Enough has been said even at this stage to show that Einstein’s theory cannot be considered a mere mathematical dream. The extraordinary difficulties with which classical science was confronted owing to the negative results of the experiments we have mentioned (and to numerous other problems) have disappeared. With them have vanished the miraculous compensations which Lorentz was compelled to invoke in order to explain these negative results. The complete relativity of Galilean motion explains all our troubles. Nature is no longer mischievous, but the ether or space is relative for Galilean motion. We had failed to recognise this relativity which was staring us in the face; in so doing we ourselves were the creators of our difficulties. Yet rather than abandon the classical concepts of space and time, physicists in general refused to follow Einstein. To-day criticisms have subsided (on the part of the great majority of scientists) as experiment after experiment has confirmed Einstein’s previsions. The majority of experiments, however, concerned the consequences of the theory and not [Pg 153] its foundations. The experiments of de Sitter and Majorana have filled this gap by proving that a ray of light always passes the observer with the same invariant speed regardless of the relative velocity of observer and source. Now one of the most important criticisms that has been directed against Einstein’s theory is that it deprives us of the possibility of conceiving of an objective ether. There is not the slightest doubt that Einstein’s theory compels us to abandon our conception of the stagnant ether of classical science, with reference to which motion could be measured. But this is not quite the same thing as holding that the theory banishes the ether entirely. Nevertheless, upon first inspection, the fact that whatever our Galilean motion may be, experiments conducted in our frame will always yield exactly the same results, would seem to relegate the ether to the realm of ghosts, making it a useless hypothesis. If this were the case we could no longer conceive of electromagnetic fields and light rays as expressing states of the ether, but should have to regard these fields as constituting independent realities of some new category, differing from matter but susceptible of existence in space without the support of a carrier, or without being the mere manifestations of its states. However, in the general theory which we shall discuss in the second part of this book we shall find that the ether is reinstated in the guise of the metrical field of space-time. But as this new ether has only its name in common with the stagnant Lorentzian ether, there does not appear to be much advantage in retaining the older appellation.