Sitting at his post in the Bern patent office one day in 1907, Albert Einstein imagined how a housepainter would experience gravity if he fell off a roof. The success of Einstein's Special Theory of Relativity had prompted requests for more articles on the subject. As he rewrote the original work, Einstein thought about ways to expand his theory to include the presence of gravity.
On that day, the physicist's daydream ended with what he later called his "happiest moment." He surmised that the unlucky painter would feel weightless when accelerating toward the ground. This clue led Einstein to reason that gravity and acceleration must be equivalent. Called the "equivalence principle," this idea was the seed that--over the next nine years--bloomed into Einstein's masterpiece, the "General Theory of Relativity." This new theory laid the foundation for relativistic astrophysics and modern cosmology.