Einstein's Law of Gravity
Then, in 1914, Albert Einstein lifted the veil. The ideas he put forward grew out of his formulation of the special theory of relativity a decade earlier. That theory is based on the postulate that no observations made inside an enclosed chamber can answer the question of whether the chamber is at rest or moving along a straight line at constant speed. Thus a person in the situation of the author as he writes these lines-in an inside cabin of the S.S. Queen Elizabeth sailing on a smooth sea-can perform no experiment, mechanical, optical or any other kind, that will tell him whether the ship is really moving or still in port. But let a storm come up and the situation changes painfully; the deviation from uniform motion is all too apparent.
In order to deal with the problem of nonuniform motion Einstein imagined a laboratory in a spaceship located far from any large gravitating masses. If the vehicle is at rest, or in uniform motion with respect to distant stars, the observers inside, and all their instruments that are not secured to the walls, will float freely. There will be no up and no down. As soon as the rocket motors are started and the ship accelerates, however, instruments and people will be pressed to the wall opposite the direction of motion. This wall will become the floor, the opposite wall will become the ceiling and the people will be able to stand up and move about much as they do on the ground. In fact, if the acceleration is equal to the acceleration of gravity on the surface of the earth, the passengers may well believe that their ship is still standing on its launching pad.
Suppose one of the passengers simultaneously releases two spheres, one of iron and one of wood, which he has been holding next to each other in his hands. What "actually" happens can be described as follows: While the spheres were held they were undergoing accelerated motion, along with the observer and the whole ship. When they are released, they are no longer driven by the rocket engines. Now they will move side by side, each with a velocity equal to that of the spaceship at the moment of release. The ship itself, however, will continuously gain speed and the "floor" of the ship will quickly overtake the two spheres and hit them simultaneously.
To the observer inside the ship the experiment will look different. He will see the balls drop and hit the "floor" at the same time. Recalling Galileo's demonstration from the leaning tower of Pisa, he will be persuaded that an ordinary gravitational field exists in his space laboratory.
Both descriptions of the observed event are correct; the equivalence of the two points of view is the foundation of Einstein's relativistic theory of gravity. This so-called principle of equivalence between observations carried out in an accelerated chamber and in a "real" gravitational field would be trivial, however, if it applied only to mechanical phenomena. Einstein's deep insight was that the principle is quite general and holds also for optical and other electromagnetic phenomena.
Imagine a beam of light propagating across the space laboratory in a "horizontal" direction. Its path can be traced by means of a series of vertical fluorescent glass plates spaced at equal distances. Again what actually happens is that the beam travels in a straight line at constant speed, while the glass plates move across its path at an ever increasing speed. The beam takes the same time to travel from each plate to the next, but the plates move farther during each successive interval. Hence the pattern of fluorescent spots shows the floor approaching the light beam at an increasing rate. If the observer inside the chamber draws a line through the spots, it will look to him like a parabola bending toward the floor. Since he considers acceleration phenomena as being caused by gravity, he will say that a light ray is bent when propagating through a gravitational field.