physics which is conformable to the general principle of relativity, since the
equations of such a theory hold for every body of reference, whatever may be its
state of motion.
Notes
*) The objection is of importance more especially when the state of motion of
the reference-body is of such a nature that it does not require any external
agency for its maintenance, e.g. in the case when the reference-body is rotating
uniformly.
A FEW INFERENCES FROM THE GENERAL PRINCIPLE OF
RELATIVITY
The considerations of Section 20 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 K1 which is accelerated relatively to K. But since a gravitational
field exists with respect to this new body of reference K1, 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 curvilinear motion with respect to the accelerated
reference-body K1 (chest). This acceleration or curvature corresponds to the
influence on the moving body of the gravitational field prevailing relatively to
K. It is known that a gravitational field influences the movement of bodies in
this way, so that our consideration supplies us with nothing essentially new.
However, we obtain a new result of fundamental importance when we carry out
the analogous consideration for a ray of light. With respect to the Galileian