SPACE AND TIME BY EMILE BOREL Honorary Director of LEcole Normalc Supcrieure Professor of the Faculte des Sciences of Paris Member of the Institute BLACKIE SON LIMITED LONDON AND GLASGOW 1926 Preface The reader will not find here a didactic account of Einsteins theories. Such an account requires the use of the formulas of mathematical physics and would be intelligible only to readers equally at liome with these formulae and with physical theories. But, failing such an account, a sort of general survey of the theories of Einstein with a description of some of their aspects seems not impracticable. As we proceed, we shall have to speak of facts and theories well known before Einstein theories which, when we reflect a little, make his new discoveries appear Jess strange and paradoxical, although not less admirable. To make a complete survey of the theories of Einstein, we should, have Jo traverse, not only the sciences of space and time, but also mechanics and electromagnetism including optics. We have, however, as the title of the book would suggest, dealt mainly with space and time, introducing mechanical and electromagnetic considerations only when they were indispensable. The introduction which precedes the eight chapters of the book is a preliminary reconnaissance. It is well for us to make this first of all in order to know where we are going in the course of the somewhat slow journey that is to follow, in which we may be in danger of losing our perspective in a maze of detail. vi PREFACE I am under no illusion as to the merits of this plan, but I indicate briefly at the beginning of the Introduction why such an exposition, however imperfect, did not seem quite useless. In fact, the essential points of Einsteins theory n jw form part of general culture, like the sphericity of the e irth and its rotation round the sun. The importance of the new theory of relativity from the point of view of culture is, however, quite distinct from its practical importance. It is of no consequence to an architect that the earth is round it is even essential for him, when he is planning a house, to treat all vertical lines as parallel, that is to say, to take the earth as flat. And yet we should be rightly shocked to find an architect ignorant of the fact that the earth is round. The theory of relativity has encountered objections and aroused controversies. I do not deny the element of truth which some of the objections may contain, nor the utility of some of the controversies but I do not believe that the best way to serve science is to adopt towards 2insteins theories the negative attitude which was adopted towards Maxwells work by a learned French physicist who died prematurely a few years ago. However ingenious and sometimes apparently well-founded these criticisms may be, Maxwells equations remain the solid foundation of electromagnetism, whilst the controversies to which they gave rise possess only an historical interest. In my opinion, one fact overshadows all the theoretical disputes Einstein not only has gone beyond the physics of the nineteenth century in the co-ordination of known phenomena, but has also added to this co-ordination the prediction of new phenomena, and his predictions have so far been confirmed by the test of experiment. Even if, by making use of his work, we succeeded, with the aid of some analytical device, in co-ordinating the old and the new results, while taking as starting-point the methods of PREFACE vii nineteenth-century physics, we should hardly prove the superiority of the old methods over the new. We should only be succeeding after the event where the eminent scientis s of the nineteenth century had failed. It will only beiwhen someone, starting from principles different from th9se of Einstein, succeeds both in foretelling new phenomena and in co-ordinating the old ones, that the new principles he uses will either take a place beside those of Einstein, or perhaps even replace the latter entirely...