In his theory of Special Relativity, Einstein, as we saw, interprets phenomena without introducing the ether, or at least without introducing the kinematic properties which are usually attributed to it. In other words, Special Relativity neither affirms nor denies the existence of the classic ether. It ignores it. But this indifference to or disdain of the ether disappears in the theory of General Relativity. We saw in a previous chapter that the trajectories of gravitating bodies and of light are directly due, on this theory, to a special curvature and the non-Euclidean character of the medium which lies close to massive bodies in the void—that is to say, ether. This, therefore, though Einstein does not give it the same kinematic properties as classic science did, becomes the substratum of all the events in the universe. It resumes its importance, its objective reality. It is the continuous medium in which spatio-temporal facts evolve. Hence in its general form, and in spite of the new kinematic attitude which is ascribed to it, Einstein’s general theory admits the objective existence of ether. Stellar aberration shows that this medium is stationary relatively to the orbital motion of the earth. The negative result of Michelson’s experiment tends, on the contrary, to prove that it shares the earth’s motion. The Fitzgerald-Lorentz hypothesis solves this antinomy by admitting that the ether does not really share the earth’s motion, but saying that all bodies suddenly displaced in it are contracted in the direction of the movement. This contraction increases with their velocity in the ether, which explains the negative result of the Michelson experiment. Lorentz’s explanation seemed to Einstein inadmissible on account of certain improbabilities which we pointed out, and especially because it assumes that there is in the universe a system of privileged references which recalls Newton’s “absolute space.” Einstein, taking his stand on the principle that all points of view are equally relative, does not admit that there are in the universe privileged spectators—spectators who are stationary in the ether—who could see things as they are, whereas these things would be deformed for every other observer. Then, while preserving the Lorentz contraction and the formulæ in which it is expressed, Einstein says that this contraction, while it really exists, is only an appearance, a sort of optical illusion, due to the fact that the light which shows us objects does not travel instantaneously, but with a finite velocity. This spread of light follows laws of such a nature that apparent space and time are changed in precise accordance with the formulæ of Lorentz. That is the foundation of Einstein’s Special Relativity. Hence the two immediate possible explanations of the negative result of the Michelson experiment are: