Motivated by the result of the Michelson-Morley experiment, Albert Einstein formulated the following principle of the constancy of light speed:
Any ray of light moves in the “stationary” system of co-ordinates with the determined velocity \(c\), whether the ray be emitted by a stationary or by a moving body.
A more common formulation of this principle is the following:
The speed of light is constant in vacuum in all inertial reference frames (i.e. it is independent from the movement of the inertial reference frame or the observer in it).
Definitions: 1
Proofs: 2
Propositions: 3