397.Exactly why is the speed of light a constant in all reference frames?

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Special & General Relativity Questions and Answers

Exactly why is the speed of light a constant in all reference frames?

Because it simply is, and this is an experimental fact of life that we have to deal with. It doesn't matter if you are on two cars moving in opposite directions flashing lights at one another, the total speed of the electromagnetic disturbance is the same, and is not augmented or reduced by your motion. The only thing that affects the speed of light is the refractive index of the medium through which it moves, and for empty space, this number is 1.000000 and gives you the maximum possible speed of light. In glass, it is 1.3333 times smaller since c/n = c/1.333 = 300,000 km/sec/1.333 = 225,000 km/sec.

The invariance of the speed of light in all uniformly moving reference frames is a postulate of special relativity, it does not derive from special relativity, which only then states how observers will experience/measure space and time given the invariance of the speed of light.

https://einstein.stanford.edu/co ... %20of%20the%20speed,of%20the%20speed%20of%20light.