Page:Relativity (1931).djvu/108

 curvilinear motion with respect to the accelerated reference-body $$K'$$ (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 reference-body $$K$$, such a ray of light is transmitted rectilinearly with the velocity $$c$$. It can easily be shown that the path of the same ray of light is no longer a straight line when we consider it with reference to the accelerated chest (reference-body $$K'$$). From this we conclude, that, in general, rays of light are propagated curvilinearly in gravitational fields. In two respects this result is of great importance.

In the first place, it can be compared with the reality. Although a detailed examination of the question shows that the curvature of light rays required by the general theory of relativity is only exceedingly small for the gravitational fields at our disposal in practice, its estimated magnitude for light rays passing the sun at grazing incidence is nevertheless 1&middot;7 seconds of arc. This ought to manifest itself in the following way.