Albert Einstein - Stafford Lectures (Meaning of Relativity)

Further, it follows that for two simultaneous events,

The invariance of the distance between the two points results from squaring and adding. From this easily follows the co-variance of Newton's equations of motion with respect to the Galilean transformation (21). Hence it follows that classical mechanics is in accord with the principle of special relativity if the two hypotheses respecting scales and clocks are made.

But this attempt to found relativity of translation upon the Galilean transformation fails when applied to electro-magnetic phenomena. The Maxwell-Lorentz electro-magnetic equations are not co-variant with respect to the Galilean transformation. In particular, we note, by (21), that a ray of light which referred to K has a velocity c , has a different velocity referred to K' , depending upon its direction. The space of reference of K is therefore distinguished, with respect to its physical propertiess, from all spaces of reference which are in motion relatively to it (quienscent ether). But all experiments have shown that electro-magnetic and optical phenomena, relatively to the earth as the body of reference, are not influenced by the tranlational verlocity of earth. The most important of these experiments are thos of Michelson and Morley, which I shall assume are known. The validity of the principle of special relativity also with respect to electro-magnetic phenomena can therefore hardly be doubted.

On the other hand, the Maxwell-Lorentz equations have proved their validity in the treatment of optical problems in moving bodies. No other theory has satisfactorily explained the facts of aberration, the propagation of light in moving bodies (Fizeau), and the phenomena observed in double stars (De Sitter). The consequence of the Maxwell-Lorentz equations that in a vaccum light is propegated with the velocity c , at least with respect to a definite inertial system K , must therefore be regarded as proved. According to the principle of special relativity, we must also assume the truth of this principle for every other inertial system.