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 Emeritus PF Gold P: 8,147 It's an empirical fact. The first hint of it was the Michelson-Morley experiment. Voigt saw right away this could be explained if the speed of light was independent of its source. Then Lorentz, trying to clean up the Maxwell theory of electromagnetism developed his Transformations that mixed time with space. Finally Einstein deduced the transformaions from the fact that light speed is independent of its source. Later it was found that the postulate that every inertial observer measures the same speed of light was equivalent and made easier derivations.