The Ultimate Tool for Understanding the Universe:

Light or the Electromagnetic Spectrum

Victor-Andrei Bodiut, S2574608

Coordinator: Prof. Dr. Marc Verheijen

Course: The Evolving Universe – Minor Astronomy through Space and Time

University of Groningen

18-01-2016

The Ultimate Tool for Understanding the Universe:

Light or the Electromagnetic Spectrum

Light is brilliant. We all love watching a reflection in the lake, a rainbow just off the

waterfall, a sunset, gazing at the moon or stars at night, a comet or a falling star. Moreover,

almost all the information we now have about the Cosmos is due to light. After some

introspection, as well as consulting others, I was left with the impression that light as a

phenomenon is somewhat taken for granted and not given enough appreciation. That is,

although we are introduced into the basic principles of how light works, its underlying

mechanisms, its complex behavior, its beauty, power and importance are often missed. What

is light, really? Where does it come from? How can it be responsible for our entire visual

perception, everything we see, each and every beam, color, nuance, shade and darkness?

Why is the sunlight warm? How did our understanding of light evolved and in turn shaped

our picture of the universe? In the present paper, I will address these questions from a

scientific point of view, with a major focus on the astronomical perspective of light and the

entire electromagnetic spectrum.

A Long Journey

Our journey starts where

every journey should start: the

Big Bang, the very beginning of

everything. An infinitely dense

and hot singularity containing all

the mass and space-time

unravels into what is going to

become the Universe. It is a

common misconception to think

of the years close to the Big

Bang as bright, luminous and

explosive. A very dense

subatomic soup of particles called

plasma prevented photons to travel freely. The matter was so dense that photons could not

go a long way before hitting an electron and being sucked by it. It is only after ~ 379.000

years when matter has expanded and cooled enough for atoms to start forming and photons

to escape: the recombination era. Arno Penzias and Robert Wilson serendipitously

discovered the thermal radiation of this very first light in 1974, the Cosmic Microwave

Background (CMB, see image). These photons have been travelling ever since.

Perhaps of a more personal relevance is the

formation of our Solar System, ~ 4.5 billion years ago.

Since then, light from the Sun has been the main source of

energy for Earth. It plays a crucial role in the preservation

of liquid water and its cycle in nature, weather patterns, day

and night times, seasons, photosynthesis and thus the

increase in oxygen and carbohydrates, basically all life as

we know it.

Humanity has then found other forms of light.

Starting with fire, moving to lamps that did not require

electricity, through incandescent and fluorescent light and

reaching nowadays technologies. While our ancestors were

gazing at the objects on the night sky, the only light source

there was, light-pollution is an increasing problem in the

big cities today (see picture for comparison). By

understanding and controlling light, lenses, mirrors,

microscopes and telescopes were invented and became

crucial tools in both the micro- and macro- worlds.

Light: Wave or Particle?

Aristotle and other ancient Greeks explained both light and gravity as vibrations in the

ether, the quintessence that filled all space. This view had strong implications in the scientific

revolution as well. Johann Bernoulli concluded that some “excessively small whirlpools” were

responsible for the elasticity of the ether through which light could travel. The Dutch

mathematician Christiaan Huygens came up with a wave theory, in which light traveled as

longitudinal waves in an elastic medium – ether. A very similar standpoint was taken by the

French philosopher, Rene Descartes.

Isaac Newton contradicted the behavior of light

as a wave by showing for the first time how it behaves

as a particle. By using prisms, Newton decomposed

white light into colors and showed that this effect is not

due to imperfections in the glass as it was previously

assumed. The ‘color spectrum’ was born (see image),

with light ranging from red to violet and mixing into

white light. Newton was also the first one to propose

what for me is a great achievement: color is not a

property of an object per se, but a property of how

light reflects from the object. While contradicting

Robert Hooke, another believer in light’s wave theory

and Newton’s ‘rival’ if we can call it that, Isaac stated the one quote that in my opinion

summarizes the whole scientific inquiry of discovery: “If I have seen further, it is by standing

on the shoulders of giants”.

Thomas Young’s “double-slit

experiment” provided the first strong evidence

in favor of the wave theory of light. In this

experiment, light was passing through two

very thin holes, 10-6 m. It was presupposed

that if light was made of particles, than one

should see 2 separate dots. Instead, Young

saw complex interference fringes that could

only be explained by the wave nature of light

(see picture). His findings were backed up

later by Augustin Fresnel and many detailed

descriptions of the wave theory followed.

Young’s brilliant experiment was of great interest in the world of quantum mechanics in the

20th century and now we know that it actually provides ground for the wave-particle duality

of light.

James Clark Maxwell was the first to realize that light is an electromagnetic radiation,

bringing together electricity, magnetism and light. He did this by demonstrating that

magnetic and electric fields travel in waves through space at the speed of light. This was seen

as the second most important breakthrough, after Newton’s laws of gravity. Heinrich Hertz

conclusively found experimental evidence for Maxwell’s theory. He was able to transmit radio

waves using instruments that ruled out all other known wireless communication. The measure

unit for the frequency of a wave was named after him: 1 cycle / second = 1 Hz. It has to be

mentioned here that most of the physical concepts in which vibrations in electricity and

magnetism could explain all aspects of electromagnetism were set before by Michael

Faraday. This rings a bell towards Newton’s infamous quote once more.

The first to acknowledge that the energy of light

waves is quantized was Max Planck. He did so by

the use of blackbodies, a theoretical ideal object

that behaves both as a perfect absorber and

emitter of light. Almost all objects of interest in

astronomy could be approximated as

blackbodies. As can be seen in Planck’s curve, at

a constant temperature, the shorter the

wavelength, the higher the intensity. Also, higher

intrinsic temperatures have higher peaks of

radiation. Based on Planck’s work, the Wien Law was able to explain why the peak moves to

shorter wavelengths when the temperature increases while the Stefan-Boltzmann Law

explained the increase in peak or the height of the curve with an increase in temperature. By

showing that the energy emitted in a unit of surface/second is proportional to an object’s

temperature, the later gave astronomers the possibility to find stellar radii from observations

of temperature and total energy emitted. All this work was a true breakthrough in astronomy.

Two other scientists play a key role in the

use of electromagnetic radiation in astronomy.

Albert Einstein firstly defined light as made of

massless particles named photons that interact

with matter as soon as in contact. Niels Bohr

brilliantly described at an atomic level how the

photons are being generated or released. When

an electron, that very tiny indivisible particle (10-16)

changes energy levels inside an atom, it can do so

by receiving or releasing a very particular amount

of energy.

What type of energy? - Light. When the

electron advances a level, photons are being

absorbed and when it regresses a level, photons

are being generated. If the amount of light energy

that is being absorbed or generated has to be

unique for each type of atom (see the “stairs” as

unique level patterns for H and He), it means that

just by observing objects we can infer their

composition. The implications of this have led to

the invention and use of spectroscopy as a way of

exploring the skies (to be discussed in more detail

later).

The Speed of Light

The speed of light is nowadays one of the most accurate and well-established

constants. However, many scientist initially taught that light travels infinitely fast.

Although Isaac Beeckman

and Galileo Galilei proposed a

couple of experiments to determine

the speed of light, the first

quantitative measurement was made

by the Danish astronomer Ole

Rømer in the 17th century. While in

Giovanni Cassini’s assistantship,

Roemer observed that the time

between Io’s eclipses (the 3rd largest

Galilean moon of Jupiter), thus Io’s

orbital period, is decreasing as the

distance between Earth and Jupiter

decreases and vice-versa (see image). This inequality was attributed to light taking time to

reach us. Based on his data, the velocity of light was calculated ~ 215.000 km/s.

French physicist Armand Fizeau came up in

the 19th century with a brilliant design that had

nothing to do with observing the celestial sphere. In

a nutshell, he placed a spinning cogwheel and a

mirror ~ 8 km apart and emitted light through the

cogs of the wheel into the mirror (see image). He

observed that if the wheel is spinning fast enough,

the reflection of light from the mirror is obscured by

one of the cogs. By knowing the rotational speed of

the wheel, as well as the distance between the light

emitter, wheel and mirror, he came up with the

speed of light ~ 315.000 km/s (5% error rate).

Afterwards, by means of cavity resonance and later laser interferometry, the speed of

light has been precisely measured at 299.792 km/h in vacuum.

Defining, understanding and measuring light was clearly not a

trivial task. All of this is to pay a tribute to the greatness of human mind

and the scientific method, the beauty and power of knowledge and

discovery and to not stand ignorant and forget the foundations we are

standing on.

In the current view, light is an electromagnetic radiation with a spectrum

much larger than the visible light. Light is described by its wave-particle

duality, travelling through space as a wave and behaving as a particle

(photons) when interacting with matter. The velocity of light is

independent of the framework of reference, i.e. the speed of the

observer. The speed of light is the highest in vacuum and has very close

values in other mediums (e.g. glass, water). The finite value of the speed

of light has of course the following implication: by looking further away,

we see further back in time. A light-year is the distance light travels in a year

and is used to express distances too far for other scales.

The Andromeda Galaxy, our neighbor, is for instance ~ 2.5 million light years away. Its light

that we see now has been travelling for 2.5 million years to reach us. On the same line of

thought, the Sun light we see at each instant, left the photosphere ~ 8 minutes before. Each

photon is an energy package, the shorter the wavelength, the more energy it carries.

The Electromagnetic Spectrum and Its Use in Observing the Universe

Human visual sensation and perception is restricted to ~ 400-700 nanometers

wavelengths, peaking in the greenish ~ 550 nm. The visual network has evolved to be

sensitive and advantageous in the given environment: type of light emitted by all stars (the

Sun is no exception), color vision has developed, night vision is scarce as we are diurnal

organism, etc. Hence, most molecular energy levels cannot be excited by long-waved light

(such as infrared light) and high energy short-waved light has the power to damage the

photosensitive cells irremediably. It is understandable thus that when we refer to light, we

usually refer to visible light. The range of visible light is but a tiny fraction of the full

electromagnetic spectrum. Just like we cannot hear a bat’s echolocation or the “call” of a

whale.

By acknowledging this fact, astronomical tools were modified to “see” the entire spectrum,

revealing information that is continuously shaping our universe like never before. Take a look

at the depiction below as it summarizes a great deal of information about light.

A wavelength is nothing else than the distance from one peak to the next one in a

wave of light (m or nm). Frequency basically counts the number of waves passing by a certain

point in 1 second 1 Hz = 1 cycle/second). The energy carried by the photons is measured

in electron volts and can be taught of as temperature.

The relationship between wavelength, frequency and energy is very important. The

shorter the wavelength, the higher the frequency and the more energy (higher temperature).

The longer the wavelength, the lower the frequency, the less energy (lower temperature). As

simple as that.

We will start with the light that makes it possible to listen our

favorite radio show while driving through the country. Radio waves are

the least energetic and have the longer wavelengths. This means they

can travel vast distances without fading away. It also means they do

not interfere with Earth’s atmosphere and can be used to map the sky

on a cloudy day as well. Radio waves revolutionized our long-distance

communication, including aircraft communication, Wi-Fi, radar,

navigation. They are also the ones we’ve sent in SETI (Search for

Extraterrestrial Life) and still waiting an answer from. In astronomy,

radio waves are used to map our own Milky-Way galaxy and penetrate

and see inside dense interstellar clouds of gas and dust (see galaxy

M33 in radio waves). They are also emitted by stars and gasses in space.

A great feature of radio telescopes is that by using a powerful computer, the data from

several radio telescopes can be combined. Thus, placing observatories at great distances will

yield the resolution of one telescope as big as the distance

between the separate telescopes. Think of the following: the

human eye is a light detector with a lens of several mm in

diameter. As a result, there only so much one can see with the

naked eye. The fainter the object, the fewer photons reach the

retina. This follows the 1/R2 rule: intensity of light from an object

is inversely proportional to the distance to that object. Doubling

the distance would lead to 4 times less light. Eyes of a few

meters in size would collect more photons and would see much

fainter objects. The larger the detector, the more light it detects.

The Very Large Baseline Array (VLBA) for example consists of

telescopes ranging from Hawaii to Puerto Rico (see image).

The next type of light cooks the popcorn we all enjoy while watching a movie.

Microwaves are the second least energetic type of light and are used to study the structure of

near-by galaxies, as well as the cosmic background

radiation discussed in the very beginning, the remnant of

the Big Bang. This radiation is in the microwave range.

Because Earth’s atmosphere block the majority of

microwave radiation, astronomers use satellite-based

telescopes. See artist’s impression of the ESA’s (European

Space Agency) Planck observatory in its orbit.

Infrared radiation is less energetic than visible light

and it’s used in night goggles and other devices that

detect heat. It can be thought of as ‘heat’ radiation. It is

also used in remote controls and security, to name some.

The major problem we encounter here is that because everything emits heat and thus

infrared radiation, it is easy to mistake its origin. The atmosphere radiates heat, the Earth and

even the telescopes and detectors themselves. This is why ground-based infrared telescopes

are placed in cold and dry climates, often on mountain tops, the telescopes are cooled and

the atmospheric radiation is controlled for.

They excel at finding dim or cool

stars, that are otherwise invisible, map

dust or gas clouds and lanes that are

not heated enough to glow in visible

light and even measure temperature

of exoplanets. The Spitzer Space

Telescope gave us new information

about the dust lanes located at the

center of the Milky Way (see image).

The successor of the Hubble Space

Telescope (HST) will be an infrared

specialized telescope, which will see

further in space and time, up to the very first

galaxies. Such an information would contribute to

our understanding of complex cosmological

questions. It will also pierce into dust and gas

clouds around current planetary nebulae. See

artist’s depiction of James Webb Space Telescope.

As mentioned before, visible light is

discerned into colors, from the lower energy,

longer wavelength red to the short-waved more

energetic violet with all the others in between.

Most of the stars in the Universe radiate in this

field of visible light. As wavelength is related to

energy, we can know the temperature of stars solely by

checking their colors (see Planck curve). A star that appears

bluish will be hotter than a star glowing reddish. Visible light

can pass Earth’s atmosphere. This is why astronomy is the

oldest science there is. The night sky was always there for us

to see. However, ground-based optical observatories

(operating in the visible spectrum) are facing the “seeing”

effect from the atmosphere. It blurs the image and affects a

telescope’s resolution (the power to distinguish between

very close objects). By using “adaptive optics”, astronomers

can somewhat control for that effect (see the Very Large Telescope in Chile projecting a fake

star on the sky in order to control for “seeing”).

Still, because of this “seeing”,

space-based telescopes

outperform by far. The HST

has given us some incredibly

beautiful and detailed

images and unique

information. Here’s a star

formation region in the

Carina Nebulae, some 8.000

light years deep in the

Southern sky. Red – sulfur,

green – hydrogen, blue –

oxygen. In this 3 light-year-

long pillar, stars are being

born from dense gas and dust

streams. On a personal note, these images give me the chills every single time and remind

me that we’ve come a long way.

It is these star birth regions and some very

energetic stars that ultraviolet or UV telescopes are

specialized upon. When viewing very distant

galaxies, most of the stars and usual dust and gas

do not reach the UV spectrum and these nebulae,

these superhot “nurseries” pop up in the sky. The

UV light from the Sun is fortunately almost entirely

absorbed by Earth’s ozone layer. Still, some gets

through and at such high energy, this type of light

attacks skin molecules causing sunburns or worse

problems. The GALEX space-based observatory

viewed ~ 500 million galaxies in ultraviolet light,

trying to observe the history of star formation (see

GALEX’s view on the Cygnus Loop nebula, hot gas

and dust).

The X-ray photons are so energetic that they penetrate matter. It is this type of light

that is used to screen the luggage in airports and medical purposes such as imaging teeth.

As in the UV case, X-ray light is blocked by the atmosphere, making it impossible to locate an

X-ray telescope on the ground. The high energy causes another problem: X-ray light does

not reflect from a mirror as lower energetic light does, requiring very long focal lengths

(several meters). Thus, the telescope has to be

very big very hard to launch in space. Objects

that emit X-rays are the accretion disks around a

black hole, millions degrees heated gas, and

some exotic neutron stars (extremely compact

and dense “core-like” stars that result from

supernovae). See the supernova remnant of

Cassiopeia A and a very tiny blue dot in the

middle as the remaining neutron star.

The most energetic light and at the same

time the most harmful to life is gamma radiation.

It is used by doctors to scan inside a body, but in

a controlled and limited amount. It is produced

in nuclear or radioactive decay, when the nucleus

of an unstable emits radiation to lose energy. The disastrous mutations after the explosions of

Nagasaki, Hiroshima or Chernobyl were caused mostly by

gamma radiation. Immense amounts of energy can be

detected in supernovae, cosmic radioactive decay, black

holes, active galaxies, pulsars (rotating neutron stars),

gamma-ray bursts (GRBs, violent flashes in distant

galaxies, thought to be the brightest electromagnetic

events in the Universe) or even destruction of antimatter

(beyond the scope of the present paper). Gamma

radiation is so energetic that no telescope can directly

observe it. Instead, astronomers use special masks that

detect the shadows of gamma radiation (Fermi Space

Telescope) or use the atmosphere itself as a detector from the ground (HESS). See Burst Alert

Telescope (BAT), a GRB

detector under construction.

The distribution of telescopes

across the full electromagnetic

spectrum.

Spectroscopy and the Doppler Effect

Following Bohr’s model of the atom

discussed earlier, we know that each type of

atom emits or absorbs light of different

wavelengths, thus different color. Our eyes

cannot detect such small differences, but a

spectrometer can. It can tell the difference

between light emitted by a H2, He or C atom

for example (see emission and absorption

spectrum for H2). While in the case of dense

stars, the color spectrum is highly influenced by temperature, the spectrum of fairly thin gas is

given by its composition. This is how we know

that most of the star and gas clouds are made

of H2 with some He and other heavy metals, that

Jupiter’s atmosphere contains methane or

Venus’s CO2.

Another phenomenon that astronomers

make use of is the Doppler Effect. The same

effect that makes a siren sound higher in pitch

when coming towards us than when moving

away. This effect happens because the

wavelengths get compressed (shorter

wavelength higher frequency) when

approaching and loosen up (longer wavelength

lower frequency) when going away. As with sound, the same is true for light. When an

object is moving towards the Earth, the wavelengths will be shorter, the frequency higher, so

more towards the blue, while if moving away from us its spectrum will be more to the red.

Astronomers use the terms: blue-shifter, moving towards us

and red-shifted, moving away from us (see image, spectrum

at rest, followed by a red-shifted and blue-shifted spectrum

respectively).

By how strong the Doppler Effect is, it is possible to know the

velocity with which object travel, either towards or away from

us. It is because of the redshift of the galaxies that grows with

distance that Edwin Hubble and others reached to the

astonishing conclusion that the Universe is expanding.

By means of other spectroscopic techniques, astronomers can determine the

magnetic field and its strength, the mass, the density, the spin and rotational speed of

objects. That is a vast amount of information and properties that can be found by solely by

dividing light into colors.

Almost everything we know about our Solar System, Milky Way, Local Group, distant

galaxies – active or inactive, star formation nebulae, interstellar medium of gas and dust,

black holes, neutron stars, pulsars, white and brown dwarfs, supernovae, binary systems,

exoplanets, star clusters, everything we know about the Universe is because of one brilliant

thing: light.

References/Bibliography

Born, Max; Emil Wolf (1997). Principles of Optics. Cambridge University Press. ISBN 0-521-

63921-2.

Fernald, Russell D. (2001). The Evolution of Eyes: How Do Eyes Capture Photons? Karger

Gazette 64: "The Eye in Focus"

Kerker, Milton. The Scattering of Light and Other Electromagnetic Radiation: Physical

Chemistry: A Series of Monographs. Vol. 16. Academic press, 2013.

Kuhn, Karl F., and Theo Koupelis. In quest of the universe. Jones & Bartlett Learning, 2004.

Madau, Piero, Lucia Pozzetti, and Mark Dickinson. "The star formation history of field

galaxies." The Astrophysical Journal 498.1 (1998): 106.

Newton, Isaac. "A letter of Mr. Isaac Newton, professor of the mathematics in the University

of Cambridge; containing his new theory about light and colors: Sent by the author to the

publisher from Cambridge, Feb. 6. 1671/72; in order to be communicated to the r.

society." Philosophical Transactions (1665-1678) (1965): 3075-3087.

http://www.astro.rug.nl/EDUCATION/Aantekeningen/05_25nov15.htm

http://www.astro.rug.nl/EDUCATION/main_en.html

http://physicalscience.jbpub.com/starlinks/7e/

https://www.astro.rug.nl/~weygaert/tim1publication/cosmic_origins/cosmic_origins.lect7.bigb

ang.pdf

https://www.youtube.com/playlist?list=PL8dPuuaLjXtPAJr1ysd5yGIyiSFuh0mIL

https://www.youtube.com/watch?v=jjy-eqWM38g

http://www.slate.com/blogs/bad_astronomy/2015/01/16/crash_course_astronomy_episode_1.h

tml

http://www.timaras.com/science/light1.html

http://www.light2015.org/Home/ScienceStories/Discoverers-of-Light--.html

http://www.light2015.org/Home/ScienceStories/A-Brief-History-of-Light.html

https://cosmology.carnegiescience.edu/timeline/1964

http://violet.pha.jhu.edu/~wpb/spectroscopy/tool.html

https://www.spacetelescope.org/

http://chandra.harvard.edu/

http://nasaviz.gsfc.nasa.gov/

http://www.eso.org/public/images/

http://www.space.com/20330-cosmic-microwave-background-explained-infographic.html

http://www.space.com/

http://earthsky.org/space/what-is-the-electromagnetic-spectrum

https://www.aps.org/publications/apsnews/201007/physicshistory.cfm

http://imagine.gsfc.nasa.gov/science/toolbox/emspectrum_observatories1.html

http://imagine.gsfc.nasa.gov/science/toolbox/emspectrum1.html

https://en.wikipedia.org/wiki/Electromagnetic_spectrum

https://en.wikipedia.org/wiki/Light

https://en.wikipedia.org/wiki/Christiaan_Huygens

https://en.wikipedia.org/wiki/Galileo_Galilei

https://en.wikipedia.org/wiki/Hippolyte_Fizeau

https://en.wikipedia.org/wiki/Ole_R%C3%B8mer

http://www.kidsastronomy.com/jupiter/moons.htm

https://en.wikipedia.org/wiki/Heinrich_Hertz

https://en.wikipedia.org/wiki/Robert_Hooke

https://en.wikipedia.org/wiki/Thomas_Young_(scientist)

https://en.wikipedia.org/wiki/Aether_(classical_element)

https://en.wikipedia.org/wiki/Evolution_of_the_eye

https://en.wikipedia.org/wiki/Wien%27s_displacement_law

https://en.wikipedia.org/wiki/Stefan%E2%80%93Boltzmann_law

https://en.wikipedia.org/wiki/Planck%27s_law

https://en.wikipedia.org/wiki/Black-body_radiation

https://en.wikipedia.org/wiki/Double-slit_experiment

https://en.wikipedia.org/wiki/Wave%E2%80%93particle_duality

https://en.wikipedia.org/wiki/Cosmic_microwave_background

https://en.wikipedia.org/wiki/Isaac_Newton

https://en.wikipedia.org/wiki/Albert_Einstein