On the Theory of Gravitation from the Standpoint of the Principle of Relativity

In the present communication, I wish to develop further several aspects of the theory of gravitation whose fundamentals I published in two previous essays and discuss it.¹ The theory presented in the last essay is not completely unambiguous. First—as emphasized on p. 509 [p. 867 in the original]—the rest density of matter was defined in a fairly arbitrary way; though a different definition of the concept of mass would not change the general laws of mechanics, it would modify the laws of gravitation. Second, in the theory of gravitation, the possibility has been left open that the gravitational factor g is not a constant, but could depend on various circumstances. One can think of this scalar quantity as being dependent on the internal state of the object as well as on the gravitational potential at the location in question. A dependence of the gravitational factor on the state of stress of the body is equivalent to a change in the definition of mass, but a dependence on the gravitational potential will have a deeper significance for the theory.