Page:Relativity (1931).djvu/139



F the reader has followed all our previous I considerations, he will have no further diffculty in understanding the methods leading to the solution of the problem of gravitation.

We start off from a consideration of a Galileian domain, i.e. a domain in which there is no gravitational field relative to the Galileian reference-body $$K$$. The behaviour of measuring-rods and clocks with reference to $$K$$ is known from the special theory of relativity, likewise the behaviour of “isolated” material points; the latter move uniformly and in straight lines.

Now let us refer this domain to a random Gauss co-ordinate system or to a ‘‘mollusk” as reference-body $$K'$$. Then with respect to $$K'$$ there is a gravitational field $$G$$ (of a particular kind). We learn the behaviour of measuring-rods and clocks and also of freely-moving material points with reference to $$K'$$ simply by mathematical transformation. We interpret this behaviour as the