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Space Physics:
Energy Traveling Through Space

Where Do the Photons Go?

When photons come out of a galaxy, where do they go? Do these photons get old? Do they change in any way?

Our galaxy is just one of many galaxies. Galaxies clump together in clusters and even "super-sized" clusters. All of the photons that escape from a galaxy enter into what is known as the "intergalactic medium" - just the space between galaxies. (Note: this space is not empty, but is filled with electromagnetic radiation, dark matter, and dark energy, for example.)

These photons travel through the universe until they encounter something to interact with.... for example, our eyes or our telescopes. They do not "age". If we are moving toward or away from the source (i.e., some other galaxy), then we will observe that photon at a slightly higher or lower frequency than it was emitted at. This is called the Doppler shift. We use this effect, in fact, to tell when sources are moving away from us or toward us!

Dr. Louis Barbier
(August 2003)


Do We Receive the Same Photons a Star Emits?

Someone from my astronomy club told me that when we see a quasar our eyes are actually being "touched" by the same actual photons that were released from that object billions of light years away and billions of years ago. Do the same photons that left a star actually make the whole trip to our eye? I thought photons interacted with atoms which released other photons, and through a series of exchanges we finally "see" photons that are released closer to us. Who is right?

The guy in your astronomy club is correct. The vast majority of the photons that we see from a distant star or quasar were actually emitted by those objects and have traveled for many light years to reach us.

Your line of reasoning is also correct, in that atoms in the line of sight (that is, between Earth and the distant object) can absorb this radiation and produce photons of their own. But these atoms can emit photons in any direction, and the chances that they emit them in our direction are small. Therefore, these photons are usually a small percentage of the light we see, compared to those emitted by the distant object itself.

Dr. Nick Sterling
(March 2007)


Redshifted Energy

When an electromagnetic wave is redshifted by the gravity of a massive object, where does the lost energy go?

The same place the kinetic energy of a ball rolling up a hill goes as the ball slows down and stops: into potential energy. The total energy of the electromagnetic wave includes the gravitational potential energy. As it moves away from the massive object, energy gets moved from the photon energy (which redshifts the wavelength) into potential energy, but its total energy is conserved. Electromagnetic waves moving towards a massive object are correspondingly blueshifted as they "fall" towards the object.

Dr. Eric Christian
(September 2001)


Expansion of a Sphere of Light

What would the rate of expansion of the surface area of a sphere of light be, and what would it look like, if the sphere's radius was increasing at a rate equal to the speed of light -- if you saw it first right at the moment of the Big Bang and then after it had been expanding for 15 billion years? And what value of pi would you use in the calculation?

This question came to me when I was reading "A Brief History of Time" by S. Hawking. He said on "....if a pulse of light is emitted at a particular time at a particular point in space, then as time goes on it will spread out as a sphere of light whose size and position are independent fo the speed of the source. After one millionth of a second the light will have spread out to form a sphere with a radius of 300 meters; after two millionths of a second, the radius will be 600 meters; and so on." I tried to calculate the increase in the size of the surface area as its radius expanded at the speed of light, and I found that when I got to the point in time where the surface area of the sphere was really big....15 billion years old...that the surface area was so big, that the actual value of its size seemed to depend on which value of pi I used.

The surface of a sphere expanding at the speed of light is A = 4/3 * pi * c * c * t * t, where c is the speed of light, and t is the time since the light was emitted. There is only one value of pi. The precision that you use on your calculator will affect the final precision of your answer, but since the speed of light is only known to about 8 significant figures; using more than that in your value of pi doesn't get you anything.

Dr. Eric Christian

Current in a Vacuum

Is electron flow through a vacuum considered current?

Yes it is. Within our own heliosphere there are currents (of electrons and also of protons) flowing toward and away from the Sun. These currents generate magnetic fields, just as currents flowing through wires generate magnetic fields around the wires.

Dr. Louis Barbier

Is Light Affected By Gravity?

Is light affected by gravity? If so, how can the speed of light be constant? Wouldn't the light coming off of the Sun be slower than the light we make here? If not, why doesn't light escape a black hole?

Yes, light is affected by gravity, but not in its speed. General Relativity (our best guess as to how the Universe works) gives two effects of gravity on light. It can bend light (which includes effects such as gravitational lensing), and it can change the energy of light. But it changes the energy by shifting the frequency of the light (gravitational redshift) not by changing light speed. Gravity bends light by warping space so that what the light beam sees as "straight" is not straight to an outside observer. The speed of light is still constant.

Dr. Eric Christian

Looking Back in Time

I was looking at some of the Hubble Space Telescope pictures, and one was listed as "Hubble peers back more than 10 billion years to reveal at least 1500 galaxies at various stages of development". How can a telescope look into the past?

The galaxies that Hubble was looking at were 10 billion light years away. That means that the light the Hubble telescope was collecting left those galaxies 10 billion years ago. 10 billion years ago, the Universe was still relatively young (maybe only 5 billion years old). So we're seeing galaxies that are young.

Dr. Eric Christian

Can We See the "Present" in Space?

I have questions about time, distance, and what we see. Since the objects we see in space can be hundreds of light years away, we are seeing the "past" of each object. Is there a way for us to know the "present" of objects that are hundreds of light years away? How do we know the things we see in space still exist in the present time? If they no longer exist and all we see is the past, what would the impact be on our Earth and lives?

This is a good question. The short answer is that we cannot see what these objects are actually like right now. We are limited to the information we can measure, which is carried by photons (light). Photons of course cannot travel any faster than the speed of light, which is about 300,000 kilometers per second. On the other hand, astronomers can search the nearby Universe to find analogues of very distant stars or galaxies. For example, the Milky Way is surrounded by a halo of very old stars that are deficient in metals (any element heavier than helium, in astronomer-speak!), relative to the Sun. These are mostly very old, low-mass stars that may be similar to stars formed in distant galaxies. In order to find analogues of distant massive stars, astronomers can look at nearby "dwarf" galaxies, which are undergoing star formation and are deficient in metals. These galaxies contain massive stars that are not found in the halo (due to their short life spans, massive halo stars have long since ended their lives as supernovae).

Other objects don't have obvious analogues in the nearby Universe (such as quasars or gamma-ray bursts). For these types of objects, we must compare observations of the present (nearby Universe) to those of the past (distant Universe), and build computer models based on our knowledge of physics to infer the evolution and fate of these bodies.

The time-lag between what we see from Earth and what is happening in distant galaxies is a mixed blessing. On the one hand, the time-lag allows us to study what the Universe was like when it was much younger, essentially a window into the past. On the other hand, we will not see how these particular galaxies and their stars actually evolve, and we must rely on computer simulations and nearby analogues to decipher their fate.

Dr. Nick Sterling
(February 2007)


Time, Light, and Motion

In January 1999, scientists mapped a deep space gamma ray burst from an object estimated to be 12 billion light years away. This is close to the time estimated for the Big Bang at 15 billion light years. How did this object get 12 billion light years away in the 3 billion years after the start of the Big Bang, when presumably everything (including the Sun) started at the same point?

3 billion years after the Big Bang the gamma ray burst was less than 12 billion light years away. But since the Milky Way galaxy is moving away from the site of the gamma ray burst, the light has had to catch up.

Dr. Eric Christian
(May 2002)


How Does Light Travel Through Space to Earth?

How do the rays of light come to Earth?

We're not sure exactly what you're asking, but this page at Imagine the Universe! covers light in terms of the visible spectrum and our eyes, and how light travels in the vacuum of space.

Dr. Louis Barbier and Beth Barbier

Spots That Have Never Seen Light?

Are there spots in space that have never seen light?

No, just after the Big Bang, the Universe was essentially nothing but light. The spots that will have been in the dark for the longest are inside the first chunks of solid matter formed, if those chunks are still around. These might be the cores of the first planets or asteroids, for example.

Dr. Eric Christian

Sound in Space

Why can't you talk in space? Is it because space is a vacuum?

Sound is caused by pressure waves in air (or something else, sound can go through water, for example). If there is no material (called a vacuum), there is no way for sound to travel, so you can't talk. Inside a space capsule or space station there is air, so the astronauts can talk there.

Dr. Eric Christian
(September 2001)


Tuning Fork in Space

Since sound cannot travel in the vacuum of space, what would happen if you had something that normally produces sound, like a tuning fork, in space? What would happen to the energy?

For a tuning fork to vibrate, it must be struck. On Earth, these vibrations compress surrounding air molecules to produce a sound wave that we can hear. If an astronaut in space were to strike a tuning fork, it would vibrate, and sound waves would occur within the tuning fork itself. However, with no air molecules around, it would not produce a sound that the astronaut could hear. The energy from these vibrations would heat the tuning fork (due to internal friction) and eventually be radiated away.

Dr. Nick Sterling
(March 2007)


Sound from the Big Bang

I understand that sound cannot be heard in space. Was there another kind of wave created during the Big Bang? Would it still be traveling through space? How would it register? And how would it dissipate if there is no "end" to space?

A sound wave in air is simply a sequence of compressions (higher than normal density) and rarefactions (regions of lower than normal density) which propagate through the air at the speed of sound (compressions like to expand because of their higher pressure, the rarefactions like to get smaller -- the result is a propagating wave).

If the sound waves are of the correct wavelength, we hear them as ordinary sound -- shorter ones are perceived as having higher pitch. If the waves are too short or too long in wavelength, we cannot hear them, but they are still called sound waves. Any gas can support similar waves.

Such waves were present in the Big Bang. A big part of the pressure of the gas was the radiation (seen as microwaves) and these determined the sound speed. Early in the expansion, the density of the gas became low enough that the radiation could no longer see it, to communicate its pressure to the gas. At this point, the sound no longer propagated and the microwave radiation retained the imprint of the sound at that epoch. The various wavelengths seen in the microwaves tell us about the physical conditions early in the Big Bang. We would not hear these waves as sound.

A place to read more about this is on the WMAP mission website. You might want to explore this site further for more information on the cosmic microwave background radiation.

Dr. Randy Jokipii
(January 2006)


Sound Telescopes?

Is there an aural equivalent to visual telescopes, so that we know something about sound in gas and atmospheres around planets, stars, etc.?

Sound waves propagate by disturbing the environment, whatever that may be (atoms or molecules). Since interstellar space has an extremely low density of particles, sound propagation is prevented. So unfortunately it is impossible for sounds from distant stars or gas clouds to propagate to Earth.

So unless Gustav Holst knew something about this matter that modern physicists do not, he must have only relied on his own creativity to describe the planets in music!

Drs. Lauren Scott and Georgia de Nolfo
(December 2004)


Shock Waves in Space

How can shock waves travel through outer space when there is no air in space to carry them?

Well, space is not in fact empty. It is filled with tenuous (and in some places not so tenuous) plasma. This plasma is much like our atmosphere and shock waves can in fact travel though it.

Dr. Louis Barbier
(November 2001)


Radio Signals in Space

We have learned that waves have to travel through a medium. If there is no medium in space (no gas, liquid, or solids), then how do people on Earth communicate with the astronauts? How are the sound waves transmitted from a space station to Earth?

Sound waves require a medium, but electromagnetic waves do not. So radio and TV (both of which send information superimposed on electromagnetic waves) work fine. The sound waves don't travel any farther than the microphone.

Dr. Eric Christian

Do radio waves (transmitted by humans for television, radio, etc.) spread continuously until they are no longer receivable? I know that a flashlight pointed at the Moon would not be viewable on the Moon, because the photons would have spread too far apart to be seen. Does the same thing happen to radio signals after a certain distance?

Yes, all electro-magnetic radiation (which includes radio waves, microwaves, visible light, ultra-violet, xrays, and gamma rays) will spread out with distance and will, at some point, become too dim to be observable. You can prove the radio wave part of it in your car. The reason why you can't hear San Francisco radio stations in New York City is because the signal gets weaker with distance.

At what speed do radio signals travel through space?

Radio waves are electromagnetic waves with a longer wavelength than visible light, but all electromagnetic waves travel at the speed of light in a vacuum (in matter they travel somewhat slower).

Dr. Eric Christian

Speed of Light in Deep Space?

Is it correct that we do not actually know how fast light travels in deep space? Can we only make inferences at this point?

All of our observations agree on what the speed of light is in a vacuum. This includes measurements of the speed of light to the Moon and planets. If by "deep space" you mean outside the solar system, while we haven't directly measured light speed there, there is no evidence that it is any different than it is in any vacuum.

Dr. Eric Christian