Einstein and the universe: A popular exposition of the famous theory

by Charles Nordmann

This single hypothesis, which seemed to be capable of extricating physicists from the dilemma into which Michelson had put them, was first advanced by the distinguished Irish mathematician Fitzgerald, then taken up and developed by the celebrated Dutch physicist Lorentz, the Poincaré of Holland, one of the most brilliant thinkers of our time. Einstein would no more have attained fame without him than Kepler would without Copernicus and Tycho Brahe.

Let us now see what this Fitzgerald-Lorentz hypothesis, as strange as it is simple, really is.

But we must first glance at a preliminary matter of some importance. A number of able men have declared—after the issue, let it be said—that the result of the Michelson experiment could only be negative a priori. In point of fact, they argue (more or less), the Classic Principle of Relativity, the principle known to Galileo and Newton, implies that it is impossible for an observer who shares the motion of a vehicle to detect the motion of that vehicle by any facts he observes while he is in it. Thus, when two ships or two trains pass each other,[4] it is impossible for the passengers to say which of the two is moving, or moving the more rapidly. All that they can perceive is the relative speed of the trains or ships.

The men of science to whom I have referred say that, if Michelson’s experiment had had a positive result, it would have given us the absolute velocity of the earth in space. This result would have been contrary to the Principle of Relativity of classical philosophy and mechanics, which is a self-evident truth. Therefore the result could only be negative.

This is, as we shall see, ambiguous. There is, if I may say so, a flaw in the argument which has escaped the notice even of distinguished men of science like Professor Eddington, the most erudite of the English Einsteinians. It was he who organised the observations of the solar eclipse of May 29, 1919, which have, as we shall see, furnished the most striking verification of Einstein’s deductions.

In the first place, if Michelson’s experiment had had a positive result, what it would have indicated is the velocity of the earth in relation to the ether. But, for this to be an absolute velocity, the ether would have to be identical with space. This is so far from being necessary that we can easily conceive a space—to put it better, a discontinuity—between two stars that contains no ether and across which neither light nor any other known form of energy would travel.

When Eddington says that “it is legitimate and reasonable,” that it is “inherent in the fundamental laws of nature,” that we cannot detect any movement of bodies in relation to ether, and that this is certain “even if the experimental evidence is inadequate,” he affirms something which would be evident only if space and ether were evidently identical. But this is far from being the case. If Michelson’s experiment had had a positive result, if we had detected a velocity on the part of the earth, should we have discovered a velocity in relation to an absolute standard? Certainly not. It is quite possible that the stellar universe which is known to us, with its hundreds of thousands of galaxies which it takes light millions of years to cross, may be contained in a sphere of ether that rolls in an abyss which is devoid of ether, and is sown here and there with other universes, other giant drops of ether, from which no ray of light or anything else may ever reach us. It is, at all events, not inconceivable. And in that case, assuming that the ether has the properties attributed to it by classic physics, even if we had detected the movement of the earth in relation to it, we should not have discovered an absolute movement, but at the most a movement in relation to the centre of gravity of our particular universe, a standard which we could not refer to some other which would be absolutely stationary. The Classical Principle of Relativity would not be violated.

Hence, whatever may have been said to the contrary, the issue of Michelson’s experiment might, in these hypotheses, be either positive or negative without any detriment to Classical Relativism. As a matter of fact, it was negative, so nothing further need be said. Experiment has pronounced, and it alone had the right to pronounce.

These distinctions were not unknown to Poincaré, and he wrote: “By the real velocity of the earth I understand, not its absolute velocity, which is meaningless, but its velocity in relation to the ether.” Therefore the possibility of the existence of a velocity discoverable in relation to the ether was not regarded as an absurdity by Poincaré. He said: “Any man who speaks of absolute space uses a word that has no meaning.”

It is worth while noticing that in all this the development of Poincaré’s ideas betrays a certain hesitation. Speaking of experiments analogous to those of Michelson, he said: