SECTION A.— MATHEMATICAL AND PHYSICAL SCIENCES. 



THE OUTSTANDING PROBLEMS 

 OF RELATIVITY. 



ADDRESS BY 



PROF. E. T. WHITTAKER, LL.D., Sc.D., F.R.S., 



PRESIDENT OF THE SECTION. 



It was in January 1914 that Einstein 1 made his great departure from the 

 Newtonian doctrine of gravitation by abandoning the idea that the 

 gravitational potential is scalar. The thirteen eventful years which have 

 passed since then have seen the rapid development of the new theory, 

 which is called General Relativity, and the confirmation by astronomers 

 and astrophysicists of its predictions regarding the bending of light-rays 

 by the sun and the displacement of spectral lines. At the same time a 

 number of new problems have arisen in connection with it ; and perhaps 

 the time has now come to review the whole situation and to indicate where 

 there is need for further investigation. 



Speaking from this Chair I may perhaps be permitted to recall that 

 my first experience of the British Association was as one of the secre- 

 taries of Section A nearly thirty years ago ; and that my secretarial duties 

 brought me the privilege of an introduction to the distinguished mathe- 

 matical physicist, Prof. G. F. FitzGerald of Dublin, who was a regular 

 and prominent member of the section until his death in 1901. FitzGerald 

 had long held an opinion which he expressed in 1894 in the words ' Gravity 

 is probably due to a change of structure of the aether, produced by the 

 presence of matter.' 2 Perhaps this is the best description of Einstein's 

 theory that can be given in a single sentence in the language of the older • 

 physics : at any rate it indicates the three salient principles, firstly, that 

 gravity is not a force acting at a distance, but an effect due to the modifica- 

 tion of space (or, as FitzGerald would say, of the aether) in the immediate 

 neighbourhood of the body acted on ; secondly, that this modification 

 is propagated from point to point of space, being ultimately connected 

 in a definite way with the presence of material bodies ; and thirdly, that 

 the modification is not necessarily of a scalar character. The mention 

 of the aether would be criticised by many people to-day as something out 

 of date and explicable only by the circumstance that FitzGerald was 

 writing thirty-three years ago ; but even this criticism will not be uni- 

 versal ; for Wiechert and his followers have actually combined the old 

 aether theory with ideas resembling Einstein's by the hypothesis that 

 gravitational potential is an expression of what we may call the specific 

 inductive capacity and permeability of the aether, these qualities being 



1 Zeits. f. Math. u. Phys. 63 (1914), p. 215. 



2 FitzGerald's Scientific Writings, p. 313. 



