 stars can

be seen using a small telescope. There are an estimated one hundred billion (100,000,000,000) stars




166in our own Milky Way galaxy, although some estimates range up to four times that many, much

depending on the number of brown dwarfs and other very dim stars.

A typical galaxy may contain anywhere between about ten million and one trillion stars. Therefore,

using a very rough estimate of a hundred billion galaxies in the observable universe, and the number

of stars in our own galaxy as a reasonable average, there may be around ten billion trillion

(10,000,000,000,000,000,000,000 or 1022) stars in the observable universe, or quite possibly any-

where up to 1024.

HOW DOES THE SUN SHINE?

The Sun, or any star for that matter, “shines” or “burns” due to a process of thermonuclear fusion,

not due to a chemical reaction like the oxygen-driven fires on Earth.

Because the Sun is so massive, it has great gravity and so its core is under tremendous levels of

pressure and heat. This pressure and heat is so high in the Sun’s core (about 15 million °C) that the

protons of the hydrogen atoms which largely make up the Sun collide into each other with enough

speed that they stick together or “fuse” to create helium nuclei. It ef fectively takes four hydrogen

nuclei to fuse together to produce one nucleus of helium, although it is actually a more complicated

three-part process (hydrogen to deuterium, deuterium to helium-3 and helium-3 to helium).

However, the net mass of the fused helium nuclei is actually slightly smaller than the sum of the

masses of its constituent hydrogen atoms, and this tiny amount of lost mass is converted into an

enormous amount of energy, according to the mass-energy equivalence relationship E = mc². To give

an idea of the scale of this process, each second of every day our Sun converts about 700 million tons

of hydrogen into about 695 tons of helium. The missing 5 million tons is converted into energy

equivalent to the detonation of about 100 billion one-megaton bombs, two hundred million times




167the explosive yield of every nuclear weapon ever detonated on Earth. And this happens every

second.

The fusion process therefore releases huge amounts of energy, initially as gamma ray photons, that

traverse the interior of the Sun through a combination of radiation and convection, and are then

radiated into space as electromagnetic energy, including visible light. The process also emits particle

radiation, known as the “stellar wind”, a steady stream of electrically charged particles, such as free

protons, alpha particles and beta particles, as well as a steady stream of neutrinos. It is the internal

pressure of this nuclear fusion process that prevents the Sun from collapsing further under its own

gravity (known as a state of hydrostatic equilibrium).

Hydrogen is by far the most common element in the Sun (and in the universe as a whole) and helium

is the second most common. A star will spend most of its life, called the “main sequence” phase,

fusing hydrogen into helium, but, in larger hotter stars, the helium which accumulates in the core

becomes more and more compressed and hot until the helium atoms begin fusing to form oxygen

and carbon. These stars are therefore continually creating heavier elements from the less heavy:

helium from hydrogen, oxygen from helium, and so on and so on. Even in the largest of stars,

however, this process stops at the ultra-stable element iron, which will not easily fuse to form

heavier elements, at which point the inward pressure of gravity takes over, crushing the core and

resulting in a supernova explosion and the creation of a neutron star or black hole.

WHAT DIFFERENT TYPES OF STARS ARE THERE?

There are several different main types of stars, depending on their size, luminosity and lifespan:




168

brown dwarfs - "failed stars", which form from clouds of interstellar gas, as other stars do,

but never reach sufficient mass, density and internal heat to start the nuclear fusion process

(i.e. less than 8% of the mass of our Sun). Although they may glow dimly when newly formed

(and are therefore in fact red not brown), they start to cool soon after and so are very

difficult to spot. They may actually be among the most common type of stars.

red dwarfs - small and relatively cool stars, bigger than brown dwarfs, but less than 40 - 50%of the mass of our Sun. Most of the stars in our galaxy (excluding possible unseen brown

dwarfs) are red dwarfs. They are much dimmer than our Sun (even the largest red dwarf has

only about 10% of the Sun's luminosity), burn much more slowly, and typically live much

longer.

yellow dwarfs - main-sequence stars like our own Sun, Alpha Centauri A, Tau Ceti, etc,

typically about 80 - 100% of the size of the Sun, and actually more white than yellow. They

are also known as G V stars for their spectral type G and luminosity class V.

white stars - bright, main-sequence stars with masses from 1.4 to 2.1 times the mass of the

Sun and surface temperatures between 7,600°C and 10,000°C, such as Sirius A and Vega.

red giants - luminous giant stars of low or intermediate mass (usually between 0.5 and 10

solar masses) in a late phase of stellar evolution, such as Aldeberan and Arcturus. When a

main-sequence star has fused all its hydrogen into helium, it then starts to burn its helium to

produce carbon and oxygen, and expands to many times its previous volume to become a

red giant. After a relatively short time (in the region of two hundred million years), the red

giant puffs out its outer layers in a gas cloud called a nebula and collapses in on itself to form

a white dwarf. The largest red giants are known as red supergiants, and are the largest stars

in the universe in terms of volume (well-known examples are Antares and Betelgeuse).

white dwarfs - small, dense, burnt-out husks of stars, no longer undergoing fusion reactions,

and representing the final evolutionary state of most of the stars in our galaxy. When a redgiant has used up its helium to produce carbon and oxygen, and has insufficient mass to

generate the core temperatures required to fuse carbon, it sheds its outer layers to form a

planetary nebula, and leaves behind an inert mass of carbon and oxygen. A white dwarf is

typically only the size of the Earth, but 200,000 times more dense.

black dwarfs - hypothetical stellar remnants created when a white dwarf becomes cool and

dark after about ten billion years of life. Black dwarfs are very hard to detect, and very few

would exist yet anyway in a universe only 13.7 billion years old.

blue giants - bright, giant stars, between 10 and 100 times the size of the Sun, and between

10 and 1,000 times its luminosity. Because of their mass and hotness, they are relatively

short-lived and quickly exhaust their hydrogen fuel, ending as red supergiants or neutron

stars. The biggest and most luminous stars are referred to as blue supergiants and

hypergiants. The best known blue supergiant is Rigel, the brightest star in the constellation of

Orion, which has a mass about 20 times that of the Sun and a luminosity more than 60,000

times greater. The biggest and brightest ever found is 10 million times as bright as the Sun.

neutron stars - stellar remnants that can result from the gravitational collapse of massive

stars during a supernova event. They are composed almost entirely of crushed neutrons, and

are very hot and very dense. Although a typical neutron star has a mass of only between 1.35

and about 2.1 times that of our Sun, it is 60,000 times smaller than the Sun (usually in the

region of just 20 - 30 kilometres across) and, because of this huge density, has a gravity of

over 200 billion times that we experience on Earth. They rotate very fast (especially soon

after the supernova explosion) and some emit regular pulses of radiation and are known as

pulsars. Smaller collapsed stars will usually become white dwarfs, and larger ones (over

about 5 solar masses) will collapse completely into a black hole singularity.




169

variable stars - stars that grow and shrink in size periodically and appear to pulsate. The

changes in apparent brightness may be due to variations in the star's actual luminosity, or to

variations in the amount of the star's light that is blocked from reaching Earth.

binary stars - two stars in close proximity which orbit around their common centre of mass.

In fact, the majority of stars are part of binary, triplet or multiple star systems, and well-

known examples are Sirius in the Canis Major constellation and Alpha Centauri.

WHAT IS THE HUMAN BODY (AND THE EARTH, THE SUN, THE UNIVERSE) MADE OF?

The human body is made up of elements in the following approximate proportions (by weight): 65%

oxygen, 18% carbon, 10% hydrogen, 3% nitrogen, 2% calcium, 1% phosphorus, and 1% other

elements such as potassium, sodium, iron, zinc, etc. By the number of atoms, however, the

proportions are: 63% hydrogen, 24% oxygen and 12% carbon, with only tiny traces of the others.

The earth’s crust is made up (by weight) of: 46% oxygen, 27% silicon, 8% aluminium, 5% iron, 4%calcium, 2% sodium, 2% potassium and 2% magnesium, plus traces of the other 84 naturally

occurring elements. The air we breathe contains roughly (by volume): 78% nitrogen, 21% oxygen, 1%

argon, 0.038% carbon dioxide, and trace amounts of other gases.

The Sun is composed of: 75% hydrogen, 24% helium and 1% oxygen, with tiny traces of carbon, neon

and iron. In fact, hydrogen and helium are estimated to make up roughly 74% and 24% respectively

of all the matter in the universe as a whole, along with tiny amounts of oxygen (107%), carbon

(0.46%), neon (0.13%), iron (0.109), nitrogen (0.1%), silicon (0.065%), magnesium (0.058%) and

sulphur (0.044%).

HOW MANY MOLECULES/ATOMS ARE THERE IN EACH CUBIC METRE?

Each cubic metre of air on Earth contains about 10 trillion trillion molecules. This falls to around 4

trillion trillion at the top of Mount Everest. A hundred kilometres up, sometimes considered to be the

border of space, there are around a million trillion molecules per cubic metre. At the International

Space Station, roughly 350 kilometres away, there are only around 10 trillion.

100,000 kilometres from the Earth (over a third of the way to the Moon, where there is absolutely no

influence from the Earth’s atmosphere), there are around seven million particles per cubic metre. Atthe edge of the Solar System, the density is down to about a thousand atoms per cubic metre. In

intergalactic space, there are only about ten atoms per cubic metre of space.

WHAT IF THE HISTORY OF THE UNIVERSE WERE SQUEEZED INTO THE PERIOD OF ONE YEAR?

the very first stars and galaxies formed at about 7:00am on January 8th;

our Sun was born at about 8:00am on September 1st and the Earth at about 2:00am on

September 11th;

the earliest life on Earth occurred at 1:00pm on September 30th, although the first multi-

cellular life did not appear until 11:00pm on December 14th;

dinosaurs appeared on Earth at 3:00am on December 27th and died out just 3 days later at

10:00am on December 30th;




170

the first humans arrived as late as 11:39pm on December 31st;

the philosophers of ancient Greece flourished about 5 seconds before midnight on December

31st and everything since Columbus “discovered” America happened within the last second

of the year.

On the same scale:

our Sun will become a red giant star, burning up the Earth in the process, next May 2nd at

around 4:00pm, and around the same time, our nearest galactic neighbour, Andromeda, will

start to crash into our own galaxy;

by 1:00pm on May 7th the Sun will have become a cold, dead white dwarf star.

WHAT ARE THE COLDEST AND THE HOTTEST OBJECTS IN THE UNIVERSE?

Absolute zero, the temperature at which thermal energy is theoretically zero and which is therefore

generally considered the coldest possible temperature, is -273.15°C Celsius (or 0°K on the absolute

Kelvin scale). The cosmic microwave background radiation which uniformly permeates all of spacehas a temperature of 2.725°K, or around -270°C.

Within the Solar System, the average temperature on Pluto is around -235°C, on Neptune around -

220°C, on Uranus -210°C, on Saturn -184°C and on Jupiter -153°C. The temperature on Mars varies

between about -87°C and -5°C, with an average of around -46°C. The lowest natural temperature on

Earth (recorded at Vostok, Antarctica in 1983) is -89°C; the highest surface temperature on Earth

(recorded at Al 'Aziziyah, Libya in 1922) is 58°C; the mean overall temperature on Earth is 14°C.

Water at standard pressure on Earth freezes at 0°C, and boils at 100°C. Lead melts at around 328°C,

iron at 1,535°C, titanium at 1,668°C, and carbon in the range of 3,550°C to 3,675°C depending on the

type. The temperature of an incandescent light bulb is round 2,200°C. A lightning bolt can reach

28,000°C. The temperature in a working fusion reactor is around 100 million °C. The highest man-

made temperature, about 2 billion °C, was generated by the so-called Z-Machine at the Sandia

National Laboratories in Albuquerque, New Mexico.

Continuing through the Solar System, the mean daytime temperature on the Moon is 107°C, while

the mean nighttime temperature is -153°C. The temperature on Venus is a relatively uniform 462°C.

The surface temperature on Mercury varies between 466°C on the sunward side and -184°C on the

other side. The surface of the Sun has a temperature of about 5,700°C, and the core of the Sun about

15 million °C (although the temperature in the Sun’s corona can rise to over 2 million °C).

Red dwarf and red giant stars typically have surface temperatures in the range of 2,500°C to 3,500°C.

Blue supergiant and hypergiant stars have surface temperatures ranging anywhere from 3,500°C to

35,000°C. The explosion of a supernova can generate temperatures in excess of 100 billion °C. The

Planck Temperature is the temperature of the universe at 1 Planck Time after the Big Bang, and is

considered the de facto maximum possible temperature. It has been calculated to be approximately

1.4 × 1032°C (140,000,000,000,000,000,000,000,000,000,000°C).

WHAT IS THE ELECTROMAGNETIC SPECTRUM?

The electromagnetic spectrum is the range of possible electromagnetic radiation frequencies. They

are usually described in terms of either their wavelengths (the distance between waves) or their

frequency (the number of waves per second). From high to low wavelengths (low to high




171frequencies), the spectrum covers: radio waves - wavelengths ranging from about 1,000 metres

down to about 1 metre, roughly on the scale of buildings and people, with radio and television

frequencies ranging from AM radio at about 1,000 to 100 metres, and FM radio at about 10 to 1

metres;

microwaves - wavelengths of between 1 metre and 1 millimetre (or 0.001 of a metre), roughly on the

scale of people and insects;infrared - wavelengths from about 1 millimetre down the edge of visible

light at about 750 nanometres (or 0.00000075 of a metre), roughly the size of the point of a needle;

visible light - wavelengths of about 750 nanometres (0.00000075 of a metre) down to about 400

nanometres (0.0000004 of a metre), roughly the size of cells, following the familiar colour spectrum

of red, orange, yellow, green, blue, indigo, violet; ultaviolet - wavelengths of 400 nanometres

(0.0000004 of a metre) down to 10 nanometres (0.00000001 of a metre), roughly the size of

molecules; x-rays - wavelengths of 10 nanometres (0.00000001 of a metre) down to 0.01 nanometres

or 10 picometres (0.00000000001 of a metre), roughly the size of atoms; gamma rays - wavelengths

below 10 picometres (0.00000000001 of a metre), roughly the size of atomic nuclei.

WHAT IS A PLANET? WHAT IS A DWARF PLANET?

While it might seem obvious what a planet is, the definition has become more complex since 2006

when the International Astronomical Union voted to reduce the status of Pluto (discovered in 1930

and considered a planet ever since then) to dwarf planet, so that it is officially no longer the ninth

planet in the Solar System.Under the new definition, a planet has to be large enough that its gravity

forces it into the shape of a sphere (smaller, oddly-shaped asteroids therefore do not quality); it has

to be orbiting a star, and not a satellite of another planet (several moons in the Solar System are

bigger than Pluto); it has to be not massive enough to cause thermonuclear fusion (in which case it

would classify as a star); and it must have cleared its neighbouring region of planetesimals and otherobjects by its gravitational pull.




172It was the last of these requirements that Pluto failed. Pluto is effectively part of the Kuiper Belt

Objects, a wide belt of space towards the edge of the Solar System (between about 4.5 billion

kilometres and 8 billion kilometres from the Sun) which contains millions of small, icy, rocky objects,

of which more than 70,000 have been identified over 100 kilometres in diameter. Haumea and

Makemake are two other dwarf planets in this region, and the dwarf planet Eris (which is actually

larger than Pluto) is even further out, roughly three times Pluto’s distance from the Sun. Ceres, thelargest object in the asteroid belt between Mars and Jupiter, is also technically a dwarf planet (which

essentially means a small, more or less spherical planetoid, whose gravity is not sufficient to have

cleared its local area of debris).

The distinction between dwarf planets, asteroids, meteoroids and comets is perhaps less clearly

defined. Simplistically, asteroids are relatively small inactive bodies composed of rock or metals;

dwarf planets are the largest asteroids; meteoroids are smaller particles of asteroids (called meteors

or "shooting stars" when they burn up in the atmospere, and meteorites if they manage to penetrate

to the Earth's surface); comets are mainly composed of dirt and ices rather than solid rock or metal,

and tend to have dust and gas tails when close to the Sun.

WHY DO THE PLANETS ORBIT THE SUN?

Paradoxically, it is the Sun's gravity that keeps the planets in orbit around it, just as the Earth's gravity

keeps the Moon and satellites in orbit around it. The reason they do not just fall into the Sun is that

they are travelling fast enough to continually "miss" it.

An analogy helps to explain this: if you throw a rock out from the top of a high tower, it will travel a

certain distance before curving down and hitting the Earth. Once thrown, the rock has inertia andwould continue in a straight line of motion if there were not some force (gravity) pulling it down. The

faster you throw the rock out, the further it travels, until eventually, if you could throw it fast enough

(and assuming no air resistance), it would travel all the way around the Earth (and hit you in the

back!). The rock is therefore now in orbit: it is still always falling towards the Earth, but the round

surface of the Earth is falling away just as fast. Throw the rock a little faster and it would still travel

around the Earth but at a higher orbit. If you could throw the rock at what is called the "escape

velocity", it would break away from the gravity of the Earth completely and never fall back.The

reason the planets are travelling at just that speed which allows them to orbit the Sun (and not spiral

into it or whirl away into space) is not a coincidence or evidence of divine intervention, but goes back

to when the Solar System was just a spinning cloud of gas and dust. Everything that was spinning

slowly was incorporated into the Sun itself under the force of gravity; everything that was spinning

too fast escaped into outer space; everything else remained in orbit around the Sun and gradually

coalesced into the planets, retaining its speed of spin and therefore its orbit (encountering little

resistance in the near-vacuum of space).

Because the Sun and planets all formed from the same spinning nebular cloud, this is also why they

all rotate in the same direction. As the nebula continued to contract under the influence of gravity it

rotated faster and faster due to the conservation of angular momentum. Centrifugal effects caused

the spinning cloud to flatten into a flattish disk with a dense bulge at its centre (which would

coalesce into the Sun). This is why the planets orbit the Sun in a more or less flat plane, known as the

ecliptic.




173




174

Glossary of Terms--

A

Abiogenesis:

The study of how life on Earth could have arisen from inanimate matter. It should not be confused

with evolution (the study of how living things change over time), biogenesis (the process of lifeforms

producing other lifeforms) or spontaneous generation (the obsolete theory of complex life

originating from inanimate matter on an everyday basis).

Absolute Zero:

The lowest temperature possible, equivalent to -273.15°C (or 0° on the absolute Kelvin scale), at

which point atoms cease to move altogether and molecular energy is minimal. The idea that it is

impossible, through any physical process, to lower the temperature of a system to zero is known as

the Third Law of Thermodynamics.

Accretion Disk:

Diffuse material orbiting around a central body such as a protostar, a young star, a neutron star or a

black hole. Gravity causes the material in the disc to spiral inwards towards the central body with

great speed, and the gravitational forces acting on the material cause the emission of x-rays, radio

waves or other electromagnetic radiation (known as quasars).

Alpha Particle (Alpha Decay):

A particle of 2 protons and 2 neutrons (essentially a helium nucleus) that is emitted by an unstable

radioactive nucleus during radioactive decay. It is a relatively low-penetration particle due its

comparatively low energy and high mass.

Angular Momentum:

A measure of the momentum of a body in rotational motion about its centre of mass. Technically,

the angular momentum of a body is equal to the mass of the body multiplied by the cross product of

the position vector of the particle with its velocity vector. The angular momentum of a system is thesum of the angular momenta of its constituent particles, and this total is conserved unless acted on

by an outside force.

Anthropic Principle:

The idea that the fundamental constants of physics and chemistry are just right (or “fine -tuned”) to

allow the universe and life as we know it to exist, and indeed that the universe is only as it is because

we are here to observe it.

Antimatter:

A large accumulation of antiparticles - antiprotons, antineutrons and positrons (antielectrons) -

which have opposite properties to normal particles (e.g. electrical charge), and which can come

together to make antiatoms. When matter and antimatter meet, they self-destruct in a burst of high-




175energy photons or gamma rays. The laws of physics seem to predict a pretty much 50/50 mix of

matter and antimatter, despite the observable universe apparently consisting almost entirely of

matter, known as the “baryon asymmetry problem”.

Atom:

The basic building block of all normal matter, consisting of a nucleus (which is itself composed of

positively-charged protons and zero-charged neutrons) orbited by a cloud of negatively-charged

electrons, so that the positive charge is exactly balanced by the negative charge and the atom as a

whole is electrically neutral. Atoms range from about 32 to about 225 picometres in size (a picometre

is a trillionth of a metre). A typical human hair is about 1 million carbon atoms in width.

B

Beta Particles (Beta Decay):

High-energy, high-speed electrons or positrons (antielectrons) emitted by some types of radioactive

decay, when an unstable atomic nucleus with an excess of neutrons or protons undergoes beta

decay (a process mediated by the weak nuclear force). The particles emitted are a form of ionizing

radiation, also known as beta rays.

Big Bang:

The huge “explosion” 13.7 billion years ago in which the universe (including all space, time and

energy) is thought to have been created. According to this theory, the universe began in a super-dense, super-hot state and has been expanding and cooling ever since. The phrase was coined by

Fred Hoyle during a 1949 radio broadcast.

Big Crunch:

One possible scenario for the ultimate fate of the universe, in which the gravity of the matter in the

universe (providing that there is in fact a “critical mass”) will one day halt and reverse the universe’s

expansion in a mirror image of the Big Bang, causing it to collapse into a black hole singularity.

However, in the light of recent evidence for an accelerating universe, this is no longer considered the

most likely outcome.

Black Body:

An idealized object that absorbs all electromagnetic radiation that falls on it, without passing

through and without reflection. The radiation emitted from a black body is mostly infrared light at

room temperature, but as the temperature increases it starts to emit visible wavelengths, from red

through to blue, and then ultraviolet light at very high temperatures.

Black Hole:

The warped space-time remaining after the gravity of a massive body has caused it to shrink down to

a point. It is a region of empty space with a point-like singularity at the centre and an event horizon

at the outer edge. It is so dense that no normal matter or radiation can escape its gravitational field,

so that nothing - not even light - can ever leave (hence its blackness). It is thought that most galaxies

have a supermassive black hole at their heart.




176

C

Classical Physics:

A general term used to describe the physics based on principles developed before the rise of general

relativity and quantum mechanics, essentially physics as it had existed up to the early years of the20th Century. It includes the mechanics of Galileo and Newton, the electrodynamics of Maxwell, the

thermodynamics of Boyle and Kelvin, and usually even the special relativity of Einstein.

Complementarity:

The idea in quantum theory that items can be separately analyzed as having several contradictory,

and apparently mutually exclusive, properties. For example, the wave-particle duality of light, where

light can either behave as a particle or as wave, but not simultaneously as both.

Copernican Principle:

The idea that there is nothing special about our position in the universe, a generalized version of

Nicolaus Copernicus’ recognition that the Earth is actually just a planet circling the Sun, and not vice

versa.

Cosmic Microwave Background Radiation:

Cosmic microwave background radiation (or CMB for short) is the “afterglow” of the Big Bang, a

microwave radiation which still uniformly permeates all of space at a temperature of around -270°C

(about 3° above absolute zero). It is considered to be the best evidence for the standard Big Bang

model of the universe.

Cosmic Inflation:

The idea that, in the first split-second after the Big Bang, the universe underwent a fantastically fast

(exponential) expansion driven by the vacuum of empty space. The theory was developed by Alan

Guth in the early 1980s to explain certain problems and inconsistencies with the basic Big Bang

theory, such as those related to the large-scale structure of the features of the universe, the “horizon

problem”, the “flatness problem” and the “magnetic monopole problem”.

Cosmic Rays:

High speed, energetic particles (about 90% of which are protons) originating from space that impinge

on Earth's atmosphere. Some are generated by our own Sun, some by supernovas, some by as yet

unknown events in the farthest reaches of the visible universe. The term "ray" is a misnomer, as

cosmic particles arrive individually, not in the form of a ray or beam of particles.

Cosmological Constant:

A term added by Albert Einstein as a modification to his original theory of general relativity, in order

to balance the attractive force of gravity and achieve a static or stationary universe. It represents thepossibility that there is a density and pressure associated with apparently empty space, and that the

overall mass-energy of the universe is actually much greater than currently estimated. Once




177dismissed as just a mathematical “fix”, it has been revived in recent years with the discovery of the

apparent acceleration of the expansion of the universe.

Cosmological Principle:

The starting point for the General Theory of Relativity and the Big Bang theory is that, that averaged

over large distances, one part of the universe looks approximately like any other part, and that,

viewed on sufficiently large distance scales, there are no preferred directions or preferred places in

the universe. Stated in more technical terms, on large spatial scales, the universe is homogeneous

and isotropic.

Critical Mass (Critical Density):

As applied to the universe as a whole, critical mass refers to the total required mass of matter in theuniverse which will allow the effects of gravity to overcome its continued outward expansion. If the

universe contains more than the critical mass of matter, its gravity will eventually reverse the

expansion, causing the universe to collapse back to what has become known as the Big Crunch. If,

however, it contains insufficient matter, it will go on expanding forever. In the same way, critical

density is that overall density of the matter in the universe which will just allow continued expansion.

In other contexts, critical mass is also used to refer to the amount of fissile material needed to

sustain nuclear fission.

D

Dark Energy:

An invisible, hypothetical form of energy with repulsive gravity that permeates all of space and that

may explain recent observations that the universe appears to be expanding at an accelerating rate. In

some models of cosmology, dark energy accounts for 74% of the total mass-energy of the universe.

Its exact nature remains a mystery, although Einstein’s hypothesized “cosmological constant” is now

considered a promising candidate.

Dark Matter:

Matter that gives out no light and does not interact with the electromagnetic force, but whose

presence can be inferred from gravitational effects on visible matter. It is estimated that there may

be between 6 and 7 times as much dark matter as normal, bright matter in the universe, although its

exact nature remains a mystery.

Decoherence:

The process by which bodies and quantum systems lose some of their more unusual quantum

properties (e.g. superposition, or the ability to appear in different places simultaneously) as they

interact with their environments. When a particle decoheres, its probability wave collapses, any

quantum superpositions disappear and it settles into its observed state under classical physics.

Density:




178The mass of an object divided by its volume, a measure of how much it is compacted or crowded

together (e.g. air is low in density, iron is high). Boyle’s Law dictates that a substance increases in

density as its pressure is increased or as its temperature is decreased.

Dimensions:

Independent directions in space-time. We are familiar with the three dimensions of space (length,width and height, or east-west, north-south and up-down) and one of time (past-future), but

superstring theory, for example, requires the universe to have ten dimensions.

DNA:

Deoxyribonucleic acid (DNA) molecules consist of two long intertwined polymers of nucleotides, with

backbones made of sugars and phosphate groups joined by ester bonds, structured as the familiar

double helix. DNA is responsible for the long-term storage of genetic information, and specifies the

sequence of the amino acids within proteins. It is organized into structures called chromosomes, and

contains the genetic instructions used in the development and functioning of all known livingorganisms and some viruses. The first accurate model of the structure of DNA was formulated by

James Watson and Francis Crick in 1953. The genetic information from DNA is transmitted into the

nucleus of cells by molecules of RNA, which controls certain chemical processes in the cell. Both DNA

and RNA are considered essential building blocks of life.

E

Electric Charge:

A property of microscopic particles, which may be either positive (e.g. protons) or negative (e.g.electrons). Particles with the same charge repel each other, and particles with opposite charges

attract each other. The field of force that surrounds an electric charge is called an electric field, and a

river of charged particles flowing through a conductor is called an electric current.

Electric Field:

The field of force that surrounds an electric charge (in the same way as a magnetic field is the field of

force that surrounds a magnet). Together, the electric and magnetic fields make up the

electromagnetic field which underlies light and other electromagnetic waves, and changes in either

field will induce changes in the other, as shown in the equations of James Clerk Maxwell.

Electromagnetic Force (or Electromagnetism):

The force that an electromagnetic field exerts on electrically charged particles. It is one of the four

fundamental forces of physics (along with the gravitational force and the strong and weak nuclear

forces), and the one responsible for most of the forces we experience in our daily lives. The

electromagnetic forces acting between the electrically charged protons and electrons inside atoms

and between atoms are essentially responsible for gluing together all ordinary matter.Although

hugely stronger (1042 times) than the force of gravity, it is a less dominant force on larger scales

because the attractive and repulsive interactions tend to cancel each other out. Like gravity, the

electromagnetic force is subject to an inverse-square law, and its strength is inversely proportional to

the square of the distance between the particles. The force is mediated or operated by the exchange

of photons between the particles. The ‘electrostatic force’ is one aspect of the electromagnetic force,

which arises when two charged particles are static (i.e. not in motion).




179Electromagnetic Radiation (or Electromagnetic Waves):

A wave that travels though space at the speed of light, consisting of an electrical field that

periodically grows and dies, alternating with a magnetic field that periodically dies and grows.

Electromagnetic waves carry energy and momentum, which may be imparted when it interacts with

matter.

In order of increasing frequency, the electromagnetic spectrum includes radio waves, microwaves,

terahertz radiation, infrared radiation, visible light, ultraviolet radiation, x-rays and gamma rays.

Electron:

A negatively-charged sub-atomic particle. It is an indivisible, elementary particle, and is usually to be

found orbiting the nucleus of an atom. Electrons in an atom (which exist in the same quantity as the

number of protons in the nucleus of the particular atom, so that the overall electric charge is zero)

are constrained to occupy certain discrete orbital positions or “shells” around the nucleus.

Interactions between the electrons of different atoms play an essential role in chemical bonding andphenomena such as electricity, magnetism and thermal conductivity. The discovery of electrons is

credited to the British physicist J. J. Thomson in 1897.

Element:

A substance that cannot be reduced any further by chemical means. It is a pure chemical substance

composed of atoms with the same atomic number (i.e. the same number of protons in its nucleus).

There are 92 naturally occurring elements on Earth, and all chemical matter consists of these

elements (although a further 25 have been discovered as products of artificial nuclear reactions).

Elements with atomic numbers 83 or higher are inherently unstable, and undergo radioactive decay.

The list of elements is usually shown in the form of a Periodic Table, in order of their atomic number

(see box at right, or click ther source link for a more detailed interactive Periodic Table).

Elementary Particle:

A particle with no substructure (i.e. not made up of smaller particles) and which is therefore one of

the basic building blocks of the universe from which all other particles are made. Quarks, electons,

neutrinos, photons, muons and gluons (along with their respective antiparticles) are all elementary

particles; protons and neutrons (which are made up of quarks) are not.

Energy:

Sometimes defined as the ability to do work or to cause change, energy is notoriously difficult to

define. In accordance with the Law of Conservation of Energy, energy can never be created or

destroyed but it can be changed into different forms, including kinetic, potential, thermal,

gravitational, sound, light, elastic and electromagnetic. The standard scientific unit of energy is the

Joule.

Entanglement:

The phenomenon in quantum theory whereby particles that interact with each other become

permanently dependent on each other’s quantum states and properties, to the extent that they losetheir individuality and in many ways behave as a single entity. At some level, entangled particles

appear to “know” each other’s states and properties.




180Entropy:

A measure of the disorder of a system and of its constituent molecules. More specifically, in

thermodynamics it is a measure of the unavailability of a system’s energy to do work. The Second

Law of Thermodynamics embodies the idea that entropy can never decrease, but rather will tend to

increase over time, approaching a maximum value as it reaches thermal equilibrium. A classic

example of increasing entropy is ice melting in water until both reach a common temperature.

Event Horizon:

A one-way boundary in space-time surrounding a black hole. Any matter or light that falls through

the event horizon of a black hole can never leave, and any event inside the event horizon cannot

affect an outside observer.

Exogenesis:

The hypothesis that life on Earth was transferred from elsewhere in the universe. A related but more

limited concept is that of panspermia, the idea that "seeds" of life exist already all over the universe,

and that life on Earth may have originated through these "seeds".

Exotic Particle:

A kind of theoretical particle said to exist by some theories of modern physics, whose alleged

properties are extremely unusual. Examples include tachyons (particles that always travels faster

than the speed of light), WIMPs (weakly interacting massive particles which do not interact with

electromagnetism or the strong nuclear force), axions (particles with no electric charge, very small

mass and very low interaction with the strong and weak forces) and neutrinos (particles that travel

close to the speed of light, lack an electric charge and are able to pass through ordinary matteralmost undisturbed).

Expanding Universe:

A universe which is constantly growing in size and in which the constituent parts (galaxies, clusters,

etc) are flying ever further away from each other. Although contrary to the static universe which had

been assumed throughout most of history, an expanding universe was confirmed by Edwin Hubble’s

1929 observations of the redshifts of distant Cepheid variable stars, and is consistent with most

solutions to Albert Einstein’s general relativity field equations. It also suggests that, in the distant

past, the universe was much smaller and ultimately had its beginning in a Big Bang type event.

F

Fundamental (or Elementary) Forces:

There are four basic forces of physics that are believed to underlie all phenomena in the universe.

Listed in order of strength they are: the strong nuclear force, the electromagnetic force, the weak

nuclear force and the gravitational force (or gravity). It is thought likely that, in extremely high energyconditions such as occurred near the beginning of the Big Bang, the four fundamental forces of

nature are actually unified in a single theoretical framework (known as the Grand Unified Theory).

According to quantum field theory, the forces between particles are mediated by other particles, and




181the fundamental forces can be described by the exchange of virtual force-carrying particles: the

strong nuclear force mediated by gluons; the electromagnetic force by photons; the weak nuclear

force by W and Z bosons; and gravity by hypothetical gravitons.

G

Galaxy:

One of the basic building block of the universe, a galaxy is a massive system of stars, stellar

remnants, gas, dust, and possibly a hypothetical substance known as dark matter, bound together by

gravity. Galaxies may be anywhere from 1 to 100,000 light years across and are typically separated by

millions of light years of intergalactic space. They are grouped into clusters, which in turn can form

larger groups called superclusters and sheets or filaments. There are many different kinds of galaxy

including spiral (like our own Milky Way galaxy), elliptical, ring, dwarf, lenticular and irregular. There

are estimated to be over a hundred billion galaxies in the observable universe.

Gamma Ray:

A form of electromagnetic radiation produced by some kinds of radioactive decay. Gamma rays have

the highest frequency and energy and the shortest wavelength in the electromagnetic spectrum, and

penetrate matter more easily that either alpha particles or beta particles.

Gamma Ray Burst:

A narrow beam of intense electromagnetic radiation released during a supernova event, as a rapidly

rotating, high-mass star collapses to form a black hole. They are the brightest events known to occur

in the universe, and can last from milliseconds to several minutes (typically a few seconds). The initialburst is usually followed by a longer-lived 'afterglow' emitted at longer wavelengths (X-ray,

ultraviolet, optical, infrared and radio).

Gas:

A state of matter consisting of a collection of particles (molecules, atoms, ions, electrons, etc)

without a definite shape or volume, and that are in more or less random motion. A gas tends to have

relatively low density and viscosity compared to the solid and liquid states of matter, expands and

contracts greatly with changes in temperature or pressure (“compressible”), and diffuses readily,

spreading and homogeneously distributing itself throughout any container.

General Theory of Relativity:

Sometimes known as the Theory of General Relativity, this was Albert Einstein’s refinement

(published in 1916) of his earlier Special Theory of Relativity and Sir Isaac Newton’s much earlier Law

of Universal Gravitation. The theory holds that acceleration and gravity are indistinguishable - the

Principle of Equivalence - and describes gravity as a property of the geometry (more specifically a

warpage) of space-time. Among other things, the theory predicts the existence of black holes, an

expanding universe, time dilation, length contraction, gravitational light bending and the curvature of

space-time. Although classical physics can be considered a good approximation for everyday

purposes, the predictions of general relativity differ significantly from those of classical physics. They

have become generally accepted in modern physics, however, and have been confirmed by all

observations and experiments to date.




183Formulated by Edwin Hubble in 1929, the law states that the redshift in light coming from distant

galaxies is proportional to their distance, so that every galaxy appears to be rushing away from us (or

from any other point in the universe) with a speed that is directly proportionate to its distance from

us. It is considered the first observational basis for an expanding universe (or the metric expansion of

space), and the most often cited evidence in support of the Big Bang theory, and arguably one of the

most important cosmological discoveries ever made.

Hydrostatic Equilibrium:

The state in which the force of gravitation working to crush a star is exactly balanced by the thermal

pressure of its hot gas pushing outwards. It is the reason that stars in general do not implode or

explode, and it also explains why the Earth's atmosphere does not collapse to a very thin layer on the

ground.

I

Inertia:

The natural tendency (as defined in Sir Isaac Newton’s First Law of Motion of 1687) of objects to

resist changes in their state of motion. Therefore, a body at rest tends to stay at rest and, once set in

motion, a body tends to stay moving at a constant speed in a straight line (or along a geodesic in

curved space) unless acted on by an outside force. An example of an inertial force is centrifugal force,

which in reality is just due to a body trying to continue in a straight line while constrained to move

along a curved path.

Inertial Frame (or Inertial System):

A reference frame in which the observers are not subject to any accelerating force. An inertial frame

is a frame of reference in which a body remains at rest or moves with constant linear velocity unless

acted upon by outside forces (as stipulated by Sir Isaac Newton’s First Law of Motion, Force = Mass ×

Acceleration). Any frame of reference that moves with constant velocity relative to an inertial system

is itself an inertial system.

Interference:

The ability of two waves passing through each other to mingle, reinforcing each other where crests

coincide and cancelling each other out where crests and troughs coincide, similar to the way ripplesin water interfere with each other. This results, for example, in an interference pattern of light and

dark stripes on a screen illuminated by light from two sources.

Ion:

An atom or molecule that has been stripped of one or more of its orbiting electrons, thus giving it a

net positive electric charge. Technically, an atom which gains an electron (thus giving it a net

negative electric charge) is also a type of ion, known as an anion.

Isotope:

A possible form of an element, distinguishable from other isotopes of the same element by its

differing mass, which is caused by a different number of neutrons in the nucleus (the number of




184protons, which gives the atomic number of the element, must be the same). Around 75% of isotopes

are stable, while some are unstable or radioactive, and will decay over time into other elements.

L

Law of Conservation of Energy:

Also known as the First Law of Thermodynamics, this is the principle that energy can never be

created or destroyed, only converted from one form to another (e.g. the chemical energy of gasoline

can be converted into the energy of motion of a car). The total amount of energy in an isolated

system (or in the universe as a whole) therefore remains constant.

Law of Universal Gravitation:

Published by Sir Isaac Newton in 1687, and sometimes also known as the Universal Law of Gravity,

this was the first formulation of the idea that all bodies with mass pull on each other across space.

Newton observed that the force of gravity between two objects is proportional to the product of the

two masses, and inversely proportional to the square of the distance between them. Although the

theory has since been superseded by Albert Einstein's General Theory of Relativity, it predicts the

movements of the Sun, the Moon and the planets to a high degree of accuracy and it continues to be

used as an excellent approximation of the effects of gravity for everyday applications (relativity is

only required when there is a need for extreme precision, or when dealing with the gravitation of

very massive objects).

Length Contraction:

The phenomenon, predicted by Albert Einstein’s Special and General Theories of Relativity, whereby,from the relative context of one observer's frame of reference, space or length appears to decrease

as the relative velocities increase.

Life:

A difficult and contentious phenomenon to define, life is usually considered to be a characteristic of

organisms that exhibit certain biological processes (such as chemical reactions or other events that

results in a transformation), and that are capable of growth through metabolism and are capable of

reproduction. The ability to ingest food and excrete waste are also sometimes considered

requirements of life (e.g. bacteria are usually considered to be alive, whereas simpler viruses, whichdo not feed or excrete, are not). The two distinguishing features of living systems are sometimes

considered to be complexity and organization (negative entropy). Some organisms can communicate,

and many can adapt to their environment through internally generated changes, although these are

not universally considered prerequisites for life.

Light:

Technically, this refers to electromagnetic radiation of a wavelength that is visible to the human eye,

although in the broader field of physics, it is sometimes used to refer to electromagnetic radiation of

all wavelengths, whether visible or not. It exhibits “wave-particle duality” in that it can behave as

both waves and particles (photons). Light travels at a constant speed of about 300,000 kilometres per

second in a vacuum.

Light Year:




185A convenient unit for measuring the large distances in the universe. It is the distance that light

travels in one year which, given that light travels at 300,000 kilometres per second, works out to

about 9,460,000,000 kilometres (9.46 trillion kilometres).

M

Magnetic Field:

The field of force that surrounds a magnet (in the same way as an electric field is the field of force

that surrounds an electric charge). Together, the magnetic and electric fields make up the

electromagnetic field which underlies light and other electromagnetic waves, and changes in either

field will induce changes in the other, as indicated by James Clerk Maxwell’s Equations of

Electromagnetism.

Magnetic Monopole:

A hypothetical particle that is a magnet with only one pole, and which therefore has a net magnetic

charge. Although the existence of monopoles is indicated by both classical theory and quantum

theory (and predicted by recent string theories and grand unified theories), there is still no

observational evidence for their physical existence.

Mass:

A measure of the amount of matter in a body. It can also be seen as a measure of a body’s inertia or

resistence to change in motion, or the degree of acceleration a body acquires when subject to a force

(bodies with greater mass are accelerated less by the same force and have greater inertia). Mass is

often confused with weight, which is the strength of the gravitational pull on the object (andtherefore how heavy it is in a particular gravitational situation), although, in everyday situations, the

weight of an object is proportional to its mass.

Mass-Energy Equivalence:

The concept that any mass has an associated energy, and that, conversly, any energy has an

associated mass. In Einstein’s Special Theory of Relativity, this relationship is expressed in the famous

mass-energy equivalence formula, E = mc2, where E = total energy, m = mass and c = the speed of

light in a vacuum. Given that c is a very large number, it becomes apparent that mass is in fact a very

concentrated form of energy.

Matter:

Anything that has both mass and volume (i.e. takes up space). Matter is what atoms and molecules

are made of, and it exists in four states or phases: solid, liquid, gas and plasma (although other

phases, such as Bose-Einstein condensates, also exist).

Molecule:

A collection of atoms glued together by electromagnetic forces. A more formal definition might be: a

sufficiently stable electrically neutral group of at least two atoms, in a definite arrangement, heldtogether by very strong chemical bonds. A molecule may consist of atoms of the same chemical

element (e.g. oxygen: O2) or of different elements (e.g. water: H2O). Organic molecules are those

which include carbon, and the others are called inorganic.




186Momentum:

A measure of how much effort is required to stop a body, defined as the body’s mass multiplied by

its velocity. Thus, a large heavy body (e.g. a train) going relatively slowly may have more momentum

than a smaller body going very fast (e.g. a racing car). The Law of Conservation of Momentum rules

that the total momentum of an isolated system (one in which no net external force acts on the

system) does not change.

Multiverse (Parallel Universes):

A hypothetical set of multiple possible universes (including our own) which exist in parallel with each

other. Our universe would then be just one of an enormous number of separate and distinct parallel

universes, the vast majority of which would be dead and uninteresting, not having a set of physical

laws which would allow the emergence of stars, planets and life.

N

Neutrino:

A sub-atomic elementary particle with no electrical charge and very small mass that travels very

close to the speed of light. They are created as a result of certain types of radioactive decay or

nuclear reaction, such as the decay of a free neutron (i.e. one outside of a nucleus) into a proton and

electron. Being electrically neutral and unaffected by the strong nuclear force or the electromagnetic

force, neutrinos are able to pass through ordinary matter almost undisturbed and are therefore

extremely difficult to detect, although when created in huge numbers they are capable of blowing a

star apart in a supernova.

Neutron:

One of the two main building blocks (along with the proton) of the nucleus at the centre of an atom.

Neutrons have essentially the same mass as a proton (very slightly larger) but no electric charge, and

are made up of one “up” quark and two “down” quarks. The number of neutrons in an atom

determines the isotope of an element. Outside of a nucleus, they are unstable and disintegrate

within about ten minutes.

Neutron Star:

A star that has shrunk under its own gravity during a supernova event, so that most of its material

has been compressed into neutrons only (the protons and electrons have been crushed together

until they merge, leaving only neutrons). Neutron stars are very hot, quite small (typically 20 to 30

kilometres in diameter), extremely dense, have a very high surface gravity and rotate very fast. A

pulsar is a kind of highly-magnetized rapidly-rotating neutron star.

Newton’s Laws of Motion:

The three physical laws, published by Sir Isaac Newton in 1687, that form the basis for classical

mechanics: 1) a body persists its state of rest or of uniform motion unless acted upon by an external

unbalanced force; 2) force equals mass times acceleration; and 3) to every action there is an equaland opposite reaction.

Nonlocality:




187The rather spooky ability of objects in quantum theory to apparently instantaneously know about

each other’s quantum state, even when separated by large distances, in apparent contravention of

the principle of locality (the idea that distant objects cannot have direct influence on one another,

and that an object is influenced directly only by its immediate surroundings).

Nuclear Fission:

A nuclear reaction in which the nucleus of an atom splits into smaller parts, often producing free

neutrons, lighter nuclei and photons (in the form of gamma rays). The process releases large

amounts of energy, both as electromagnetic radiation and as kinetic energy of the resulting

fragments.

Nuclear Fusion:

The welding together of two light nuclei to make a heavier nucleus, resulting in the liberation of

nuclear energy. An example of this kind of nuclear reaction is the binding together of hydrogen nuclei

in the core of the Sun to make helium. In larger, hotter stars, helium itself may fuse to produceheavier elements, a process which continues up the periodic table of elements as far as iron. The

fusion of ultra-stable iron nuclei actually absorbs energy rather than releasing it, and so iron does not

easily fuse to create heavier elements.

Nucleosynthesis:

The process of creating new atomic nuclei from pre-existing protons and neutrons by a process of

nuclear fusion. The primordial nucleons (hydrogen and helium) themselves were formed from the

quark-gluon plasma in the first few minutes after the Big Bang, as it cooled to below ten million

degrees, but nucleosynthesis of the heavier elements (including all carbon, oxygen, etc) occurs

primarily in the nuclear fusion process within stars and supernovas.

Nucleus:

The tight cluster of nucleons (positively-charged protons and zero-charged neutrons, or just a single

proton in the case of hydrogen) at the centre of an atom, containing more than 99.9% of the atom’s

mass. The nucleus of a typical atom is about 100,000 smaller than the total size of the atom

(depending on the individual atom).

O

Oscillating Universe:

A cosmological model, in which the universe undergoes a potentially endless series of oscillations,

each beginning with a Big Bang and ending with a Big Crunch. After the Big Bang, the universe

expands for a while before the gravitational attraction of matter causes it to collapse back and

undergo a “bounce”.

P

Panspermia:

The hypothesis that "seeds" of life exist already all over the universe, and that life on Earth may have

originated through these "seeds", driven by a steady influx of cells or viruses arriving from space via




188comets. It is a more limited form of the related hypothesis of exogenesis, which also proposes that

life on Earth was transferred from elsewhere in the universe, but makes no prediction about how

widespread it may be.

Pauli Exclusion Principle:

The prohibition on two identical fermions from sharing the same quantum state simultaneously.Among other implications it stops electrons (which are a kind of fermion) from piling on top of each

other, thereby explaining the existence of different types of atoms and the whole variety of the

universe around us.

Photoelectric Effect:

The phenomenon in which, when a metallic surface is exposed to electromagnetic radiation above a

certain threshold frequency (typically visible light and x-rays), the light is absorbed and electrons are

emitted. The discovery of the effect is usually attributed to Heinrich Hertz in 1887, and study of it

(particularly by Albert Einstein) led to important steps in understanding the quantum nature of lightand electrons and in formulating the concept of wave-particle duality.

Photon:

A particle (or quantum) of light or other electromagnetic radiation, which has no intrinsic mass and

can therefore travel at the speed of light. It is an elementary particle and the basic unit of light, and

effectively carries the effects of the electromagnetic force. The modern concept of the photon as

exhibiting both wave and particle properties was developed gradually by Albert Einstein and others.

Planck Constant:

The proportionality constant (h) which provides the relation between the energy (E) of a photon and

the frequency (v) of its associated electromagnetic wave in the so-called Planck Relation E = hv. It is

essentially used to describe the sizes of individual quanta in quantum mechanics. Its value depends

on the units used for energy and frequency, but it is a very small number (with energy measured in

Joules, it is of the order of 6.626 × 10-34 J·s).

Planck Energy:

The super-high energy (approximately 1.22 × 1019 GeV) at which gravity becomes comparable in strength

to the other fundamental forces, and at which the quantum effects of gravity become important.

Planck Length:

The fantastically tiny length scale (approximately 1.6 × 10 -35 metres) at which gravity becomes

comparable in strength to the other fundamental forces. It is the scale at which classical ideas about

gravity and space-time cease to be valid, and quantum effects dominate.

Planck Temperature:

The temperature of the universe at 1 Planck Time after the Big Bang, approximately equal to 1.4 ×

1032°C.

Planck Time:




189The time it would take a photon travelling at the speed of light to cross a distance equal to the Planck

Length. This is the “quantum of time”, the smallest measurement of time that has any meaning, and

is approximately equal to 10-43 seconds.

Planck Units:

“Natural units” of measurement (i.e. designed so that certain fundamental physical constants arenormalized to 1), named after the German physicist Max Planck who first proposed them in 1899.

They were an attempt to eliminate all arbitrariness from the system of units, and to help simplify

many complex equations in modern physics. Among the most important are the Planck Energy, the

Planck Length, the Planck Time and the Planck Temperature.

Plasma:

A partially ionized gas of ions and electrons, in which a certain proportion of the electrons are free

rather than being bound to an atom or molecule. It has properties quite unlike those of solids, liquids

or gases and is sometimes considered to be a distinct fourth state of matter. An example of plasmapresent at the Earth's surface is lightning.

Positron:

The antiparticle or antimatter counterpart of the electron. The positron, then, is an elementary

particle with a positive electric charge, and the same mass and spin as an electron. The existence of

positrons was first postulated in 1928 by Paul Dirac, and definitively discovered by Carl Anderson in

1932.

Primeval (or Primordial) Soup:

The theory of the origin of life on Earth first put forward by Alexander Oparin, whereby a “soup” of

organic molecules could be created in a “reducing” oxygen-less atmosphere through the action of

sunlight, creating the necessary building blocks for the evolution of life.

Principle of Equivalence:

The idea that no experiment can distinguish the acceleration due to gravity from the inertial

acceleration due to a change of velocity (or acceleration).

Principle of Relativity:

The idea, first expressed by Galileo Galilei in 1632 and also known as the principle of invariance, that

the fundamental laws of physics are the same in all inertial frames and that, purely by observing the

outcome of mechanical experiments, one cannot distinguish a state of rest from a state of constant

velocity. Thus, all uniform motion is relative, and there is no absolute and well-defined state of rest.

Probability Wave (or Wave Function):

A description of the probability that a particle in a particular state will be measured to have a given

position and momentum. Thus, a particle (an electron, photon or any other kind of particle), when

not being measured or located, takes the form of a field or wave of probable locations, some being

more probable or likely than others.

Prokaryotes and Eukaryotes:




190Prokaryotes are primitive organisms that lack a cell nucleus or any other membrane-bound

organelles. Most prokaryotes are single-celled (although some have multicellular stages in their life-

cycles), and they are divided into two main domains, bacteria and archaea.

Eukaryotes, on the other hand, are organisms whose cells contain a nucleus and are organized into

complex structures enclosed within membranes. Most living organisms (including all animals, plants,

fungi and protists) are eukaryotes.

Proton:

One of the two main building blocks (along with the neutron) of the nucleus at the centre of an

atom. Protons carry a positive electrical charge, equal and opposite to that of electrons, and are

made up of two “up” quarks and one “down” quark. The number of protons in an atom’s nucleus

determines its atomic number and thus which chemical element it represents.

Pulsar:

A highly-magnetized rapidly-rotating neutron star that sweeps regular pulses of intense

electromagnetic radiation (radio waves) around space like a lighthouse. The intervals between pulses

are very regular, ranging from 1.4 milliseconds to 8.5 seconds depending on the rotation period of

the star. A pulsar generally has a mass similar to our own Sun, but a diameter of only around 10

kilometres.

Q

Quantum:

The smallest chunk into which something can be divided in physics. Quantized phenomena are

restricted to discrete values rather than to a continuous set of values. Some quanta take the form of

elementary particles, such as photons which are the quanta of the electromagnetic field. Quanta are

measured on the tiny Planck scale of the order of around 10 -35 metres.

Quantum Electrodynamics:

Sometimes shortened to QED, it is essentially the theory of how light interacts with matter. More

specifically, it deals with the interactions between electrons, positrons (antielectrons) and photons. It

explains almost everything about the everyday world, from why the ground is solid to how a laser

works to the chemistry of metabolism to the operation of computers.

Quantum Gravity (or Quantum Theory of Gravity):

A so-called “theory of everything” which combines the General Theory of Relativity (the theory of

the very large, which describes one of the fundamental forces of nature, gravity) with quantum the-

ory (the theory of the very small, which describes the other three fundamental forces, electromag-

most promising candidates, like superstring theory and loop quantum gravity, still need to overcome

major formal and conceptual problems, and this is still very much a work in progress.

Quantum State:




191The set of characteristics describing the condition a quantum mechanical system is in. It can be

described by a wave function or a complete set of quantum numbers (energy, angular momentum,

spin, etc), although, when observed, the system is forced into a specific stationary "eigenstate". If a

particle within a quantum system (such as an electron within an atom) moves from one quantum

state to another, it does so instantaneously and in discontinuous steps (known as quantum leaps or

jumps) without ever being in a state in between.

Quantum Theory (or Quantum Physics or Quantum Mechanics):

The physical theory of objects isolated from their surroundings. Because it is very difficult to isolate

large objects, quantum theory (also known as quantum mechanics or quantum physics) is essentially

a theory of the microscopic world of atoms and their constituents. Among its main principles are the

dual wave-like and particle-like behaviour of matter and radiation (wave-particle duality), and the

prediction of probabilities in situations where classical physics predicts certainties. Classical physics

provides a good approximation to quantum physics for everyday purposes, typically in circumstances

with large numbers of particles.

Quantum Tunnelling:

The quantum mechanical effect in which particles have a finite probability of crossing an energy

barrier, or transitioning through an energy state normally forbidden to them by classical physics, due

to the wave-like aspect of particles. The probability wave of a particle represents the probability of

finding the particle in a certain location, and there is a finite probability that the particle is located on

the other side of the barrier.

Quark:

A type of elementary particle which is the major constituent of matter. Quarks are never found on

their own, only in groups of three within composite particles called hadrons (such as protons and

neutrons). There are six different types (or “flavours”) of quarks - up, down, top, bottom, charm and

strange - and each flavour comes in three “colours” - red, green or blue (although they have no

colour in the normal sense, being much smaller than the wavelength of visible light). Quarks are the

only particles in the standard model of particle physics to experience all four fundamental forces, and

they have the properties of electric charge, colour charge, spin and mass.

Quasar:

Short for QUAsi-StellAr Radio source, a quasar is an extremely powerful and distant active galacticnucleus (a compact region at the centre of a galaxy which has a much higher than normal

luminosity). It derives most of its energy from very hot matter swirling into a central supermassive

black hole, and can generate as much light as a hundred normal galaxies from a much smaller

volume. It is one of the most powerful objects in the universe, and among the most distant things

ever seen in space.

R

Radioactivity (Radioactive Decay):

The disintegration of unstable heavy atomic nuclei into lighter, more stable, atomic nuclei,

accompanied in the process by the emission of ionizing radiation (alpha particles, beta particles or




192gamma rays). This is a random process at the atomic level but, given a large number of similar atoms,

the decay rate on average is predictable, and is usually measured by the half-life of the substance.

Redshift:

The shifting of emitted electromagnetic radiation (such as visible light) towards the less energetic

red end of the electromagnetic spectrum when a light source is moving away from the observer. Thisoccurs as the wavelengths of light stretch as an object moves away (as opposed to being squashed by

an approaching object), similar to the familiar Doppler effect on sound waves. Among other things, it

can be used as a measure of the speed with which galaxies throughout the universe are moving away

from us.

Relativity:

The theory, formulated essentially by Einstein’s theory has two main parts: the Special Theory of

Relativity (or special relativity) which deals with objects in uniform motion, and the General Theory

of Relativity (or general relativity) which deals with acclerating objects and gravity.

RNA and DNA:

Ribonucleic acid (RNA) is a type of single-stranded molecule that consists of a long chain of

nucleotide units, each of which consists of a nitrogenous base, a ribose sugar and a phosphate. RNA

transmits the genetic information from DNA into the nucleus of cells, and controls certain chemical

processes in the cell. Both DNA and RNA are considered essential building blocks of life.

S

Second Law of Thermodynamics:

The idea that entropy (the microscopic disorder of a body) can never decrease, but rather will tend

to increase over time. In practice, this results in an inexorable tendency towards uniformity and away

from patterns and structures, and means, for example, that heat always flows from a hot body to a

cold one, and that differences in temperature, pressure and density tend to even out in an isolated

physical system (or in the universe as a whole).

Simultaneity:

The idea, disproved by Einstein in his Special Theory of Relativity, that events that appear to happenat the same time for one person should appear to happen at the same time for everyone in the

universe.

Singularity (or Gravitational Singularity):

A region of space where the density of matter, or the curvature of space-time, becomes infinite and

the concepts of space and time cease to have any meaning. At this point, the whole fabric of space-

time ruptures and the precepts of Einstein’s General Theory of Relativity (and physics in general)

break down and no longer apply. According to general relativity, the Big Bang started with a

singularity, and there is a singularity at the centre of a black hole.

Space-Time:




193Space-time (or spacetime or the spacetime continuum) is any mathematical model that combines

space and time into a single construct. The fourth dimension of time is traditionally considered to be

of a different sort than the three dimensions of space in that it can only go forwards and not back

but, in Albert Einstein’s General Theory of Relativity, space and time are seen to be essentially the

same thing and can therefore be treated as a single entity.

Special Theory of Relativity:

Albert Einstein’s first major theory, dating from 1905, special relativity builds on Galileo's more

simplistic principle of relativity and relates what one person sees when looking at another person

moving at constant speed relative to them. “Special” indicates that the theory restricts itself to

observers in uniform or constant relative motion, a restriction Einstein addressed later in his General

Theory of Relativity. The theory incorporates the principle that the speed of light is the same for all

inertial observers, regardless of the state of motion of the source. Among other things, it reveals that

the moving person appears to shrink in the direction of their motion (length contraction)and their

time slows down (time dilation), effects which are ever more marked as speeds approach the speed

of light. The theory also leads to some famous paradoxes like the so-called Time Travel Paradox and

the Twin Paradox.

Speed of Light:

In a vacuum, light travels at a speed of exactly 299,792,458 metres per second, or about 300,000

kilometres per second, a speed which remains constant irrespective of the speed of the source of the

light or of the observer (one of the cornerstones of Albert Einstein’s Special Theory of Relativity). It is

the term c in Einstein’s famous equation E = mc2.

Spin:

A fundamental property of sub-atomic elementary particles that means that behave as though they

are spinning or rotating (although in reality they are not spinning at all). The concept has no direct

analogue in the everyday world. Particles of spin ½ (e.g. electrons, positrons, neutrinos and quarks)

make up all the matter in the universe, while particles with integer spin (0, 1 or 2) give rise to, or

mediate, the forces operating between the matter particles (e.g. photons, gluons, W and Z bosons).

Star:

A massive, luminous ball of gas or plasma, held together by its own gravity, that replenishes the heat

it loses to space by means of nuclear energy generated in its core. Almost all of the elements heavier

than hydrogen and helium were created by the nuclear fusion processes in stars. There are many dif-

ferent types of stars including binary stars, proto-stars, dwarf stars (like our nearest star which we call

the Sun), supergiants, supernovas, neutron stars, pulsars, quasars, etc. There are a roughly estimated

10,000 billion billion stars (1022) in the observable universe.

Steady State Universe:

A cosmological model developed by Fred Hoyle, Thomas Gold and Hermann Bondi in 1948 as the

main alternative to the standard Big Bang theory of the universe. Steady state theory holds that the

universe is expanding but that new matter and new galaxies are continuously created in order tomaintain the perfect cosmological principle (the idea that, on the large scale, the universe is essen-

tially homogenous and isotropic in both space and time), and therefore has no beginning and




194no end. The theory was quite popular in the 1950s and 1960s, but fell out of favour with the

discovery of distant quasars and cosmic background radiation in the 1960s.

String:

An object with a one-dimensional spatial extent, length (unlike an elementary particle which is zero-

dimensional, or point-like). According to string theory, the different fundamental particles of thestandard model can be considered to be just different manifestations of one basic object, a string,

with different vibrational modes. The characteristic length scale of strings is thought to be on the

order of the Planck Length (about 10-35 metres, still too small to be visible in current physical

laboratories), the scale at which the effects of quantum gravity are believed to become significant.

Cosmic string is a similar but separate concept which refers to one-dimensional topological defects,

extremely thin but immensely dense, which are hypothesized to have formed as a result of phase

changes soon after the Big Bang (analogous to the imperfections that form between crystal grains in

solidifying liquids or the cracks that form when water freezes into ice). According to some theories,

such cosmic strings grew as the universe expanded and were instrumental in the accretion of matterand the formation of galaxy clusters and large-scale structures in the universe.

String Theory (Superstring Theory):

A theory which postulates that the fundamental ingredients of the universe are tiny strings of matter

(on the tiny scale of the Planck Length of around 10-35 metres) which vibrate in a space-time of ten

dimensions. It is considered one of the most promising of the quantum gravity theories which hope

to unite or unify quantum theory and the General Theory of Relativity, and apply to both large-scale

structures and structures on the atomic scale.

Superstring theory (short for supersymmetric string theory) is a refinement of the more general

theory of strings.

Strong Nuclear Force:

Also known as the strong interaction, this is the powerful but short-range force that holds protons

and neutrons together in the nucleus of an atom despite the electromagnetic repulsion of same-

charge particles, as well as holding together the constituent quarks which comprise neutrons and

protons. It is one of the four fundamental forces of physics (along with the gravitational force, the

electromagnetic force and weak nuclear force), and the most powerful, being 100 times the strength

of the electromagnetic force, about 1013 times as great as that of the weak force and about 1038 times

that of gravity.

The force is mediated by elementary particles called gluons which shuttle back and forth between the

particles being operated on and "glue" the particles together. Unlike the other forces, the strength of

the strong force between quarks becomes stronger with distance, acting like an unbreakable elastic

thread. However, it only operates over a very small distance (less than the size of the nucleus), out-

side of which it fades away abruptly.

Supernova:

A cataclysmic explosion caused by the collapse of an old massive star which has used up all its fuel.

For a short time, such an explosion may outshine an entire galaxy of a hundred billion ordinary stars.

It leaves behind a cloud of brightly coloured gas called a nebula, and sometimes a highly compressed

neutron star or even a black hole.




195Superposition:

The ability in quantum theory of an object, such as an atom or sub-atomic particle, to be in more

than one quantum state at the same time. For example, an object could technically be in more than

one place simultaneously as a consequence of the wave-like character of microscopic particles.

T

Time Dilation:

The phenomenon, predicted by Albert Einstein’s Special and General Theories of Relativity, whereby,

from the relative context of one observer's frame of reference, another’s time (for example, an

identical clock) appear to run slower. Thus, moving clocks run more slowly compared to stationary

clocks and, the closer the speed of movement approaches to the speed of light, the greater the

effect. Gravitational time dilation is a related phenomenon, whereby time passes more slowly the

higher the local distortion of space-time due to gravity (such as near a black hole, for example).

U

Uncertainty Principle:

The principle in quantum theory, formulated by Werner Heisenberg in 1926, which holds that the

values of certain pairs of variables cannot BOTH be known exactly, so that the more precisely one

variable is known, the less precisely the other can be known. For example, if the speed or

momentum of a particle is known exactly, then its location must remain uncertain; if its location is

known with certainty, then the particle’s speed or momentum cannot be known. Formulatedanother way, relating the unvertainties of energy and time, the uncertainty principle permits the

existence of ultra-short-lived microscopic particles (virtual particles) in apparently empty space,

which briefly blink into existence and blink out again.

Universe:

Everything that physically exists, including the entirety of space and time, all forms of matter, energy

and momentum, and the physical laws and constants that govern them. The universe (or cosmos) is

usually considered to have begun about 13.7 billion years ago in a gravitational singulary commonly

known as the Big Bang, and has been expanding ever since. Some have speculated that this universeis just one of many disconnected universes, which are collectively denoted as the multiverse.

W

Wave-Particle Duality:

The idea that light (and indeed all matter and energy) is both a wave and a particle, and that

sometimes it behaves like a wave and sometimes it behaves like a particle. It is a central concept of

quantum theory.

Weak Nuclear Force:

Also known as the weak interaction, it is one of the forces experienced by protons and neutrons in

the nucleus of an atom, the other being the strong nuclear force. It is one of the four fundamental




196forces of physics (along with the gravitational force, the electromagnetic force and the strong nuclear

force). It is called the weak force because it is about 1013 times weaker than the strong nuclear force

and 1011 times weaker than the electromagnetic force, and it is also very short range in its effect.

The weak interaction is mediated by the exchange of heavy elementary particles known as W and Z

bosons. It is responsible for radioactive beta decay (as it converts neutrons into protons) and for the

production of neutrinos.

White Hole:

The theoretical time reversal of a black hole, which arises as a valid solution in general relativity.

While a black hole acts as a vacuum, drawing in any matter that crosses its event horizon, a white

hole acts as a source that ejects matter from its event horizon.

Wormhole:

A hypothetical “tunnel” through space-time that connects widely distant regions, thus providing a

kind of short-cut through space-time. Although there is no observational evidence for wormholes,

they are known to be valid solutions under the General Theory of Relativity.

the physics of the universe-ii - siddhant singh

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