C in a Vacuum: Evidence of Constant?

In summary, the speed of light in a vacuum is a constant, and is not affected by such things as mass or velocity.
  • #1
pkc111
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TL;DR Summary
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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  • #2
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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  • #3
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.
 
  • #4
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").
 
  • #5
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.
 
  • #7
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.
 
  • #8
Thanks Weirdoguy and Ibix I think that's exactly what I am after.
 
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1. What is "C in a Vacuum"?

"C in a Vacuum" refers to the speed of light in a vacuum, which is a fundamental constant in physics. It is denoted by the letter "c" and has a value of approximately 299,792,458 meters per second.

2. Why is the speed of light in a vacuum considered a constant?

The speed of light in a vacuum is considered a constant because it has been measured to be the same value regardless of the observer's frame of reference. This means that no matter how fast an observer is moving or where they are located in the universe, the speed of light will always be the same.

3. How is the constant "c" measured?

The constant "c" is typically measured using experiments involving the interference of light or the behavior of electromagnetic waves. These experiments have been refined over many years and have consistently shown that the speed of light in a vacuum is a constant value.

4. What is the significance of the constant "c" in physics?

The constant "c" has a significant role in many areas of physics. It is a fundamental constant in Einstein's theory of relativity and is used to define the units of length and time in the International System of Units (SI). It also plays a crucial role in understanding the behavior of electromagnetic waves and the structure of the universe.

5. Is there any evidence that the speed of light in a vacuum is not constant?

To date, there has been no evidence that the speed of light in a vacuum is not constant. Numerous experiments and observations have consistently shown that the speed of light is a constant value. However, there are theories that suggest the possibility of the speed of light varying in certain extreme conditions, such as near a black hole, but these theories are still being studied and have not been confirmed.

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