Page:Eddington A. Space Time and Gravitation. 1920.djvu/128

112 by experiments with the Eötvös torsion-balance that the ratio of weight to mass for uranium is the same as for all other substances; so the energy of radio-activity has weight. Still even this experiment deals only with bound electromagnetic energy, and we are not justified in deducing the properties of the free energy of light.

It is easy to see that a terrestrial experiment has at present no chance of success. If the mass and weight of light are in the same proportion as for matter, the ray of light will be bent just like the trajectory of a material particle. On the earth a rifle bullet, like everything else, drops 16 feet in the first second, 64 feet in two seconds, and so on, below its original line of flight; the rifle must thus be aimed above the target. Light would also drop 16 feet in the first second ; but, since it has travelled 186,000 miles along its course in that time, the bend is inappreciable. In fact any terrestrial course is described so quickly that gravitation has scarcely had time to accomplish anything.

The experiment is therefore transferred to the neighbourhood of the sun. There we get a pull of gravitation 27 times more intense than on the earth; and—what is more important—the greater size of the sun permits a much longer trajectory throughout which the gravitation is reasonably powerful. The deflection in this case may amount to something of the order of a second of arc, which for the astronomer is a fairly large quantity.

In Fig. 16 the line $$EFQP$$ shows the track of a ray of light from a distant star $$P$$ which reaches the earth $$E$$. The main part of the bending of the ray occurs as it passes the sun $$S$$; and the initial course $$PQ$$ and the final course $$FE$$ are practically straight. Since the light rays enter the observer's eye or telescope in the direction $$FE$$, this will be the direction in which the star appears. But its true direction from the earth is $$QP$$, the initial