# Speed of light in different frames

1. Dec 16, 2005

### michael879

ok one of the postulates of special relativity is that the speed of light is constant in any intertial reference frame right? Has there been any experimental proof of this? When I learned this, my teacher mentioned the michalson-moray (spellings probly wrong) experiment that disproved the aether theory but I dont see how it has anything to do with this question.

2. Dec 16, 2005

### Garth

If you swim up a river and back between two trees on the riverbank the speed of flow of the river affects the time it takes you to make the return trip. Going with the flow does not compensate for the extra time taken going against the flow, If you cannot swim as fast as the river flow then you go down stream very quickly but cannot get back at all.

If there was an aether then the time taken for light to travel in one direction would be different to the time taken in another direction as the Earth is moving around the Sun, the Sun around the galaxy and the galaxy through space against the CMB surface of last scattering.

Michelson and Morley performed an experiment in 1887 that could not detect any such variation. This more or less falsified the aether hypothesis and their result was finally explained by Einstein's theory of Special Relativity in 1905.

Garth

3. Dec 16, 2005

### michael879

I understand the experiment. I was saying how that experiment has nothing to do with my question, which as u explained it, clearly doesnt. My questionis simply, what experimental proof do we have that the speed of light is constant in all interial reference frames? (special relativity postulate #2)

4. Dec 16, 2005

### michael879

the fact that two perpendicular beams of light travel at the same speed doesn't prove the speed of light is constant in any reference frame, it just disproves an aether

5. Dec 16, 2005

### JesseM

But one of the features of the experiment was that they specifically made sure to repeat it at different times of year, when the earth was in different positions in its orbit and thus moving in different directions relative to any given inertial observer. So, it did show that the round-trip speed of light is the same in different directions in multiple different reference frames (whether you define the one-way speed of light to be the same too depends on how you synchronize your clocks--if you use Einstein's synchronization convention it will be).

6. Dec 16, 2005

### michael879

oo got it, but does the earth rly move at a speed comparable to light?

7. Dec 16, 2005

### michael879

and if you repeated this experiment with sound waves or water waves would you get different results?

8. Dec 16, 2005

### dicerandom

Relative to what? There are all kinds of objects in the universe which are moving at a good fraction of the speed of light relative to us, various energetic particles which encounter our atmosphere, distant galaxies, etc.

I think that your question hints at a deeper problem though, one of the hardest things about SR is removing the concept of absolute velocity from your thoughts. Speaking of velocities is only meaningful when you speak of relative velocities between two frames.

9. Dec 16, 2005

### michael879

I was talking about the experiment. Two perpendicular rays of light. If SR is wrong, light coming out of a moving flashlight should be > c, so the beam pointing with the earth's motion would be moving faster than the one perpendicular to it. AFter they bounce off all the mirrors they would destructivly interfere. However, in the experiment, the constructivly interfer, showing that the speed of light doesnt change. However, does this experiment rly prove that? I mean if you tried something like this with sound waves would you get destructive interference? and does is the earth's velocity great enough to make a noticeable difference? and I mean, the wavelengths of visible light is so small, if they had the distances off by a tiny bit, the results would be off right?

10. Dec 16, 2005

### dicerandom

Again, the Earth's velocity relative to what? The postulate of the experiment was that there is some ether background which absolute velocity can be measured with respect to, the result of the experiment disproved that. The accuracy of the experiment was such that they should have been able to pick any difference in the speed of light due to the earth's changing velocity (relative to some stationary frame) as we rotate around the sun. That is why they ran the experiment multiple times over the course of a year, to get a sampling of different directions that the earth could be moving relative to the ether.

I believe that an analagous situation can be set up with water or sound waves, yes. Don't ask me to describe how it could be done though, I'm no experimentalist

11. Dec 16, 2005

### Garth

My analogy of swimming in a river was to illustrate that the M-M experiment showed, after repeating the experiment at different times of the year, that it is not like "an analagous situation can be set up with water or sound waves". In SR there is no aether, no analogous 'water' or 'air' against which an absolute velocity can be measured, all velocities are relative to the observer. As well as vwelocities measurements of length and time durations area also observer, i.e. 'frame', dependent.

Garth

12. Dec 16, 2005

### Staff: Mentor

13. Dec 16, 2005

### Staff: Mentor

Yes! But there is really no need: It is easy enough to see in everyday situations that the speed of sound is realtive to the stationary air/water. If it weren't, supersonic planes wouldn't produce a shock wave and there'd be no thermoclines in water for sonar. Heck, string and wind instruments wouldn't work, - a great many situations where there we see mechanical waves are affected by the fact that these waves travel on a medium.

Prior to M&M, it was simply assumed that light must work the same way.

Last edited: Dec 16, 2005
14. Dec 16, 2005

### Staff: Mentor

"earthrly"?
Anyway, as others have noted, there's a question of "speed relative to what?" M&M were thinking in terms of the speed relative to the supposed all-pervading ether. There was no reason for expecting the solar system to have any particular velocity relative to the ether, but the earth does change its direction of motion as it goes round its orbit. So one would expect the earth's speed relative to the ether to vary during the course of a year, by an amount comparable to the earth's orbital speed around the sun. This speed is about 30 km/sec which is .0001 times the speed of light. A rather small fraction, to be sure, but the M&M apparatus should have been able to detect it. You can find the derivation of the expected shift in their interference pattern in many textbooks, and probably many web pages.

Last edited: Dec 16, 2005
15. Dec 16, 2005

### JesseM

If you were moving at different velocities relative to the medium, you'd get different results. On the other hand, if the medium was always at rest relative to you you'd always see the waves move at the same speed in your frame.