Speed of Light Invariant in Vacuum: Experiment Proof

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Discussion Overview

The discussion revolves around the experiments that demonstrate the invariance of the speed of light in a vacuum. Participants explore various experiments, including the Michelson-Morley experiment, and question how light behaves in a vacuum, particularly in relation to moving observers.

Discussion Character

  • Exploratory
  • Debate/contested
  • Technical explanation

Main Points Raised

  • Some participants express confusion regarding which experiment proved the invariance of the speed of light, initially suggesting the Michelson-Morley experiment but noting its lack of a moving observer.
  • Others clarify that the Michelson-Morley experiment was indeed affected by Earth's motion and primarily demonstrated the isotropy of light speed.
  • There is a discussion about the limitations of observing light in a vacuum and how experiments can be conducted to study light's behavior without atmospheric interference.
  • Participants mention the use of vacuum chambers and mirrors to extend the path of light, referencing LIGO as an example.
  • One participant requests specific experiments that demonstrate the invariance of light speed with a moving observer in a vacuum, leading to a discussion about the nature of observers and the relevance of GPS technology as an example of light speed invariance in practice.
  • Another participant emphasizes that all experiments on Earth involve moving observers, as every point on Earth is in motion, and clarifies the meaning of "moving observer" in the context of light experiments.
  • There is mention of the importance of understanding "optical extinction" in evaluating the quality of vacuum in experiments.

Areas of Agreement / Disagreement

Participants do not reach a consensus on which specific experiment best demonstrates the invariance of the speed of light, and there are multiple competing views regarding the interpretation of existing experiments and the conditions under which they were conducted.

Contextual Notes

Participants highlight the complexity of defining "moving observer" and the implications of Earth's motion on experimental results. There are also discussions about the adequacy of vacuum conditions in experiments, which remain unresolved.

freshnfree
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I am getting confused as to which experiment proved that the speed of light was invariant in a vacuum. At first I thought it was the Michelson-Morley experiment but that does not have a moving observer.
 
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The MM experiment was attached to the earth, so it was moving at some point during the day and year. However, the MM experiment only detected the Isotropy of c. For a more complete account of the relevant experiments you should read the sticky thread at the top of the forum.
 
Dale said:
The MM experiment was attached to the earth, so it was moving at some point during the day and year. However, the MM experiment only detected the Isotropy of c. For a more complete account of the relevant experiments you should read the sticky thread at the top of the forum.
Hi, I have gone to the top of this forum but I am not sure of which sticky thread you are referring to.
 
freshnfree said:
Hi, I have gone to the top of this forum but I am not sure of which sticky thread you are referring to.
Oops, I thought this was in the relativity forum. I have moved the thread. I was referring to the "experimental basis of relativity" thread, but here is a direct link

http://www.edu-observatory.org/physics-faq/Relativity/SR/experiments.html
 
Last edited:
Just wondering, also, how we do we know what light is doing in a vacuum because we don't see light until it reaches our atmosphere?
 
freshnfree said:
how we do we know what light is doing in a vacuum
We pull a vacuum and do experiments.
 
What kind of experiments?
 
freshnfree said:
What kind of experiments?
This is described in detail in the link above.
 
we cannot create very large vacuums and light travels incredibly fast. i cannot see any specific experiment that refers to this type of scenario.
 
  • #10
freshnfree said:
we cannot create very large vacuums and light travels incredibly fast. i cannot see any specific experiment that refers to this type of scenario.
what type of a scenario?

There are many listed experiments done in vacuum. I don't know what being large or small has to do with anything. Just search with the word vacuum.
 
  • #11
Also, you can put mirrors at each end of a vacuum chamber and let the light bounce multiple times to increase the effective length of the chamber - as LIGO does with its 4km long high-vacuum arms.
 
  • #13
Dale said:
what type of a scenario?

There are many listed experiments done in vacuum. I don't know what being large or small has to do with anything. Just search with the word vacuum.
The scenario I am referring to is one which would show that the speed of light is invariant. So we would have a vacuum and a moving observer and a light wave. Could you please tell me of a known experiment of that type of scenario.
 
  • #14
freshnfree said:
The scenario I am referring to is one which would show that the speed of light is invariant. So we would have a vacuum and a moving observer and a light wave. Could you please tell me of a known experiment of that type of scenario.

I provided a link to several in my post above.
 
  • #15
freshnfree said:
The scenario I am referring to is one which would show that the speed of light is invariant. So we would have a vacuum and a moving observer and a light wave. Could you please tell me of a known experiment of that type of scenario.
Every experiment performed on Earth involves a moving observer, as every point on the surface of the Earth is moving. Do you mean an observer moving relative to the source of the light wave?

If so, the correct operation of the GPS system is an example. All the GPS satellites are moving relative to one another and the surface of the Earth while continuously exchanging light (at radio wavelengths) signals. Furthermore, different points on the surface of the Earth are moving in different directions at different speeds (because of the Earth's rotation). GPS positions are calculated using a constant speed of light, and would be way off if the motion of either the transmitters or receivers affected the speed of light.
 
  • #16
freshnfree said:
The scenario I am referring to is one which would show that the speed of light is invariant.
All of the experiments in sections 3.1-3.4 do that.

freshnfree said:
So we would have a vacuum and a moving observer and a light wave.
The phrase "a moving observer" doesn't mean anything because every observer is at rest in one frame and moving in all other frames. I think that you mean "an observer moving relative to the source". This would include all of the experiments in 3.3. Be sure to read the explanation of "optical extinction", which is the key criticism regarding whether an experiment has a good enough vacuum.
 

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