C in a Vacuum: Evidence of Constant?

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Discussion Overview

The discussion revolves around the concept of the speed of light in a vacuum, specifically questioning the evidence supporting its status as a constant or invariant quantity. Participants explore the implications of this idea within the context of Einstein's theories and empirical testing.

Discussion Character

  • Exploratory
  • Debate/contested
  • Technical explanation

Main Points Raised

  • Some participants express uncertainty about direct experimental evidence confirming the speed of light as a constant.
  • One participant emphasizes that in empirical science, theories are not proven but are tested against predictions, citing the Michelson-Morley experiments as foundational.
  • Another participant seeks clarification on what is meant by "constant," suggesting that it may refer to invariance across different inertial frames rather than constancy over time.
  • There is a reference to a FAQ that may provide additional context on the experimental basis of special relativity, particularly regarding moving sources of light.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the existence of direct experimental evidence for the speed of light as a constant. Multiple viewpoints are presented regarding the interpretation of "constant" and the nature of supporting evidence.

Contextual Notes

There is a lack of clarity regarding the definitions of "constant" and "invariant," which may affect the discussion. The conversation also highlights the dependence on historical experiments and the implications of the theory of relativity.

pkc111
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TL;DR
What evidence is there that the speed of light in a vacuum is an absolute constant. "Absolute" in the sense of invariant to an observer in any inertial frame of reference.
I understand that it is one of Einstein's postulates, however I don't know of any direct experiments which show that it is true.
 
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In physics (as well as any other empirical science), you generally do not prove things. What you do is that you take a theory that makes certain predictions and then you go out and test those predictions empirically by performing experiments that could, a priori, falsify the theory.

In the case of the speed of light in vacuum, the original introduction as an assumption in the theory relied on experiments such as the Michelson-Morely experiments. However, the resulting theory also makes countless other predictions that have been tested to extreme levels of accuracy, such as the prediction that any massive particle cannot reach the speed of light in vacuum regardless of how much energy you provide. For example, observing an electron traveling faster than c would have falsified the theory.
 
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Thanks I understand your points.
I wasnt very clear.My question is what is the evidence that supports that c in a vacuum is a constant. I am not really talking about the experimental evidence for Special Theory of Relativity and the evidence for time dilation, mass dilation (electrons having speed limits etc) or length contraction etc. Although I can see your point if the STR was found not supported then perhaps one of the assumptions was not true.
Im referring more to specific evidence which has been gathered about the speed of light as an absolute constant.
 
Just to clarify, what do you mean by "constant" here? Do you mean that it doesn't change over time, or that it is the same in all inertial frames? Strictly, "constant" means the former, but I suspect you mean the latter (which is properly called "invariant").
 
Ibix said:
Just to clarify, what do you mean by "constant" here? Do you mean that it doesn't change over time, or that it is the same in all inertial frames? Strictly, "constant" means the former, but I suspect you mean the latter (which is properly called "invariant").
I mean "absolute constant", in the sense invariant.
 
pkc111 said:
I mean "absolute constant", in the sense invariant.
See section 3 of the FAQ linked from the thread weirdoguy linked, particularly the moving sources section.
 
Thanks Weirdoguy and Ibix I think that's exactly what I am after.
 
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