ABSTRACT
Imagine that at a point deep in space a cable accelerates an elevator upwards at a constant rate. A scientist trapped inside wants to find out whether the elevator is accelerating. So he releases a ball, which falls to the floor just as it should if the elevator is indeed accelerating. Does he now know that it is accelerating? Einstein answers in the negative because, roughly, for all the scientist knows the elevator is hanging at rest near the Earth’s surface and the ball falls due to gravity. Einstein took this to be good grounds for what is effectively the equivalence principle of the General Theory of Relativity (Einstein 2009, 80–83; Einstein and Infeld 1938, 230–235).
