If you ask a physicist to explain ether to you, he will either be embarrassed or he will say that ether does not exist, that it was rejected by Einstein in 1905. But, in1921, Einstein published "The ether and the theory of relativity". After his theory of general relativity, he became aware of the problem posed by deformable space and gravitational waves. He conferred with other physicists and recognized the existence of the Lorentz's ether, with no physical characteristics except immobility. An immobility that Einstein removed, if not a relative immobility in relation with the referential from which it is observed. We have found a hypothesis that could explain this mysterious movement: the speed of light would be everywhere the same because the ether would be seen as motionless everywhere. This hypothesis is very simple, and we propose experiments that are also quite simple to verify it.