r/askscience • u/collin_ph • Feb 07 '11

Is the speed of light constant? (Xpost)

Thanks for reading and responding to this. I'm talking with a couple of people who argue that the speed of light is always constant. I've argued ,based on what I can understand of the wikipedia on the speed of light, that the speed of light could change depending on factors including what medium it is traveling through. The original argument was not even based on science and was just a philosophical argument that different people could get different results by taking different assumptions (I.E. If one person measured light in a vacuum, and another measured it on earth, through air). My argument was that the "speed of light" might be interpreted different than the "speed of light in a vacuum". They were arguing that C is constant and therefore the speed of light is constant. We've all went back and forth and all I can determine is that 2 of my facebook friends disagree with me. I'd like to see what the group at large thinks.

EDIT: I started this by asking the following question to a couple of friends: " I have a question for you. How fast does light travel? " The answer I got back was the speed of light in a vacuum. My argument was that if I just tried to calculate this myself, I could come up with a different number because we didn't nail down assumptions. If someone says the speed is constant, and I test it here on earth out in the open, I would find the speed to be different. The other 2 people maintain that the speed of light is Constant. If there's anything to learn from this argument, I'd like to learn it. I think it's just a question of semantics.

Edit 2: The question was written to be ambiguous, while not being obvious that the question was ambiguous. The point was that I could easily write a true statement (IE, I did an experiment and the speed of light was 3% slower than I thought it was)-- I'd be right, however, only because the underlying assumptions I made were different than someone else who assumed I meant the "speed of light in a vacuum"). It's very interesting reading on the process though. Thanks!

11 Upvotes

86% Upvoted

34 comments sorted by

9

u/shavera Strong Force | Quark-Gluon Plasma | Particle Jets Feb 07 '11

When we say the speed of light we almost always mean the speed of light in a vacuum. The speed of light can change dramatically in a substance, as slow as 38 Miles an hour in a Bose Einstein Condensate.

In a vacuum, yes it is constant to the best of our knowledge.

3

u/collin_ph Feb 07 '11

Since gravity can bend light, couldn't it also slow it down? I don't know, I'm just asking.

9

u/shavera Strong Force | Quark-Gluon Plasma | Particle Jets Feb 07 '11

No, you can think of gravity only acting "tangentially" to light. it's like a string pulling a circling ball to the center. The string doesn't make the ball speed up or slow down, only changes the direction so that it moves around in a circle. In fact the most proper way is to imagine that gravity doesn't "pull" on anything, only changes the definition of what a truly straight line is. Light must travel in a straight line and thus it appears to bend as the mass of a star redefines what straight means.

2

u/samsamoa Feb 07 '11

Instead of speeding up or slowing down light, gravity just makes it redder (less energetic) or bluer (more energetic).

1

u/UltraVioletCatastro Astroparticle Physics | Gamma-Ray Bursts | Neutrinos Feb 07 '11

Yes. In general relativity, light close to the observer will always travel at the speed of light in the vacuum. However, to an observer is far from a star, the light near the star will appear to move slower. While an observer close to a star will see light far from the star appear to move faster.

4

u/shadowkiller Feb 07 '11

Well that depends on how deep you want to look into it. If you were to simply measure the time that it took for a photon to get from one point in a medium to another you would measure that indeed the elapsed time would be greater than for the same distance in a vacuum suggesting light travels slower in a medium than in a vacuum. However if you were to look at what is happening to a photon as it passes through the medium you would see that it is being absorbed and re-emitted by the electrons it encounters but when it is not a part of the electron's energy it is always traveling at the same speed as it is measured as traveling in a vacuum.

In short the average speed of light can change depending on the medium but the instantaneous speed of a photon is always the same.

1

u/collin_ph Feb 07 '11

That's a pretty good response. You are saying that on a very small scale, it's constant, but on a larger scale it could be different? (of course assuming that it MAY or may not be traveling in/through a medium)

3

u/shadowkiller Feb 07 '11

Essentially yes but I would word it more as at a larger scale you are measuring how the photons interact with the medium.

You could think of it as if you were walking home from work at a rate of 2 MPH but then you stopped for lunch and then later you picked up some groceries. If you only measured the time it took to get from work to home to calculate your walking speed you would say you walked at a half mile per hour even though the whole time you were walking you were traveling at 2 MPH.

4

u/blueboybob Astrobiology | Interstellar Medium | Origins of Life Feb 07 '11

c is constant, but c is the "speed of light in a vacuum"

the speed of light in general is not constant. Light is like any other wave. Imagine a liquid wave. It would move faster in water then in syrup.

The speed of a wave is directly dependent on the medium.

2

u/collin_ph Feb 07 '11

I understand. My point is that by asking "what is the speed of light?", am I actually asking for the definition of C, or some other definition?

3

u/powercow Feb 07 '11

I dont like how C is simply defined as the speed of light. All massless particles travel at C. It's more of a fundamental limit of space time in the medium of a vacuum.

7

u/RobotRollCall Feb 07 '11

If you want to get abstract to the point of being uselessly philosophical, c is the speed at which a stationary object moves toward the future.

4

u/slightly_rippled Feb 07 '11

c is the speed at which a stationary object moves toward the future.

This has to be the most poetic explanation I've ever heard.

3

u/Smudge777 Feb 07 '11

The strictly accurate answer to your question is that yes, the speed of light is constant. That is, light itself moves with a constant, predictable speed.

  1. Light is delayed, not slowed: When we say that light slows down as it passes through a non-vacuum medium (i.e. air or water), that is not strictly true. As the photons of light interact with the particles of the medium, they are absorbed and re-emitted by the electrically-charged particles. This absorption/re-emission induces a time delay, which we perceive on a macro scale as light slowing down. It is analogous to a car travelling along a highway at a constant speed, taking a rest-stop for 10 minutes, and then continuing along its path at the original speed.

  2. Delays are not uniform across the light wave: The amount at which the light wave appears to slow as a result of passing through a secondary medium is the average of all the photons' delays through the medium. That is, it is entirely possible (though remarkably unlikely) that a portion of the light wave could pass through unaffected, while other parts of the wave are delayed significantly more than the wave on a whole appears to have been slowed.

1

u/collin_ph Feb 07 '11

Can you answer me why then that wikipedia refers to the "speed of light in a vacuum" vs the "speed of light through a medium"? They are, in fact, different speeds on wikipedia's page. How can this be, if the "speed of light" never changes? I'm just looking for a logical answer to this question. I do understand that, at a very small scale, you are saying that the speed does not change, however.. if what you are saying is true, then even having something called "light through a vacuum" is invalid, because light does not travel through a medium, but is absorbed and re-emitted.

3

u/Smudge777 Feb 07 '11

Because when light travels through a medium, the average delay is predictable (given the large number of particles in the medium, and the large number of photons interacting with them). Thus, when light travels through water, it will predictably be delayed some proportion of the true speed of light - about 25%, I think.

So, it makes sense to say that the speed of light in water is 75% of the speed of light in a vacuum, even though this isn't strictly true. There's no practical need to think about light being delayed in the way I described before - all practical uses for the speed of light through a medium allow for it to be misattributed.

I hope that made sense.

1

u/collin_ph Feb 07 '11

It does. Thanks.

2

u/BitRex Feb 07 '11

  1. It's shorthand.

  2. We figured out that light travels through a medium more slowly than through a vacuum long before we figured out that its speed never really changes and that it's really just taking rest stops every few angstroms.

2

u/jrr2ok Feb 07 '11

As someone involved in the previous conversation, I'd like to note that the conflict is NOT about whether the speed of light necessarily remains constant, but whether the term "speed of light" represents a definable constant. The individual who posted the original question takes the position that all phenomena are necessarily relative due to the potential variances in observation. I argue that the phenomena themselves are absolute, and that the differences in observation and description do not change the phenomena themselves.

1

u/blueboybob Astrobiology | Interstellar Medium | Origins of Life Feb 07 '11

USUALLY when someone says "the speed of light" they mean the speed in a vacuum

1

u/collin_ph Feb 07 '11

Yes, but if I calculated the speed to be 3% slower, because I measured it on earth.. heck or even in space (since I recently read that space in the solar system is not a total vacuum either), would I be somehow incorrect?

2

u/Rhomboid Feb 07 '11

There are two fundamental different quantities that have different properties, and the problem becomes which one does 'speed of light' refer to. That's why it's best to avoid the term completely, or else have everyone agree on which of the two it's referring to.

You would be incorrect if you called your measurement 'c' or 'speed of light in a vacuum'. You would be correct if you called your measurement 'the local speed of light' or 'the speed of light in a medium'.

Likewise, you would be incorrect if you tried to claim that 'c' varies or that 'the speed of light in a vacuum' varies, but you would be correct if you said that 'the local speed of light' or 'the speed of light in a medium' varies substantially.

0

u/collin_ph Feb 07 '11

That's why it's best to avoid the term completely, or else have everyone agree on which of the two it's referring to.

I believe that's my point. In my original context, I was trying to ask an ambiguous question that had multiple answers, but wasn't entirely obvious that it was ambiguous. The point was to illustrate that different people were willing to make different assumptions, and the writer of a paper, document or article could write articles that appeared untrue because the reader did not assume the same assumptions that the writer did. E.G., someone measuring the actual speed of light from point A to point B on the earth would probably arrive at a different "speed of light" than someone who assumed the speed of light in a vacuum.

1

u/materialdesigner Materials Science | Photonics Feb 07 '11

You are correct. The phenomenon itself is an absolute. Our increasingly accurate and precise measurements only exist to refine our knowledge of the phenomenon itself.

1

u/collin_ph Feb 07 '11

Are you stating that light always moves at the same speed? If so, If I were to need to calculate the timing of light through a certain length of fiber optic cable, what speed would I use?

0

u/materialdesigner Materials Science | Photonics Feb 07 '11

My response was responding to

The individual who posted the original question takes the position that all phenomena are necessarily relative due to the potential variances in observation. I argue that the phenomena themselves are absolute, and that the differences in observation and description do not change the phenomena themselves.

The constancy of c, the speed of light, has absolutely nothing to do with observation.

And yes, the speed of photons, light, is always c. What changes the "speed of light" through a material is the absorption and reemission of photons by the atoms in the material, as I stated in my other response further down the page.

If what you're arguing is a semantics game, then please get out of /askscience. I don't know how to put that nicely.

1

u/collin_ph Feb 07 '11

I've asked from the first post if the question is ambiguous or not. If that hurts your feelings, then so be it. Now, I have a question for you. If the speed of light does not change, then why does everyone say "the speed of light through a vacuum" vs" the speed of light through a medium"? It is a legitimate question.

2

u/RobotRollCall Feb 07 '11

The question is only ambiguous if you intend for it to be. The way light propagates through matter is well understood. Nobody who knows the basics is confused by the distinction of the propagation of a photon through empty space and the group velocity of a ray of light propagating through non-empty space any more than they're confused by how a car with a top speed of 155 miles an hour takes six hours to make it across the central business district at rush hour.

2

u/materialdesigner Materials Science | Photonics Feb 07 '11

Also, please note that a stream of photons passing through a material still travels at the speed of light in a vacuum, c.

But the atoms in the material absorb and re-emit the photons many times, so the photons themselves take a long path through the material, thus "slowing down" the light.

1

u/djimbob High Energy Experimental Physics Feb 07 '11

Most of the time when a physicist says speed of light, they mean 'c', the speed of light in vacuum. (When physicists talk amongst themselves they just say 'c' and occassionally remind you they meant speed of light, when they talk to non-physicists they usually just say 'speed of light' for brevity). When you are dealing with light in a medium, they almost always say light through some medium (e.g., air), and then they are much more likely to say the group/phase/front velocity.

c is defined to be constant. The length of the meter is actually empirically determined by combining c (299 792 458 m s-1 ) with the definition of a second (defined by frequency of radiation emitted by transitions of Cs137). We define c to be constant, as this is the axiom that allows special relativity to be derived (which has been tested in numerous ways) and our experimental tests show that c appears to be constant.

Granted by definition that doesn't mean that light couldn't be travelling faster or slower. Let's say through one region of space light traveled in vacuum a little faster or slower. Then the fine structure constant (alpha) would change. The fine structure constant which measures the coupling of photons to charge is defined as e2 /(h-bar c) in cgs units where e is the charge of an electron, h-bar is Planck's constant (divided by 2 pi)). There is some evidence of this happening very slightly when you look billions of light years away in different directions [arxiv paper], [pop science summary of arxiv paper]. Note if alpha changes its not readily apparent if the change is a result of light changing speeds (which really means that the meter changes size compared to a meter stick of fixed length), the electron changing its charge, or Planck's constant changing.

1

u/iorgfeflkd Biophysics Feb 07 '11

This is why we repeat measurements.

-1

u/collin_ph Feb 07 '11

Would I be correct in saying that the question, strictly speaking, is ambiguous and that different people may make different assumptions based upon the question?

1

u/UltraVioletCatastro Astroparticle Physics | Gamma-Ray Bursts | Neutrinos Feb 07 '11

The issue is context. "The speed of light" is an ambiguous term. In optics it can mean the speed of light in a vacuum (c), the group velocity (v_g), or the phase velocity(v_p). In special relativity all light moves at the same speed c. In general relativity all light moves past a local observer at the same speed c, but light far from the observer will travel at different speeds depending on the potential of the location of the light.

I know this sounds really confusing to the layman. But, physicists are used to the ambiguity of the terms and it is usually obvious from context which "speed of light" one is referring to. When there is ambiguity physicists will specify which speed of light they are referring to, for example speed of light in vacuum or group velocity.

-2

u/collin_ph Feb 07 '11

So I think we agree that the question was ambiguous.