XXII
A FEW INFERENCES FROM THE GENERAL PRINCIPLE OF RELATIVITY
THE 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 . 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 which is accelerated relatively to . But since a gravitational field exists with respect to this new body of reference , 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 (in accordance with the law of Galilei) is executing an accelerated and in general
