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No fairer destiny could be allotted to any physical theory, than

that it should of itself point out the way to the introduction of

a more comprehensive theory, in which it lives on as a limiting

case.

In the example of the transmission of light just dealt with, we

have seen that the general theory of relativity enables us to

derive theoretically the influence of a gravitational field on

the course of natural processes, the laws of which are already

known when a gravitational field is absent. But the most

attractive problem, to the solution of which the general theory

of relativity supplies the key, concerns the investigation of the

laws satisfied by the gravitational field itself. Let us consider

this for a moment.