Page:Relativity (1931).djvu/71

 time variables $$x$$, $$y$$, $$z$$, $$t$$ of the original co-ordinate system $$K$$, we introduce new space-time variables $$x'$$, $$y'$$, $$z'$$, 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. Ifa 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.