Lecture 33:

How Special Are We?

Astronomy 1143 – Spring 2014Astronomy 1143 – Spring 2014

Key IdeasWhy do laws of physics have the properties they do?

• Gravity as a inverse square law• Values for the physical constants

How many independent laws of physics are there?

String theory• Candidate for theory of everything• May reduce the number of independent variables

Anthropic Principle -- if critical laws of physics different, we wouldn’t be here to observe them, so we have a biased view.

Possible explanations of how we ended up with a Universe where life could exist

Things that make Physicists Unhappy

Needing to appeal to luck to explain observations

Coincidences in fundamental properties

Observable infinities

Non-relativistic theories

Disagreement between theory and observations

The Necessities of Life

Stable source of energy• Provide liquid water• Needs to exist for a reasonable period of time• Sun – nuclear fusion of light to heavy elements• Earth has stable orbit around Sun

Complex molecules• Elements like carbon good for multiple bonds

Long-lived Universe

Low-entropy Universe

Review of Gravity

Gravity is a long-range attractive force

It is an inverse-square law

G is the Gravitational constant – tells us the strength of gravity = 6.67x10-11 N m2/kg2

Orbits in an Inverse Square Law

The force of gravity in our Universe allows a variety of orbits

Including elliptical and circular orbits

Distance between Earth and Sun relatively constant for a long period of time.

Orbits in an Inverse Cubic Law

Summary about Gravity

The answer to the question:

Why is gravity an inverse-square law?

May be:

Because if it wasn’t, we would not be here to observe it

However, there may be a deeper reason for the behavior of gravity from work on theories of everything.

Physical constants, e.g.

G = Gravitational constant

h = Planck’s constant

c = speed of light

k = Boltzmann’s constant

mp = mass of proton

me = mass of electron

e = charge on electron

Ratio of gravity to strong to weak to electromagnetic in the present Universe

G can’t be too big

At the temperatures present in the Sun, a hydrogen nucleus takes on average billions of years

If G were larger, stars would be squeezed more tightly and central temperatures would be higher

Sun would fuse hydrogen really fast

No Sun for us!

G can’t be too small

If the gravitational force is too weak, then hydrostatic equilibrium will happen for low thermal pressure

Low thermal pressure – low speeds – weak or no fusion.

No liquid water, little energy from the Sun

In addition, if gravity is too weak, we can’t form galaxies, stars, planets, etc.

The Periodic Table

Balance of Forces in a Nucleus

Strong force vs. electromagnetic

Strong force:

• If too weak compared to EM force, then protons could not come together to form atoms. Entire Universe is hydrogen.

• If too strong compared to EM force, then fusion would be easier and elements could fuse at lower temperatures. Elements lighter than iron rare.

Electromagnetic Force

Electromagnetic force has to be just right on its own too!

Too weak: no atoms because electrons not attracted to protons strongly enough

Too strong: Electrons will not bond with other atoms to form molecules

Does the Universe have that many options?

Many lists of physical constants

However, many of them are not independent• Lots of quantities are unified in our current

understanding of physics• Changing a single property about the Universe

(such as the strength of the electromagnetic force) is probably not fair

• With the unification of forces under the theories of quantum chromodynamics and supersymmetry, a lot fewer parameters can be anything they want!

Example: Electromagnetic Force

The 19th century saw the unification of the electric and magnetic forces

• Passing an electric current through a wire created a magnetic field

• Moving a metal wire through a magnetic field created a current

Therefore, you can’t adjust the equations for electric fields w/o affecting the equations for magnetic fields

Gravity – had to be Inverse Square?

For example: the latest models suggest that gravity is carried by virtual particles called gravitons

Strength of force is related to how many gravitons an object interacts with

Gravitons being emitted by an object spread out and change in number per m2 by an inverse square law

So maybe we just have to explain the properties of the graviton?

Forces in Particle Physics

Gravity getting weaker with distance

How many important constants/laws are there?Does inflation always happen? Then not

surprising that our Universe is flat

The important part of particle physics boils down to

• The masses of the electron and proton

• the current strengths of the electromagnetic and strong interactions.

But why should their determined values be hospitable to life?

Fewer Options or No Options?

Physicists are still searching for the “Theory of Everything” that will unify gravity to the other three forces

How far can the unification go?

How many properties can have arbitrary values in a particular Universe?

Will it turn out that everything in the Universe is determined and that there is only one way for physics to work in our Universe and in other Universes?

Not all of these properties are independent

Describing the Physical World

Our concept of physical objects and physical laws has changed over time

Light – wave, then particle too!

Elementary Particles• indivisible points, could not change• wait! can be treated as fields

Gravity• Force transmitted instantaneously somehow• Curvature of spacetime• Carried by the graviton?

String Theory & Particles

String Theory involves a new way for looking at particles

Physicists are very familiar with strings. Pythagoras thought about this!

Nice mathematical description of their frequencies and their motions

The search for a way to combine the strong with the weak and electromagnetic force that worked with relativity led theoreticians to write down equations that looked like the equations for strings

Website Visit

So where does it get us, actually?

Thinking of “particles” as “frequencies of a vibrating string” has consequences

One of those frequencies has all the properties of the graviton, for example

String theory interactions take place over finite distances, which is great from blending gravity and quantum mechanics

What are the observational consequences of string theory?

Scientific Method

Relies on observations to test theories

If we can’t observe, we can’t apply the scientific method• Many ways of testing• Many important questions do not have scientific

answers (currently, and perhaps never)• Quantum gravity may help us answer Big Bang

questions

Philosophy and other areas of thought are important human achievements as well!

Testing String Theory

String theory now incorporates supersymmetries• Each boson has a symmetric fermion particle• Each fermion has a symmetric boson particle• Detected in future experiments?

The mass of the Higgs boson is predicted in superstring theories (more than one version exist)

Particles have fields – could one of them be responsible for inflation.

A supersymmetric particle may be dark matter

More particles

Dark Matter Candidates

Detection of the dark matter particles (other than by gravity) would also help investigate supersymmetric theories

Unique Universe

There is only one possible way for a Universe to be

Constants and laws are not arbitrary. They have to take on certain values/forms. We just haven’t figured out why yet

But why should the only way for a Universe to be allow for the existence of life?

Any Old Universe will do

We know how to make our form of life, with water and complex molecules around a long-lived star

But this is probably a very limited view of life

Other forms of energy! Other forms for the organization of matter! Other particles that are not important to our form of life could be important in other Universes

Therefore, chances that a Universe can host some form of life = high

Multiverse (=many Universes)

Laws of physics can take on many forms

• Many Universes are born, some hospitable to life, some not

• A large variety of combinations found throughout these Universes

• No particular luck needed! We are in the Universe that could make us (weak anthropic principle)

Lucky Universe

There are a whole bunch of physical constants/laws that had to be just so for any life (not just us) to exist

There is only Universe

This Universe was lucky enough to be a Universe in which life could exist

This is deeply unsatisfying

Testing the Multiverse Idea

How many tunable values are there out there?• How lucky do we have to be?• Can the laws of physics change in our own

Universe?

Can collisions between Universe “bubbles” lead to radiation that can be observed?

Is our Universe open or closed?

Does the CMB contain information on the Universe we can from?

Anthropic Principle

Weak Anthropic Principle states the obvious:• We can only exist in a Universe that could make

us

How surprising is it that there is an “us”?• Many important properties are the result of

physical laws/constants• However, many of them are connected, so

maybe we only had to get “lucky” a few times?• Or maybe some kind of life is possible in most

Universes?

ConclusionThe idea that there are reality beyond our observable

Universe is not controversial

Controversial• The existence of distinct Universes in the multiverse• Other Universe having different laws of physics• Eternal inflation giving continual birth to other

Universes• Can we ever observe any of this, if it does exist?

I look forward to the continued observation and investigation of the Universe(s)