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.
Isaac Newton's 1687 description of gravity was considered scientific law until Einstein's General Theory of Relativity, published more than two centuries later. Einstein mathematically showed that objects, such as the Sun and planets, bend "space-time," or the four-dimensional arena in which all things exist.
You don't have to be the size of a planet to do some space-time warping.
Large objects such as the Sun and planets aren't the only masses that warp the fabric of space-time. Anything with mass—including your body—bends this four-dimensional cosmic grid.
We can't see them, but we know that black holes can exist thanks to the groundwork laid by Einstein's General Theory of Relativity. A black hole forms when the mass of an object, like a star, suddenly collapses down to a tiny volume. A small object with a large mass causes a gaping dent in space-time. This enormous warp creates a gravitational field so strong that nothing—not even light—can escape from it.
By 1911, gravity had finally lured Einstein from other topics, and his thoughts culminated in 1916 with his masterpiece General Theory of Relativity.