A NEW VIEW OF GRAVITY
Einstein’s vision of a man falling from a roof marked the beginning of a great struggle
Once while Einstein was working on the equations for general relativity, which would take him eight years to complete, he went mountain-climbing with the French-Polish chemist Marie Curie. Seemingly oblivious to the crevasses as well as to her difficulty in understanding his German, Einstein spent much of the time talking about gravitation. “You understand,” Einstein said to her, suddenly gripping her arm, “what I need to know is exactly what happens in an elevator when it falls into emptiness.”
In Einstein’s imagination, the man suspended midway between roof and earth was now inside an elevator. In a certain set of circumstances, the passenger would have no way of knowing whether he was experiencing gravity or upward acceleration. If the elevator were standing on the surface of the earth, the man would feel gravity’s force there, which causes falling objects to accelerate at a rate of 32 feet per second squared. But if the elevator were accelerating through deep space at that same rate, he would experience precisely the same downward force.
Einstein imagined a beam of light piercing the elevator. If the elevator were rising relative to the source of light, the beam would enter at a certain height on one side of the elevator and appear to curve on its way to a lower height on the opposite wall. Einstein then imagined that the elevator were stationary on the surface of the earth. Since he postulated that the two circumstances are the same, Einstein concluded that the same effect would have to hold true for both. In other words, gravity must bend light.
He wouldn’t have the math to support this idea until 1915, and he wouldn’t have the proof until the eclipse expeditions of 1919. But by then he was so confident of his calculations that when a student asked what he would have done if he’d heard the eclipse observations hadn’t validated his math, Einstein told her, “Then I would have been sorry for the dear Lord. The theory is correct.”