December 4, 1919] 



NATURE 



355 



parations for this expedition were in progress 

 before the war had ceased. 



Before attempting to understand the theory 

 which, if we are to believe the daily Press, has 

 dimmed the fame of Newton, it may be worth 

 while to recall what it was that he did. It was 

 not so much that he, first among men, used the 

 differential calculus. That claim was disputed by 

 Leibniz. Nor did he first conceive the exact 

 relations of inertia and force. Of these, Galileo 

 certainly had an inkling. Kepler, long before, 

 had a vague suspicion of a universal gravitation, 

 and the law of the inverse square had, at any 

 rate, been mooted by Hooke before the " Principia " 

 saw the light. The outstanding feature of New- 

 ton's work was that it drew together so many 

 loose threads. It unified phenomena so diverse 

 as the planetary motions, exactly described by 

 Kepler, the everyday facts of falling bodies, the 

 rise and fall of the tides, the top-like motion of 

 the earth's axis, besides many minor irregularities 

 in lunar and planetary motions. With all these 

 drawn into such a simple scheme as the three 

 laws of motion combined with the compact law 

 of the inverse square, it is no wonder that flights 

 of speculation ceased for a time. The universe 

 seemed simple and satisfying. F"or a century at 

 least there was little to do but formal develop- 

 ment of Newton's dynamics. In the mid-eighteenth 

 century Maupertuis hinted at a new physical doc- 

 trine. He was not content to think of the universe 

 as a great clock the wheels of which turned in- 

 evitably and irrevocably according to a fixed rule. 

 Surely there must be some purpose, some divine 

 economy in all its motions. So he propounded a 

 principle of least action. But it soon appeared 

 that this was only Newton's laws in a new guise; 

 and so the eighteenth century closed. 



The nineteenth saw great changes. When it 

 closed, the age of electricity had corne. Men were 

 peering into the secrets of the atom. Space was 

 no longer a mighty vacuum in the cold emptiness 

 of which rolled the planets. It was filled in every 

 part with restless energy. /Ether, not matter, 

 was the last reality. Mass and matter were elec- 

 trical at bottom. A great problem was set for 

 the present generation : to reconcile one with the 

 other the new laws of electricity and the classical ' 

 dynamics of Newton. At this point the principle 

 of least action began to assume greater import- 

 ance ; for the old and the new schemes of the uni- 

 verse had this in common, that in each of them the 

 time average of the difference between the kinetic 

 and the potential energies appears to be a minimum. 



One of the main difficulties encountered by the . 

 electrical theory of matter has been the obstinate 

 Vrefusal of gravitation to come within its scope. 

 |:<3uietly obeying the law of the inverse square, 

 fit heeded not the bustle and excitement of the 

 [new physics of the atom, but remained, indepen- 

 Ident and inevitable, a constant challenge to rash 

 "claimants to the key of the universe. The elec- 

 trical theory seemed on the way to explain 

 r-cvery property of matter yet known, except the 

 lone most universal of them all. It could trace to 

 NO. 2614, VOL. 104] 



its origins the difference between copper and 

 glass, but not the common fact of their weight ; 

 and now the aether began silently to steal away. 



One matter that has seriously troubled men in 

 Newton's picture of the universe is its failure to 

 accord with the philosophic doctrine of the rela- 

 tivity of space and time. The vital quantity in- 

 dynamics is the acceleration, the change of motiorfc 

 of a body. This does not mean that Newton 

 assumed the existence of some ultimate frame- 

 work in space relative to which the actual 

 velocity of a body can be uniquely specified, 

 for no difference is made to his laws if any 

 arbitrary constant velocity is added to the velocity 

 of every particle of matter at all time. The seri- 

 ous matter is that the laws cannot possibly have 

 the same simplicity of form relative to two frame- 

 works of which one is in rotation or non-uniform 

 motion relative to the other. It seems, for in- 

 stance, that if Newton were right, the term "fixed 

 direction" in space means something, but "fixed' 

 position " means nothing. It seems as if the two 

 must stand or fall together. And yet the physical 

 relations certainly make a distinction. Why this 

 should be so has not yet been made known to us. 

 Whatever new theory we adopt must take account 

 of the fact. 



It was with some feeling of relief that men 

 hailed the advent of the aether as a substitute 

 for empty space, though we may note in passing 

 that some philosophers — Comte, for example — 

 have held that the concept of an aether, infinite 

 and intangible, is as illogical as that of an abso- 

 lute space. But, jumping at the notion, physicists 

 proposed to measure all velocities and rotations 

 relative to it. Alas ! the aether refu.sed to dis- 

 close the measurements. Explanations were soon 

 forthcoming to account for its reluctance ; but 

 these were so far-reaching that they explained 

 away the a;ther itself in the sense in which it was 

 commonly understood. At any rate, they proved 

 that this creature of the scientific imagination was 

 not one, but many. It quite failed to satisfy the 

 cravings for a permanent standard against which 

 motion might be measured. The problem was 

 left exactly where it was before. This was pre- 

 war relativity, summarised by Einstein in 1905. 

 The physicists complained loudly that he was 

 taking away their aether. 



Let it not be thought, however, that the results 

 of the hypothesis then advanced were purely nega- 

 tive. They showed quite clearly that many current 

 ideas must be modified, and in what direction this 

 must be done. Most notably it emphasised the 

 fact that inertia is not a fundamental and invari- 

 able property of matter ; rather it must be sup- 

 posed that it is consequent upon the property of 

 energy. And, again, energy is a relative term. 

 One absolute quantity alone remained ; one only 

 stood independent of the taste or fancy of the 

 observer, and that was "action." While the 

 aether and the associated system of measurement 

 could be selected as any one of a legion, the prin- 

 ciple of least action was satisfied in each of them, 

 and the magnitude of the action was the same in all. 



