# Speed of light

Did Einstein ever say how he came up with the fact that the speed of light is always constant .

Maxwell's equations

Did Einstein ever say how he came up with the fact that the speed of light is always constant .
It's a postulate, not a fact. And not that it is always a constant, but only in a vacuum. I am not aware of any experiment that contradicts this postulate. That's the best you can do with a postulate.

A. Einstein said:
We ... introduce another postulate, which is only apparently irreconcilable with the former, namely, that light is always propagated in empty space with a definite velocity c which is independent of the state of motion of the emitting body.

http://www.fourmilab.ch/etexts/einstein/specrel/www/" [Broken]

Unfortunately, I cannot answer your question. I don't know how he came up with that idea. He was aware of the failure of some experiments to measure the speed of the earth through the 'luminiferous ether' and that may have had something to do with it.

A. Einstein said:
Examples of this sort, together with the unsuccessful attempts to discover any motion of the earth relatively to the "light medium,'' suggest that the phenomena of electrodynamics as well as of mechanics possess no properties corresponding to the idea of absolute rest.

From the same article.

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i see thanks for the responses.