The main assumption of the theory is that in any inertial frame of reference the form of the laws of nature are the same, in particular, the velocity of light has the same, constant value in any of these systems, that move with a constant velocity with respect to each other.
The consecuences of this postulate are far reaching: simultaneity between two events depends on the frame of reference, and time ceases to have an absolute meaning, it depends, too, on the frame of reference where it is being measured. (See [Time dilation]? and [Lorentz contraction]?).
There are a handful of people opposed to relativity on various grounds who have proposed various alternatives, mainly [ether theories]?. See also Theory of relativity, General relativity, and Albert Einstein.
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