The Ol' Switcheroo

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The bit about how the speed of light being constant was established before Einstein was born refers to Maxwell’s equations, according to which electromagnetic radiation has to travel at the same speed in all frames. Physicists eventually realized the light and EM waves were the same thing. This is something that isn’t always stressed in the teaching of the development of relativity, and one might get the impression that the postulate of c being invariant was just a guess.