February 17, 192 1] 



NATURE 



7«5 



in the atfreement of clock-time or solar time with 

 absolute time. It is in the mutual agreement of 

 the motions of clocks, earth, planets, moon, tides, 

 and the rest ; so that it matters not a whit whether 

 the time variable used in the calculations flows 

 uniformly on, or whether it is the merest variable 

 number devoid of any physical significance what- 

 «ver. 



Thus the great paradox, the stumbling-block 

 of absolute rotation, is not one of logic. Nature 

 thrusts it upon us. While we would all admit 

 that a priori rotation relative to empty space is a 

 meaningless term, yet there remain Newton's 

 bucket and I'oucault's pendulum; and, strive as 

 we will, Newton's way of looking at them remains 

 the simplest. We would all fain think that "ac- 

 celeration relative to empty space " is an empty 

 phrase, and yet Nature is such that Newton's 

 system of dynamics made modern science pos- 

 sible. Whatever we may say of Newton's defini- 

 tions, therefore, his space and time are natural 

 products of Nature's laboratory, to be purified, 

 perhaps, but not to be rejected as spurious. 



The nineteenth century was the ather age. 

 From .'\rago, in 1820, to Michelson and Morley 

 (1881-1905), followed by Trouton, Noble, 

 Kankine, and Brace, vain attempts were made 

 to discover the earth's velocity through the 

 aether. But what, after all, was this 

 elusive medium? To Faraday doubtless it 

 was as real as anything in the universe. His 

 resolute insistence on the need for a medium of 

 transmission of electrical action, his discovery of 

 the induction of currents by changing magnetic 

 fields, Maxwell's advocacy of his views, and the 

 demonstration of the electromagnetic nature of 

 light, led up directly to the electron theory of 

 Lorentz and Larmor, in which the aether appears 

 as the background of all material phenomena. 

 Vet the a-ther, after all, was but a name, and 

 the electron theory a formal scheme of relations; 

 like Newton's scheme, it was to be verified by its 

 correspondence with actual observation. When it 

 comes to observation, the only possibility is to 

 note the behaviour of material bodies. Thus the 

 a;ther with the as.scx-iated ideas of electric and 

 magnetic intensity ranks pari passu with New- 

 ton's absolute time and space. It has no likeness 

 with matter ; it is even doubtful whether it is of 

 such substantiality that any element of it has 

 identity. It has been .said of Larmor's account 

 of it that it reduces the universe to a .set of dif- 

 ferential equations, which, of course, is to a great 

 extent true of Newton's work also. 



But the notable thing about this theory is that 

 it leaves the problem of absolute motion and abso- 

 lute rotation exactly at the point where Newton 

 left it. It gives a reasonable account of the failure 

 of Michelson and Mj)rlcy and the rest to discover 

 the earth's velocity relative to the scthcr. just as 

 Newton's theory would show why its velocity in 

 absolute space cannot be determined. The athcr- 

 builders succeeded too well, and constnictcd, not 

 one. but an infinity of athcrs, any one having a 

 uniform transtatory motion relative to any other. 

 NO, 2677, VOL. 106] 



But in the new theory, as in the old, a body may 

 be said in an absolute sense to be devoid of rota- 

 tional motion. 



With the lack of determinateness in the 

 ajther goes a similar ambiguity in the 

 measures of time and space. Each of these 

 sethcrs has its proper scale of time and 

 space. Events which are simultaneous^ in 

 one aether are not simultaneous in another, and, 

 since none can tell which is the true aether, none 

 can tell whether two events are simultaneous or 

 not. This is where the theories of Lorentz and 

 Larmor lead us ; and yet they only suggest it 

 without demonstrating it. For neither of them 

 professed to give a complete account of the struc- 

 ture of matter. The relations they propose for 

 the aithereal action need to be supplemented by 

 hypotheses as to the nature of the electron before 

 they make a complete scheme ; and when it comes 

 to this Lorentz suggests a hypothesis, that of the 

 contracting electron, which is devised specially for 

 the purpose of getting the aether to conceal itself. 



This brings us to the point at which Einstein 

 makes his very modest suggestion (1905) of test- 

 ing the hypothesis that in the nature of things 

 we must not expect ever to find evidence of an 

 absolute velocity of the earth through space. The 

 ground had been well prepared. The electron 

 theory as generally accepted was taken as it 

 stood. The only new idea was that whereas his 

 predecessors clung to the thought that one a-ther 

 with its associated system of space-time measure- 

 ment was real and the rest fictitious, Einstein, 

 in the absence of any distinguishing features, 

 ranks them all alike. To which some replied, 

 "Give us back our absolute time and our aether." 

 Yet Einstein had logic on his side, for the defini- 

 tions of aether and of time are definitions quid 

 twminis. not quid rci. 



Looking back, we realise two great gaps in the 

 special principle of relativity as Einstein pro- 

 pounded it in 1905. The first is the same which 

 revealed itself in Newton's theory. "Uniform 

 motion in a straight line " is left as a phrase with 

 meaning, a situation as intolerable to the philo- 

 sopher as the recognition of "absolute rest." 

 Further, while it accepted the electromagnetic 

 theory intact as a common basis of the 

 structure of matter, it took no account of 

 the other common property of matter, gravi- 

 tation, of which so far the electromagnetic 

 theory f)ffered no explanation. Einstein at once 

 realised the gaps, however, and saw a relation 

 between them. Gravitational problems arc prob- 

 lems of acceleration. The magnitude of the velo- 

 city of an observer does not affect his sense of 

 gravity, but the experience of the man in the lift 

 is that his acceleration does so. Thus Einstein 

 perceived that to settle his difference with the 

 philosopher he must first reconcile gravitation. 

 Whether he would ever have done it without the 

 genius of Minkowski we cannot tell. But as a 

 matter of history the pure mathematician, who 

 surely had learned much from Riemann and Helm- 

 holtz. was the first to emphasise the profound 



