What was the thought experiment that led Einstein to general relativity?
He imagined a man in free fall. To understand what he saw, imagine a man in a closed elevator chamber that is falling toward the earth. He would float feely in the chamber, and anything he pulled from his pocket and dropped would float freely next to him — just as if he were in a closed chamber sitting still in a gravity-free region of deep outer space. On the other hand, imagine a woman in a closed chamber who is accelerating upward in outer space, far from any gravity. She would feel pulled down to the floor, just as if she were being pulled down by gravity. From the equivalence of gravity and acceleration, he constructed his general theory of relativity.