Light Speed Invariance: Experiments, Difficulties & Clarification

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Discussion Overview

The discussion centers around the invariance of the speed of light as it relates to observers, particularly whether there are experiments that demonstrate this invariance independent of the motion of the observer. Participants explore the implications of Special Relativity (SR) and the equivalence of source and observer motion, while questioning the existence of direct experimental evidence for these concepts.

Discussion Character

  • Debate/contested
  • Exploratory
  • Technical explanation

Main Points Raised

  • Some participants inquire about experiments that directly prove the invariance of light speed to observers, distinguishing this from experiments showing invariance to source motion.
  • Others argue that the motion of the source and observer is equivalent due to the principle of relativity, suggesting that any experiment demonstrating invariance to source motion also applies to observer motion.
  • A participant points out the Doppler effect as a relevant consideration, noting that it depends on relative velocity, but others challenge its applicability to the discussion of light speed invariance.
  • Some participants assert that the principle of relativity has been experimentally validated, implying that it supports the equivalence of source and observer motion.
  • There are repeated calls for specific experimental evidence that distinguishes between the invariance of light speed to source versus observer motion, with some expressing skepticism about the existing evidence.
  • One participant references the Michelson-Morley experiment as a historical example of testing light speed invariance, while others question whether such experiments adequately address the observer's motion.

Areas of Agreement / Disagreement

Participants do not reach a consensus on whether there are specific experiments proving the invariance of light speed to observers. The discussion reflects multiple competing views regarding the interpretation of existing experiments and the implications of the principle of relativity.

Contextual Notes

Participants express uncertainty about the definitions and interpretations of experiments related to light speed invariance, particularly concerning the roles of source and observer motion. There is also a noted lack of clarity on whether existing evidence sufficiently addresses the specific question of observer motion.

georgechen
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TL;DR
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?
 
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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.
 
georgechen said:
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.
 
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georgechen said:
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.
 
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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.
 
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georgechen said:
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.

georgechen said:
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.

georgechen said:
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.
 
georgechen said:
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.
 
  • #10
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.
 
  • #11
georgechen said:
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.
 
  • #14
Nugatory said:
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.
 
  • #15
georgechen said:
Think about the scenario of the sound wave

I already pointed out why considering sound is not relevant in this discussion. See post #7.
 
  • #16
georgechen said:
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.
 
  • #17
PeroK said:
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.
 
  • #18
georgechen said:
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.
 
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  • #19
georgechen said:
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.
 
  • #20
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?
 
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  • #21
georgechen said:
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.

georgechen said:
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.

georgechen said:
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.
 
  • #22
georgechen said:
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.
 
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  • #23
georgechen said:
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?
 
  • #24
Yes. That is exactly my question from the begining. Is there any experiment of testing light speed with moving observers?
 
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  • #25
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.
 
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  • #26
georgechen said:
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.
 
  • #27
georgechen said:
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.
 
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  • #28
georgechen said:
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.
 
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  • #29
PeroK said:
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?
 
  • #30
Nugatory said:
Yes. The successful operation of the GPS system.
Thank you. Could you please provide a link?
 
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