General Relativity

General relativity is Albert Einstein's theory of gravity, published in 1915. Its central idea is that gravity is not a force in the ordinary sense but the result of mass and energy bending the fabric of spacetime. Objects moving through curved spacetime follow paths that look, to us, like the pull of gravity. As the physicist John Wheeler put it, matter tells spacetime how to curve, and curved spacetime tells matter how to move.

The theory has passed every experimental test for over a century, from the bending of starlight by the Sun to the slowing of clocks in strong gravity to the detection of gravitational waves.

Why it mattered to Hawking

General relativity is the foundation of nearly everything Hawking did. It predicts black holes and the expansion of the universe, and it was by analysing its equations that Hawking and Penrose proved their singularity theorems. But general relativity is a theory of the very large and does not include quantum mechanics, the physics of the very small. Hawking's great achievement, Hawking radiation, came precisely from forcing the two together at the edge of a black hole, exposing the cracks where general relativity alone is not enough and a deeper theory of quantum gravity must take over.