The evolution of scientific thought from Newton to Einstein

by A. D'Abro

This Lorentz-FitzGerald contraction hypothesis was a hypothesis formulated ad hoc for the sole purpose of explaining the null result of Michelson’s experiment; and like all such hypotheses, it could be accepted only provided it was justified by a wide variety of experiments. At the time this hypothesis was suggested, scientists were busying themselves, with the study of the electron when in motion through the ether. Abraham assumed that an electron moving at high speed through the ether would remain spherical and rigid; and he worked out mathematically the path a rigid electron would pursue when acted upon by electric and magnetic forces. Lorentz, holding to his view of the FitzGerald contraction, attacked the same problem, assuming the electron to contract more and more as its speed through the ether increased. Experimenters were called upon to decide between the hypotheses of the rigid and the contractile electron. Accordingly, highly refined experiments were conducted on the electrons emitted by radium (

rays). Kaufmann, by photographing the paths followed by the electrons, came to the conclusion that Abraham’s rigid electron corresponded to reality. But, still more precise experiments conducted by Bucherer reversed the verdict, and little doubt remained that the electron suffered the FitzGerald contraction in the precise degree demanded by Lorentz.

Inasmuch as matter was assumed to be electronically constituted, these experiments of Bucherer lent weight to the FitzGerald contraction of matter, hence to the Lorentz-FitzGerald explanation of Michelson’s null result. This contraction being seemingly established, attempts were made by experimenters to detect it in bodies which were attached to the earth. Could this result be achieved, we should have a means of ascertaining the earth’s velocity through the ether; and it was confidently expected that this time success would attend us.

Now, as we have mentioned, if in our frame of reference, on the earth’s surface, for instance, all things, including our measuring rods and our own bodies, contracted in exactly the same way owing to the motion of our planet through the ether, it would, of course, be idle for us to expect this contraction to be observable by direct measurement. But although not observable in a direct way, it was argued that this contraction of a body should modify its optical and electrical properties so that a study of the variations in these properties when the body was placed successively in different directions should yield us the earth’s motion through the ether. In particular the contraction of a solid transparent body, owing to its motion through the ether, should give rise to a phenomenon of double refraction, much as would follow from an ordinary physical compression of the body. The experiment of Rayleigh and of Brace was based on this idea, but the result was a complete disappointment. The FitzGerald contraction gave rise to no such effect, and a negative result was registered. The experiment of Trouton and Rankine aimed to detect a variation in the electrical conductivity of a strip of metal as a result of the FitzGerald contraction. Once more a negative result was the outcome. These experiments having failed, still another was suggested and carried out by Trouton and Noble. The essence of this experiment was based on the well-known phenomenon verified by Rowland, that an electrified body in motion produces all the effects of an electric current. If, therefore, two electrified bodies were placed side by side, the motion of the earth through the ether should produce two parallel electric currents; and since parallel electric currents attract one another, a very sensitive torsion balance should be able to detect this attraction. Again, the result of this experiment was negative; the velocity of the earth through the ether evaded us completely.

Now, it should be noticed that the last two experiments pertain purely to electricity and electrodynamics; there is nothing of an optical nature about them. Hence, it would appear that all electromagnetic phenomena behave in exactly the same way, regardless of the velocity of the ether hurricane. In other words, they, too, yield negative results. But there is yet another aspect to these experiments.

If we regard the FitzGerald contraction as physically real—and we must do so in order to explain Michelson’s negative result—we must recognise that it is certainly compensated in a miraculous way; since no experiments, whether optical or electrical, appear to be capable of revealing it. Furthermore, in the last experiment, that of Trouton and Noble, we are not dealing primarily with the FitzGerald contraction; we are here in the presence of an experiment of electrodynamics, and we are endeavouring to detect a purely mechanical force arising as a result of the motion of our electrified bodies through the ether. The laws of electrodynamics require that such a force be generated, and yet no force can be observed; so we must assume that the mysterious compensations that conceal our motion through the ether are of so perfectly general a nature that they affect mechanics as well as electromagnetics and optics.

True, none of the experiments we had been able to execute exceeded the second order in precision; and there was nothing in Lorentz’s theory, even when the FitzGerald contraction was taken into consideration, to suggest that all experiments, whatever their nature and however precise they might be, would fail to detect our velocity through the ether. A very simple hypothetical experiment will make this point clear. Suppose, for instance, that the earth were rushing through the ether with a speed approximating that of light; and suppose that we suddenly stepped in front of a mirror. All that the FitzGerald contraction hypothesis would ensure would be that the time required by the light rays to leave our body and be reflected back to our eyes should be independent of the orientation of the mirror with respect to the earth’s motion. But the actual duration which the light rays would require, hence the time it would take for our image to appear to us, would depend essentially on the earth’s speed through the ether; so that theoretically, at least, a measure of duration should reveal this mysterious speed.

However, in view of the fact that the highly precise experiments that had been performed were incapable of detecting this velocity through the ether, it appeared legitimate to extrapolate, to assume that something deeper was at stake, and that in all probability this velocity would remain undetected, however far we might extend the precision of our electrodynamic or optical experiments, and however much we might vary their nature. Lorentz accepted what appeared to him as the inevitable, and asserted that the time had come to recognise that nature seemed to have entered into some giant conspiracy to defraud us of a knowledge of our velocity through the ether. Accordingly, he laid down his celebrated principle of correlation, according to which adjustments were so regulated in nature that the velocity of our planet through the ether could never be detected, however precise our experiments.

Lorentz applied his mathematical talents to the discovery of the necessary adjustments which would have to exist in nature for this correlation to be satisfied completely so far as electrodynamics and optics were concerned. In order to understand the significance of his investigations, we must recall what we have said of the laws of nature and of the space and time transformations.