5 MAY 1923, Page 24

Modern Electrical Theory : Relativity. By N. R. CampbKi.

(Cambridge : at the University Press. 7s. 6d. net.) -

This is the second monograph devoted to recent research in physics, which serves also as a supplementary chapter to Mr. Campbell's big book. Here for the first time we have relativity for the experimental physicist by the experimental physicist, to the exclusion—so far as is possible—both of the mathematician and of the philosopher. In Mr. Campbell's interpretation of relativity there are no alternatives to the special theory; the question of their existence does not even arise. For the fact, that from certain assumptions certain results follow, does not exclude the possibility that the same results may follow from a different set of assumptions. Thus alternatives become supplements. The general theory is on a different footing, however ; and its importance lies rather in the ideas which it introduces in order to arrive at its conclu- sions, than in the conclusions themselves. Until very recently, no alternatives to the general theory have been proposed. Mr. Campbell's concluding suggestions leave us with more than a hint of unknown worlds to conquer. Thus, as regards the intellectual satisfaction of the general theory., he says "it may repel, not because it is revolutionary, but because it is not revolutionary enough. But where shall we find anything more revolutionary ? The answer is beginning to appear. Contemporaneous with relativity, and following a curiously parallel course, is the quantum theory of energy. . . . Surely the two streams must join into a wider flood."