Page:Relativity (1931).djvu/107



HE considerations of Section XX show that the general principle of relativity puts us in a position to derive properties of the gravitational field in a purely theoretical manner. Let us suppose, for instance, that we know the space-time “course” for any natural process whatsoever, as regards the manner in which it takes place in the Galileian domain relative to a Galileian body of reference $$K$$. By means of purely theoretical operations (i.e. simply by calculation) we are then able to find how this known natural process appears, as seen from a reference-body $$K'$$ which is accelerated relatively to $$K$$. But since a gravitational field exists with respect to this new body of reference $$K'$$, our consideration also teaches us how the gravitational field influences the process studied.

For example, we learn that a body which is in a state of uniform rectilinear motion with respect to $$K$$ (in accordance with the law of Galilei) is executing an accelerated and in general Rh