The reason for this statement is easily understood. The fundamental law of Newtonian mechanics states that the force acting on a body is equal to the mass of the body (an invariant) multiplied by the acceleration. Now, the acceleration of a body as computed in any Galilean frame is always the same; for the various Galilean frames differ only in their velocity through absolute space, and a velocity added to or subtracted from an acceleration can never modify an acceleration. The mass of the body being an invariant according to classical mechanics, the force must also measure out the same in all Galilean frames. In other words, it was essential to classical science that the fundamental law of mechanics should remain the same in all Galilean frames, and similar conclusions would have to apply to all the mechanical laws. But then no mechanical experiment, however precise, executed in one Galilean frame or another, could ever yield varying results; so that absolute velocity could never be known. In short, if Newtonian science were to stand, the failure of mechanical experiments to detect the absolute velocity of our Galilean frame through space could never be attributed to their lack of refinement, since any positive results would have overthrown the entire structure of Newtonian dynamics.
Here, then, was an absolute velocity which was a reality since space was absolute, but which on no account could ever manifest itself!
It was, of course, possible to suggest that space had a dual structure, partly absolute and partly relative; absolute for rotation and acceleration, but relative for velocity. But this intrinsic duality in the nature of space and motion was hard to accept; it seemed as though space and motion should be one thing or the other: either entirely absolute or else entirely relative. Moreover, it was extremely difficult to conceive of a duality in the structure of space, so the entire situation was most mysterious.
There was, however, a possible way out of the difficulty, and that was to assert that absolute velocity was real, truly enough, and that it might perfectly well be detected provided we appealed to other types of physical experiment—to electrodynamic and optical experiments, for instance, and not solely to mechanical ones. But we shall see that precise experiment has since proved that these expectations were also doomed to disappointment; hence it appeared perfectly clear that no manner of experiment, mechanical, optical or electrodynamic, would ever reveal this elusive absolute velocity.
Even had optical and electromagnetic experiments succeeded in detecting absolute velocity, everything would not have been plain sailing. It would then always have been possible to assert that the absolute velocity detected in this way was not absolute velocity through space, but velocity through the ether floating in space. But we may point out that this last objection carries very little weight. For reasons which we shall explain in the next chapter, it does not appear legitimate to dissociate the ether from space. Had optical experiments detected velocity through the ether, we should have had to assume that they had also disclosed the long-sought-for absolute velocity through space; and the Newtonian belief in absolute space would have been considerably strengthened.
However, it is unnecessary to dwell on what might have happened, seeing that optical and electromagnetic experiments have failed to reveal the slightest trace of absolute velocity. Accordingly we are led to Einstein’s special principle of relativity, which states that not only mechanically, but in every way, absolute velocity through space must escape empirical detection. Under the circumstances, unless we wish to retain in science an absolute velocity which can never be detected and which is therefore any one’s guess, there is no alternative but to state that space together with its ether has a dual structure: relative for velocity, absolute for acceleration. Einstein appears to have thrown us back on the extremely difficult task of conceiving of this dual nature for space. But the solution of this particular problem was forthcoming as soon as Minkowski discovered space-time. With four-dimensional space-time the reason for the duality becomes apparent; for the absoluteness of space-time (as will be explained in a later chapter) accounts for the relativity of velocity and the absoluteness of acceleration.
Thus in Einstein’s theory, in its original form at least, velocity remains relative and acceleration remains absolute, as in classical science,[37] the only difference being that the relativity of velocity is now more thorough, applying as it does to all manner of experiments and not merely to mechanical ones. It is only when we consider the more speculative part of the theory, that pertaining to the form of the universe, that the complete relativisation of all motion in the Machian sense occurs. Space-time is then found to be no longer an absolute existing per se but to be conditioned entirely by the matter of the universe. But, as we have already had occasion to mention, this part of the theory is still highly speculative, and in order to avoid unnecessary confusion, we would strongly advise the reader to eliminate it entirely from his mind until such time as he has acquired a proper understanding of the special and of the first part of the general theory. Accordingly we will ignore it for the present; and we may state that in Einstein’s theory motion possesses a dual nature, just as was the case in classical science.
CHAPTER XI
THE ETHER
CLASSICAL science assumed that, those manifestations we called electricity, magnetism, and light were nothing but strains, compressions and wavelike motions in an imponderable medium, the stagnant ether, floating in space. This belief was not altogether blind guesswork. In view of the stresses and strains that clearly appeared to surround electrified bodies, electric currents and magnets, and in view of the well-established wave nature of light propagation, some medium had to be postulated in order to give these stresses, strains and waves physical significance. Of this mysterious ether itself, practically nothing was known; but from what scant information could be gathered, it appeared to possess strange and contradictory properties.
Its resistance to motion was practically nil—far less than that of a gas like the atmosphere; since the frictional effects due to the atmosphere are sufficient to cause shooting stars to become incandescent, while no such effect was observed in those regions of ether-filled space extending beyond the limits of the atmosphere. Besides, if motion through the ether generated friction, mechanical phenomena should be influenced by the velocity of our Galilean frame through space and hence through the ether; but so far as experiment was able to decide, no such influences had ever been detected. (This was indeed the essence of the Newtonian principle of relativity.) It appeared, therefore, that the ether must be assimilable to some ultra-rarefied gas. On the other hand, the transverse vibrations of light waves were compatible only with the existence of a rigid medium; but between the properties of rigidity and of extreme rarefaction there was an obvious contradiction. Yet, however baffling and artificial the ether appeared to be, it was extremely hard to banish it from science owing to the great mass of experimental evidence that appeared to lend support to its existence.