NA TURE 



781 



THURSDAY, FEBRUARY 17, 1921. 



Editorial and PublUhing OffUts: 



MACMILLAN 6- CO., LTD.. 



ST. MARTIN'S STREET, LONDON, W.C.I. 



Advertisements and business letters should be 



addressed to the Publishers. 



Editorial communications to the Editor. 



Telegraphic Address: PHUSIS, LONDON. 

 Telephone Number: GERRARD 8830. 



Introduction. 



SI.N'CK 1905, when Prof. X. Einstein published 

 his Principle of Relativity in its special 

 form, much attention has been devoted to the 

 subject in scientific circles, and during the last 

 two or three years keen interest has been .shown 

 in it by many jjcneral readers of intellectual type. 

 Nearly a thousand books, pamphlets, and papers 

 have appeared in which the principle as a whole 

 is de.scribed, or .some of its aspects are discussed, 

 and among these are a few which aim at making 

 ;i> character and consequences clear without the 

 mathematical expressions which give it precise 

 significance. A prai-seworthy essay of this kind is 

 that by Mr. L. Bolton, a senior examiner in 

 the Patent Ollice, published in the Westminster 

 (iazetle on Monday by arrangement with the 

 'Scientific Americnn, which awarded it a prize of 

 ive thousand dollars as the clearest explanation 

 )f Kinstoin's principle for general readers. 



It is not difTicuit to understand why such wide 

 interest is taken in this principle. .No special 

 i.nowledge is required to realise that measurc- 

 iicnts of space and time are essentially relative, 

 ind as all thinking people have pondered over 

 ilie metaphysics of infinity and eternity, they are 

 attracted by a conception in which these ideas are 

 involved. The physicist is concerned with the prin- 

 ciple because it developed out of experimental 

 results of a negative kind, and is intimately asso- 

 ciated with electromagnetic theory ; the astro- 

 nomer because it gives a new interpretation of 

 effects not explained by the Newtonian law ; and 

 the mathematician In-cause it provides him with 

 a new space-time geometry. The principle has 

 thus points of contact with many fields of scien- 

 NO. 2677, VOL. 106] 



tific activity, and it is on this account that the 

 present issue of N.\ture is devoted to it. 



We are gratified that .so many leading authori- 

 ties have been able to favour readers of this 

 journal with surveys of the foundations of the 

 principle or with their views as to the stability of 

 the framework based upon them. Our own func- 

 tion has been limited to suggesting the scope of 

 each article in the series, and, so far as possible, 

 securing that the range of the whole covers the 

 chief points around which discussion has centred. 

 The writers have not seen one another's con- 

 tributions, so that each article is an independent 

 statement complete in itself so far as it goes. 

 The order of the articles in the series is, there- 

 fore, important, and we believe that adopted will 

 be considered appropriate to these columns. 



Prof. Hinstein describes the natural sequence of 

 ideas which led to the conception of his principle, 

 and -Mr. Cunningham follows with a historical 

 sketch of the conditions which demanded a revision 

 of aether theories in relation to problems of abso- 

 lute motion. The astronomical consequences — 

 the displacement of light by the gravitational field 

 of the sun, the movement of the perihelion of 

 Mercury's orbit, and the displacement of solar 

 spectrum lines (not yet established) — are dealt with 

 by .Sir Frank Dyson, Dr. Crommclin, and Dr. 

 St. John respectively. The relation of Riemann's 

 geometry of n-dimensions to the principle is out- 

 lined by Prof. Mathews ; and the four articles 

 which follow, by Mr. Jeans, Prof. Lorentz, Sir 

 Oliver Lodge, and Prof. Weyl, are concerned 

 mainly with physical aspects. How differentlv 

 philosophers and astronomers regard the mean- 

 ing and measurement of time is described by 

 Prof. Kddington, and this article, with those by 

 Dr. Norman Campbell and by Miss Wrinch and 

 Dr. Jeffreys, leads naturally to the metaphysical 

 conceptions presented by Prof. Wildon Carr. 



Whatever may be the ultimate place taken by 

 the principle of relativity in the history of science, 

 no idea has ever proceeded by more logical steps 

 from the rank of hypothesis to theory. In two 

 cases predicted phenomena for which no satis- 

 factory alternative explanatit)n is forthcoming 

 have been confirmed by observation, and the third 

 is still a subject of inquiry. In this journal we 

 are concerned mainly with the bearing of the 

 principle upon physical science, and only incident- 

 ally with its metaphysical aspects. We may re- 

 mark, however, that the absolute in Nature is not 

 abolished by the principle. Measurements of time 

 and space cease to be ab.solutc and depend upon 

 the motion of the observer, but things like energy 

 and the velocity of light are independent of such 

 motion and remain as absolute as ever they were. 



DD 



