February 24, 192 1] 



NATURE 



82: 



use of symbols beyond the simplest type and in 

 small number, to tell us what .v conveys to a rela- 

 tivist. This is the task of Prof. Eddington, who has 

 already done more than any other authority in this 

 country to clarify the whole subject, and to reduce 

 it from a somewhat arid desert of mathematical 

 symbolism, of a type unfamiliar even to most 

 mathematicians, to something which can appeal 

 to the mind of one used to the concepts of physics. 

 This work is an attempt to make the appeal yet 

 more general in its scope, and at the outset we 

 should say that it is difficult to imagine a greater 

 degree of success possible in regard to such a 

 theory. As it leaves the pen of the author, this 

 theory, while yet the same, is scarcely recognisable 

 as the same, for in place of a scheme of pure 

 mathematics we now have a fully developed 

 physical theory of the universe expressed in 

 physical terms. 



The reader is first presented with a symposium 

 — between an experimental physicist, a pure 

 mathematician, and a relativist, who discuss the 

 nature of space and of geometry — which brings 

 out vividly the defects of our mental scaffolding 

 in so far as it consists of ideas of space and time. 

 He passes to the Michelson-Morley experiment 

 and the FitzGerald contraction of matter in the 

 direction of its motion. These are now almost 

 matters of general knowledge. But there is a 

 wealth of illustration from everyday events to 

 g-ive point to rather recondite or merely unusual 

 ideas. The confusion of the aviator and his spec- 

 tator, with their cigars, on pp. 24-25, cannot well 

 fail to give to any reader, in a humorous way, 

 an exact idea of a great part of the simpler re- 

 stricted principle of relativity ; and this type of 

 illustration, which at once captures the mind, 

 abounds throughout. It is made quite clear that 

 in accepting this principle we are merely giving 

 up some unproved and unprovable hypotheses 

 which are an actual barrier to the comprehension 

 of Nature. 



The principle itself follows, with a survey of 

 aethers of various historical types, and a general 

 summary of the reasons which make the principle 

 necessary. We do not, in this review, give any 

 account of the theory itself. The world of four 

 dimensions follows, and of this it is sufficient to 

 say that it will probably focus the reader's interest 

 more than anything else, and that it is by far the 

 best realisation of the analysis into ordinary prose 

 — if the author's vivid prose is ordinary — which we 

 have seen or imagined. It is a relief to find that 

 the author does not enter, or deem it useful to 

 enter, into metaphysical speculations on the mean- 

 ing of "imaginary time" as it appears in the 

 NO. 2678, VOL. 106] 



Minkowskian world, but frankly descends to real 

 time for his exposition. 



After chapters on fields of force and types of 

 space, we come to a comparison of the new and 

 the old laws of gravitation. To a mathematician 

 or physicist called upon by his lay friends to ex- 

 plain, several times in a week, just what awful 

 fate has overtaken Euclid and Newton, this dis- 

 cussion seems completely indispensable, for the 

 author has found words in which to express the 

 matter to a lay friend, and such words have always 

 failed the reviewer and probably many others. In- 

 cluded is a simple indication of the nature of the 

 calculation — of deflection of light by the field of 

 the sun — which observers, including Prof. Edding- 

 ton, went far afield to test. 



Much space could be occupied by a mere table 

 of contents of this volume. Every aspect of the 

 theory is touched upon, and all the alternative 

 interpretations of the phenomena which have at 

 various times been suggested receive due and fair 

 consideration. We have a clear statement on all 

 such points as "mass is a measure of energy," 

 and so forth, which puzzle the uninitiated. The 

 "weight of light" has a chapter to itself. The 

 final chapter is of a highly metaphysical type, to 

 follow on the conclusion of the theory. Apparently 

 addressed to philosophers, we can see it raising 

 many heated controversies. Perhaps we may, as 

 we began with a quotation, also end with one — the 

 penultimate paragraph of this chapter : — 



"The theory of relativity has passed in review 

 the whole subject-matter of physics. It has uni- 

 fied the great laws, which by the precision of their 

 formulation and the exactness of their application 

 have won the proud place in human knowledge 

 which physical science holds to-day. And yet, in 

 regard to the nature of things, this knowledge is 

 only an empty shell — a form of symbols. It is 

 knowledge of structural form, and not knowledge 

 of content. All through the physical world runs 

 that unknown content which must surely be the 

 stuff of our consciousness. Here is a hint of 

 aspects deep within the world of physics, and yet 

 unattainable by the methods of physics. And, 

 moreover, we have found that where science has 

 progressed the farthest, the mind has but regained 

 from Nature that which the mind has put into 

 Nature." 



A disappointing conclusion, perhaps, to many 

 ■ — but we congratulate the author on the way he 

 has dealt with a task all the harder because the 

 results read so easily and simply ; we also con- 

 gratulate the Cambridge University Press on the 

 technical excellence of the volume, which is every- 

 thing that we usually associate with the series tO' 

 which this work belongs. J. W. N. 



