Ritz remarked that Michelson’s experiment could be explained by assuming that, contrary to the requirements of the electromagnetic theory of light, waves of light were shot out from the luminous source just as bullets are shot out from a gun. With this species of ballistic theory, the speed of the light ray with respect to the observer would be given by the constant speed
of the ray with respect to its source, compounded with the relative speed of the observer and source. Now, there is not the slightest doubt that Ritz’s hypothesis affords an explanation of Michelson’s negative result, but, on the other hand, it leads us into serious difficulties when we wish to interpret a number of other experiments, Bucherer’s, for example. This can be understood when we realize that the Ritz hypothesis compels us to deny the existence of the FitzGerald contraction, since were the contraction to subsist in addition to Ritz’s assumption, Michelson’s experiment would no longer yield a negative result. But, as we have said, the FitzGerald contraction permits a very simple explanation of the moving electron’s increase in mass in Bucherer’s experiment. This difficulty would not be insuperable, but still it would constitute an argument adverse to the acceptance of Ritz’s idea. Other experiments which it would be extremely difficult to account for are those connected with the Fresnel convection coefficient, e.g., Fizeau’s experiment.
Furthermore, with Ritz’s conception of the velocity of a light wave through the ether being compounded with the velocity of the source, the frequency of light issuing from a source, moving towards us through the ether, would be increased, whereas the wave length would remain unchanged. Now, it so happens that the angle of refraction of a monochromatic light ray passing through a prism depends on the frequency of the incident light, whereas the dispersion caused by a grating depends on the wave length. It should, therefore, be possible to determine whether frequency and wave length truly behaved like independent variables as demanded by Ritz’s hypothesis. Experiments conducted by Stark in 1910 on canal rays disproved Ritz’s anticipations. Subsequently other experiments led to the same results.
Besides these objections to Ritz’s views, it must be realised that Michelson’s experiment was only one of many. Here it cannot be emphasised too strongly that the gravity of the situation arose from the fact that as a result of a large number of different experiments (many of which we have not mentioned), the entire structure of classical electrodynamics seemed to be crumbling on every side. Ritz’s hypothesis consists in shoving one individual brick back into place. Lorentz, thanks to his mathematical knowledge, realised that something much deeper was at the root of all the trouble and that more radical means would have to be adopted. Accordingly, he proceeded to tackle the very foundations of science, namely, the space and time transformations which had endured for centuries. But Lorentz only proceeded in a half-hearted way and did not have the courage to push his discoveries to their logical conclusion, contenting himself with patching up rather than reconstructing. Einstein, as we shall see, took the bull by the horns, abandoned the classical structure, and proceeded to build up an entirely new edifice, superbly coherent and free from all artificial support and scaffoldings. Thus it was that even before Ritz’s hypothesis was disproved by numerous direct investigations (experiments of Majorana and of Sagnac, and astronomical observations on double stars by de Sitter), scientists had ceased to give it serious consideration.
PART II
THE SPECIAL THEORY OF RELATIVITY
CHAPTER XIII
EINSTEIN’S SPECIAL THEORY OF RELATIVITY
HOWEVER unsatisfactory, for the numerous reasons already discussed, Lorentz’s theory might appear, it constituted for some years the only possible explanation of the negative results of all electromagnetic experiments performed on the earth’s surface. But in 1905 Einstein published a paper on the “Electrodynamics of Moving Bodies,” a document destined to prove one of the most epochal events in the history of human thought. In this paper, all the cobwebs of electrodynamics were swept aside.
The essence of Einstein’s position was that the difficulties which had beset the study of electrodynamics (we have mentioned only a small number of them) had arisen from our retention of the stagnant ether as a fundamental frame of reference; but that in view of the anomalies this attitude had brought about, the time had come to submit it to a critical analysis. After all, we knew little or nothing about the ether; why then start by stating it to be an absolute medium floating in space and hampering thereby all future progress? Why state that velocity through the ether must have a physical significance, then under the evidence of the negative experiments proceed to postulate complicated hypotheses in order to explain away the absoluteness we had ourselves introduced? Would it not be simpler to adopt a more cautious attitude, deriving knowledge from experiment rather than trying to reconcile experiment with a series of a priori beliefs which, for all we knew, might be totally erroneous?
If with Einstein we follow this line of argument, we must assume that the large number of negative experiments prove conclusively that velocity through the stagnant ether or through space is physically meaningless, not only from a purely mechanical point of view, as was the belief of classical science, but from every point of view. If we accept this relativity of the ether and space for Galilean motion as a fundamental fact of nature, holding for all manner of experiments, our past difficulties are seen to be of our own making. We had endeavoured to discover that which was meaningless, and then blamed nature for tricking us and hiding it from us. Instead of saying that velocity through the ether appeared meaningless because it was meaningless, we had complicated matters by saying that velocity appeared meaningless but of course was not meaningless.