[47] These results had already been anticipated by Lorentz, since as we have seen, he considered that his transformations were applicable to purely electromagnetic magnitudes. But Lorentz was always dealing with a fixed ether and with motion through the fixed ether; hence the interpretation of these discoveries was much less satisfactory.
[48] We must remember that it is only light in vacuo which is propagated with the speed
, hence which possesses an invariant velocity. Light passing through a transparent medium moves with a velocity which is less than
; hence in this case its velocity ceases to be invariant and is affected by the velocity of the medium, as also by that of the observer.
[49] The objective world of science has nothing in common with the world of things-in-themselves of the metaphysician. This metaphysical world, assuming that it has any meaning at all, is irrelevant to science.
[50] The identification of mass with energy necessitated by the Einstein theory allows us to obviate the paradoxical appearance of the following example: Consider an incandescent sphere radiating light in every direction. If placed at rest in some Galilean frame, it should remain there undisturbed, since the backward pressure exerted by the light radiation is distributed symmetrically round the sphere. But suppose now we view the incandescent sphere from some other Galilean frame moving away from the first with a velocity
. From this new frame the incandescent sphere will be moving with a velocity