But once more the question arises: What could be done by an optical experiment?
Einstein assumed that the principle of relativity still applied in this case, so that it would be impossible to distinguish between the conditions of the observers in the two boxes by any optical experiment.
It can easily be seen that it follows from this new generalized relativity that light cannot travel in a straight line in a gravitational field.
Imagine that the first observer sets up three slits, all in a straight line. A ray of light which passes through the first and second will obviously pass exactly through the third.
Suppose the observer in the freely falling system attempts the same experiment, having his slits P, Q, R, equally spaced, and placing them at right angles to the direction in which he is falling. When the light passes through P, the slits will be in certain position
(Figure). By the time it reaches Q, they will have fallen to a lower level,
, and when it reaches R, they will be still lower,