June" i 7, 1922] 



NA TURE 



771 



been one as much of spirit as of letter. It is the 

 lucidity of the French author, in combination with 

 liis own gift of expression, that has made it possible 

 for the translator to succeed so well in overcoming 

 the obstacles to giving the exposition in our own 

 tongue this book contains. The rendering seems to 

 me, after reading the book both in French and in 

 English, admirable." 



The book is certainly readable. The language is not 

 only clear, but also picturesque. " Einstein may be a 

 treasure, but there is a fearsome troop of mathematical 

 reptiles keeping inquisitive folk away from it. Let 

 us drive them off with the whip of simple terminology, 

 and approach the splendour of Einstein's theory." 

 This is the author's intention, like that of many 

 others ; how does he succeed } 



In the first two chapters we may admit a considerable 

 success. Here due recognition is given to the valuable 

 work done by Poincare in preparing the minds of 

 physicists for the theory by his insistence on the 

 relativity of space. The reader is brought to the point 

 of seeing the confusion wrought by the unexpected 

 result of the Michelson-Morley experiment. 



At the third chapter the author begins to feel the 

 excitement of Einstein's new thoughts, and gives us a 

 version of the explanation advanced in 1905 of the 

 true significance of the Fitzgerald contraction-hypo- 

 thesis. With the gesture of a conjurer he produces a 

 " simple " version of Einstein's argument, remarking, 

 however, that its elementary simplicity has not been 

 attained without difficulty. Unfortunately the ex- 

 planation advanced has nothing whatever to do with 

 the argument of Einstein or the Fitzgerald hypothesis, 

 but refers to the first order effect which would arise 

 if the old theory were true. Later in the chapter the 

 footnote giving as the best definition of the second, 

 " the time which light takes to cover 186,000 miles 

 in empty space, far from any strong gravitational 

 field " suggests again quite a wrong view to the unwary 

 and inexperienced reader. In spite, however, of these 

 inaccuracies the writer does convey something of the 

 impression of the insufficiency of the old absolute 

 time and space ideas, and describes the situation 

 thus : " We have before our eyes merely a battlefield 

 strewn with corpses and ruins." " Time and space lie, 

 torn and crumbled, among the rubbish of ancient 

 theories." 



Now comes the task of describing in " simple 

 terminology " the work of reconstruction. The reader 

 must judge whether the following is simpler than a 

 brief algebraic equation : " The distance in time and 

 the distance in space are numerically to each other 

 as the hypotenuse and another side of a rectangular 

 triangle are to the third side, which remains invariable. 

 Taking this third side for base, the other two will 

 NO. 2746, VOL. 109] 



describe above it, a triangle more or less elevated 

 according as the velocity of the observer is more or 

 less reduced. This fixed base is a quantity independent 

 of the velocity. It is this which Einstein has called 

 the ' Interval ' of events." A few lines later we read 

 that this interval is the " sole perceptible part of the 

 real. Apart from it there is nothing we can know." 

 With this the writer passes on to the next chapter. 

 Here the mechanics arising out of the restricted 

 principle of relativity is described clearly and without 

 rhetoric. 



We have now to see how the author deals with the 

 generalised relativity and its consequences. If the 

 reader has digested the idea of interval so rawly 

 presented to him in the early chapter, and has not 

 the quickness to perceive that this has not been 

 extended to the generalised theory, he may get a 

 general impression from the next two chapters which 

 is of the right type, but which again is far from accurate 

 in detail. It appears from the account given that the 

 only new feature in this stage is the introduction of 

 gravitation. Not a word is said about the application 

 of the general doctrine that all physical laws must have 

 a form which is independent of the arbitrary choice of 

 variables in the four - dimensional continuum. The 

 mathematical reptiles are certainly driven off, and 

 the treasure is not only left unguarded but it dis- 

 appears. What is left is not the genuine article. Here 

 again there are inaccurate statements, e.g. " the 

 universe is not Euclidean because in it light does not 

 travel in a straight line." 



It is no pleasure to write thus critically of a valiant 

 attempt to bring this theory into a form suitable for 

 the layman. It is a supremely difficult task that is 

 being attempted by so many writers. M. Nordmann 

 is well equipped in many ways for it. But he has 

 fallen into the very common error of supposing that 

 the essential truth can be given while omitting the 

 demand for concentrated thinking on vital details. 

 The world suffers far too much from loose thinking 

 already. Vague generalisations, misleading analogies, 

 superficial manifestations are made to do duty for 

 precise statements, logical reasoning, and fundamental 

 principles. It is not necessarily true that mathematical 

 skill is the only way of approach to an understanding 

 of Einstein's fundamental ideas. But it is certain 

 that if such an understanding is to be reached it can 

 only be by going down co a patient analysis of our 

 own preconceived notions until we find them in- 

 sufficient. Einstein's success has come from a deep- 

 rooted conviction that those thinkers were right who 

 would not admit that a point in empty space could 

 be labelled " stationary " or " moving uniformly in a 

 straight line." He followed the logic of his conviction 



