. The times which the light takes to move from P to Q and Q to R will be the same: but, since the system is falling ever faster and faster the distance
will be greater than
. Hence, if the light which has passed through P and Q moves in a straight line, it will strike above R, as is illustrated by the straight line in the figure. But, on Einstein’s assumption, the light must go through the third slit, as it would do in the system at rest, and must therefore move in a curved line, like the curved line in the figure, and bend downward in the direction of the gravitational force.
The Tests
Calculation shows that the deviation of light by the moon or planets would be too small to detect. But for a ray which had passed near the sun, the deflection comes out 1.7″, which the modern astronomer regards as a large quantity, easy to measure. Observations to test this can be made only at a total eclipse, when we can photograph stars near the sun, on a nearly dark sky. A very fine chance came in May, 1919, and two English expeditions were sent to Brazil and the African coast. These photographs were measured with extreme care, and they show that the stars actually appear to be shifted, in almost exactly the way predicted by Einstein’s theory.
Another consequence of “general relativity” is that Newton’s law of gravitation needs a minute correction. This is so small that there is but a single case in which it can be tested. On Newton’s theory, the line joining the sun to the nearest point upon a planet’s orbit (its perihelion) should remain fixed in direction, (barring certain effects of the attraction of the other planets, which can be allowed for). On Einstein’s theory it should move slowly forward. It has been known for years that the perihelion of Mercury was actually moving forward, and all explanations had failed. But Einstein’s theory not only predicts the direction of the motion, but exactly the observed amount.