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Every general law of nature must be so constituted that it is

transformed into a law of exactly the same form when, instead of

the space-time variables x, y, z, t of the original coordinate

system K, we introduce new space-time variables x′, y′, z′, t′ of

a co-ordinate system K′. In this connection the relation between

the ordinary and the accented magnitudes is given by the Lorentz

transformation. Or in brief: General laws of nature are

co-variant with respect to Lorentz transformations.

This is a definite mathematical condition that the theory of

relativity demands of a natural law, and in virtue of this, the

theory becomes a valuable heuristic aid in the search for general

laws of nature. If a general law of nature were to be found which

did not satisfy this condition, then at least one of the two

fundamental assumptions of the theory would have been disproved.

Let us now examine what general results the latter theory has

hitherto evinced.

XV.

GENERAL RESULTS OF THE THEORY