Light Speed Invariance: Experiments, Difficulties & Clarification

[georgechen]

TL;DR Summary

There are many experiments proving the constant light speed independent of the motion of source. Is there any experimental proof of the invariance of light speed to observers?

Let me clarify my question, is there any experiment directly proved the invariance of light speed to observers? Let's not get to the argument of equivalence between source and observer.

SR was based on the postulate that the light speed is constant and independent of both the motions of source and observer. There are many experiments proving the constant light speed independent of the motion of source. Is there any experimental proof of the invariance of light speed to observers? If not, what is the difficulty?

 

[PeroK]

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

 

[georgechen]

Thanks. That was the place that I got my question. You can see only the experiments for
No dependence on source velocity or energy

But not really experiments proving the light speed is invariant to observer velocity.

 

[PeroK]

Thanks. That was the place that I got my question. You can see only the experiments for
No dependence on source velocity or energy

But not really experiments proving the light speed is invariant to observer velocity.

It's the same thing isn't it? If the source is moving relative to the observer, then the observer is moving relative to the source.

 

[Nugatory]

Is there any experimental proof of the invariance of light speed to observers?

Every experiment that shows that the motion of the source doesn’t affect the speed is also a test of the receiver moving - we can choose to interpret either source or receiver to be at rest.

The GPS system is a working example of a large real-life system in which sources and receivers are all in motion relative to one another, and it wouldn‘t work if the speed of light were not invariant.

 

[georgechen]

No. That is not correct. Think about Doppler effect. The source movement is not equivalent of the observer's.
Even this is correct, why no one perform an experiment of moving observer vs. source.

 

[PeterDonis]

The source movement is not equivalent of the observer's.

Yes, it is; that is what the principle of relativity says. Only relative velocity is physically meaningful.

Think about Doppler effect.

The Doppler effect for light in vacuum depends on relative velocity.

If you are thinking of the Doppler effect for sound, sound doesn't travel in vacuum, only in a medium, so velocities relative to the medium become meaningful.

why no one perform an experiment of moving observer vs. source.

There is no such thing. There are only experiments in which the source and observer are in relative motion. The fact that Wikipedia describes such experiments as varying "source velocity" is a matter of Wikipedia's choice of words, not physics.

 

[Nugatory]

Think about Doppler effect. The source movement is not equivalent of the observer's.

We’re talking about the speed of a light in a vacuum, and the Doppler effect there is symmetrical and depends only on the relative speed between source and receiver.

 

[PeroK]

Think about Doppler effect.

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

 

[georgechen]

Think about the scenario of the sound wave from a supersonic jet and an observer in another supersonic jet in front. The source jet movement doesn't impact the speed of sound, otherwise the sound will be supersonic. But the observer jet movement does impact, because the sound wave will never reach the observer.

 

[PeroK]

Think about the scenario of the sound wave from a supersonic jet and an observer in another supersonic jet in front. The source jet movement doesn't impact the speed of sound, otherwise the sound will be supersonic. But the observer jet movement does impact, because the sound wave will never reach the observer.

SR postulates the invariance of the speed of light in vacuum; not the speed of sound in air.

 

[georgechen]

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

Yes. The equivalence is based on the postulates of Special Relativity. But is there any experiment proving the equivalence? Otherwise, it is cross-reference.

 

[PeroK]

Yes. The equivalence is based on the postulates of Special Relativity. But is there any experiment proving the equivalence? Otherwise, it is cross-reference.

The relativistic Doppler effect is well tested. E.g.

https://www.osapublishing.org/josa/abstract.cfm?uri=josa-52-5-529

 

[georgechen]

We’re talking about the speed of a light in a vacuum, and the Doppler effect there is symmetrical and depends only on the relative speed between source and receiver.

Yes. You are right. Doppler effect doesn't prove there is no equivalence. My question is any experimental proof of the equivalence? Doppler effect is an example that we should not take it for granted.

 

[PeterDonis]

Think about the scenario of the sound wave

I already pointed out why considering sound is not relevant in this discussion. See post #7.

 

[PeterDonis]

any experimental proof of the equivalence?

Every experimental proof of the principle of relativity--that the laws of physics are the same in all inertial frames--is a proof of the equivalence. There are lots of experimental proofs of the principle of relativity.

 

[georgechen]

SR postulates the invariance of the speed of light in vacuum; not the speed of sound in air.

That is exactly my point. It is a postulate. Is there any experimental proof? There are many experiments for the invariance to source motion. But I didn't see any to observer motion. I used the example only to show that we should not take equivalence for grant.

 

[PeroK]

That is exactly my point. It is a postulate. Is there any experimental proof?

Not any experiments that would convince you, I imagine!

Seriously, there's no point in asking for experimental evidence of SR and then rejecting all evidence provided. You're just wasting our time, really.

 

[Nugatory]

Yes. You are right. Doppler effect doesn't prove there is no equivalence. My question is any experimental proof of the equivalence? Doppler effect is an example that we should not take it for granted.

I don’t understand what you mean when you say the Doppler effect should not be taken for granted - are you asking whether the symmetrical Doppler effect for light has been experimentally proven? It has been, many times over.

 

[georgechen]

I don't know why you are irritated. I am here for a sincere technical discussion.
1. There are many experiments proving the invariance of light speed to source.
2. Is there any experiment proving the invariance of light speed to observer? If not, what is the difficulty?
3. The claim that 1 is equivalent with 2 should not be taken for grant, I gave several unrelated examples. If that is true, any experiment proving the equivalence?

 

[PeterDonis]

It is a postulate. Is there any experimental proof?

Yes. The Michelson-Morley experiment has been re-run many times with more accurate equipment. Same result.

There are many experiments for the invariance to source motion. But I didn't see any to observer motion.

It has already been repeatedly pointed out to you that the principle of relativity, which has also been tested and confirmed by many experiments, is sufficient to show that there is no such thing as "source motion" or "observer motion", only relative motion.

I used the example only to show that we should not take equivalence for grant.

We aren't. We've demonstrated it in many experiments. See above.

 

[PeterDonis]

I don't know why you are irritated. I am here for a sincere technical discussion.

You're not behaving like it. You keep repeating the same misconceptions even after they have been repeatedly corrected. For the latest corrections, see my post #21.

 

[PeroK]

2. Is there any experiment proving the invariance of light speed to observer?

Would an experiment where the observer is not at rest satisfy you?

 

[georgechen]

Yes. That is exactly my question from the begining. Is there any experiment of testing light speed with moving observers?

 

[georgechen]

If there is such an experiment, it not only proved the light invariance to the observers. It also proved the equivalence. Then we don't need to have any argument.

 

[PeroK]

If there is such an experiment, it not only proved the light invariance to the observers. It also proved the equivalence. Then we don't need to have any argument.

That's an evasive answer! A simple "no" would have been more sincere.

 

[Nugatory]

It is a postulate. Is there any experimental proof?

It is a postulate, but we choose to use that postulate when we reason about how the real world works for the same reasons that when I choose to use the postulates of Euclidean geometry when I’m laying out plans for a construction project - there is an enormous amount of evidence to show that these postulates are also accurate statements about how things really are.

 

[Nugatory]

Yes. That is exactly my question from the begining. Is there any experiment of testing light speed with moving observers?

Yes. The successful operation of the GPS system.

 

[georgechen]

That's an evasive answer! A simple "no" would have been more sincere.

No. I am not evasive. If there is any doubt about SR, this is the only one. I believe you are very knowledgeable in SR. How about a direct answer? Is there any? If no, what is the reason?

 

[georgechen]

Yes. The successful operation of the GPS system.

Thank you. Could you please provide a link?

 

[weirdoguy]

Could you please provide a link?

https://www.google.com/search?client=firefox-b-d&q=GPS

 

[georgechen]

https://www.google.com/search?client=firefox-b-d&q=GPS

No. I meant the explanation of how GPS proves the invariance of light speed to moving observer.

 

[PeterDonis]

I meant the explanation of how GPS proves the invariance of light speed to moving observer.

This is like knowing that $2 + 3 = 5$ but asking for a proof that $3 + 2 = 5$. Nobody is going to bother to write down an explicit proof since it is already a well established fact that addition is commutative. Similarly, nobody is going to bother to write down an explicit explanation of how GPS proves that only relative motion is physically meaningful, since it is already a well established fact that the principle of relativity is true.

In other words, your question has been answered. The fact that you obstinately refuse to accept the answer does not mean the answer is invalid. It's valid.

Thread closed.

 

[Dale]

Is there any experimental proof of the invariance of light speed to observers?

I am going to abuse my mentor power to post the Experimental Basis of SR FAQ:

http://www.edu-observatory.org/physics-faq/Relativity/SR/experiments.html

All of the isotropy experiments and several of the Doppler experiments directly answer your question.