362 



NATURE 



[December 4, 1919 



through a field of gravitational attraction depends 

 upon the gravitational potential, and diminishes as 

 the potential diminishes. Thus the gravitational field 

 round the sun acts like a refracting atmosphere, the 

 refraction diminishing as the distance from the sun 

 increases. 



Though there are some hundreds of theories of 

 gravitation, Einstein's is the only one which has pre- 

 dicted a result which has been verified by experience. 

 On Einstein's, as on several other theories, changes 

 in gravitational attraction travel with the velocity of 

 light, and also the mass of a body varies with the 

 proximity of other bodies. 



In view of the statements in the Press about the 

 overthrow of the Newtonian law, it may be well to 

 point out that it is only in most exceptional cases — 

 cases which are very difficult to realise — that the 

 difference between the effects of the two laws is 

 appreciable. 



The modified theory of relativity by which Einstein 

 arrived at this result is of remarkable interest and 

 subtlety. The space around matter is on this theory 

 distorted by an amount which diminishes as the dis- 

 tance from the matter increases, so that an observer 

 in an aeroplane, if he were provided with infinitely 

 delicate instruments, would, as he rose in the air, 

 find the shapes of objects on the ground continually 

 changing; and again the ratio of the circumference to 

 the diameter of a circle would be changed to a minute 

 amount by placing a weight at the centre of the 

 circle. The laws of morality have been said to be a 

 question of latitude; on Einstein's view those of geo- 

 metry are a question of altitude. 



Ori Einstein's view, gravitation is due to a particle 

 trying to find the easiest way through space distorted 

 and disturbed in this w-ay. We may put it as 

 follows : — The dynamical principle of least action, 

 when applied to a particle moving through a space 

 of this kind, would lead to a different path from that 

 which would be pursued if the space were Euclidean, 

 and this difference in path is that which would be 

 produced if we supposed the space to remain Euclidean 

 and the particle to be acted upon by an appropriate 

 force. This force is what we call gravitational attrac- 

 tion. Thus we can represent the effect of this dis- 

 torted space by the effects of suitable forces, and I 

 expect it will be found that even the most enthusiastic 

 relativitists will be tempted to think in terms of 

 forces rather than in those of the geometry of non- 

 Euclidean space. 



If the distortion of space were very great, the cus- 

 tomary methods of dynamics might lose their signi- 

 ficance; and the question arises: Will, on Einstein's 

 theory, the space inside an atom be so far from 

 Euclidean that ordinary dynamical methods are un- 

 justifiable? The answer to this question is, "No." 

 There are two lengths which have special significance 

 in connection with the atom ; one of these is what 

 we call the radius of the atom, and is of the order 

 10-' cm. ; the other we call the radius of the electron, 

 and is about lo-" cm. Even at the smaller of these 

 distances the gravitational potential due to the mass 

 of the atom, and therefore the distortion from 

 Euclidean space, would be exceedingly small com- 

 pared with the corresponding quantities due to earth 

 at its surface, so that there is no special distortion 

 inside the atom, except at distances from the centre 

 which are infinitesimal even when compared with the 

 radius of an electron. 



One point of interest in connection with any view 

 we take about mass is that, on the electrical theory 

 of matter, the massive part of the atom is invariably 

 positively charged, so that any state of space which 

 we associate with mass ought to involve something 

 corresponding to a positive charge of electricity. 

 NO. 2614, VOL. 104] 



The determination of the consequences of Einstein's 

 theory on the principles of relativity, where the ideas 

 of space and time are so intimately correlated that 

 time has to be treated as a fourth-space dimension, 

 introduces us into a space of four dimensions which 

 we cannot visualise, and the properties of which are 

 very remote from our experience. It is this which 

 makes any general explanation of Einstein methods 

 so difficult. To the analyst the difficulties presented 

 by space of four dimensions are mainly those of an 

 increase in the number of his symbols and equa- 

 tions ; his difficulties begin when he has to explain 

 his results to someone who is not an analyst. It is 

 a remarkable and most interesting fact, from the point 

 of view of either physics or metaphysics, that from 

 such transcendental considerations as those I have 

 indicated should have emerged a result so closely con- 

 nected with such a prosaic thing as that it is more 

 tiring to go upstairs than down. 



According to Einstein's theory, the Fraunhofer lines 

 in the sun must be displaced towards the red. This 

 effect, though looked for bv several observers, has 

 not been confirmed ; but even should it turn out that 

 thQ theory has greatly to be modified, or even aban- 

 doned, its conception and development will, I think, 

 always be regarded as one of the great triumphs of 

 human thought. 



Another interesting consequence of Einstein's 

 theory is the exceeding minuteness of structure which 

 it demands from matter. The electron, with a radius 

 of 10- " cm., carried our notions of the minuteness of 

 some constituents of the universe far beyond those 

 associated with the older atomic theory, but the size 

 of the centres of disturbance, which in Einstein's 

 theory are associated with matter, bears to the size 

 of the electrons about the same proportion as the 

 size of the smallest particle visible under the most 

 powerful microscope to that of the earth itself. 



I am afraid that the termination of the war has 

 not brought to an end the difficulties in the way of 

 scientific research in this country. Not the least of 

 these is the difficulty and expense of procuring ap- 

 paratus ; it is perhaps surprising that in these circurn- 

 stances the Government should have put obstacles in 

 the way of the importation of philosophical instru- 

 ments. Another very real difficulty is that the large 

 increase in the number of students in our universities 

 has greatly increased the educational duties of many 

 of our most active workers, and so diminished the 

 time they can devote to research. 



The demands of war required large_ quantities of 

 substances which previously were obtainable only in 

 small quantities and at great expense. Prominent 

 among these' is helium, which can now be procured 

 on a scale which, measured by laboratory standards, 

 is unlimited. Such supplies of helium put cryogenic 

 research on a nevy footing, and render possible in- 

 vestigations which promise to be of the greatest irn- 

 portance to many different branches of science. It is 

 greatly to be regretted that in this country, the birth- 

 place of cryogenic research, we have no adequately 

 equipped cryogenic laboratory. 



The Medallists. 



The Copley Medal is awarded to William Maddock 

 Bayliss. 



Prof. W. M. Bayliss has been engaged in the 

 investigation of physiological problems for the last 

 thirty-five vears. His work has ranged over a wide 

 field.' His" paper with Starling on the electrical 

 phenomena of the mammalian heart was the first to 

 give the correct form of the normal variation, as con- 

 firmed by later investigations with the spring galvano- 

 meter. Again, he and Starling showed that the pan- 

 creatic secretion was effected by the production of a 



