general relativity

General relativity is Einstein's 1915 theory of gravity. Its radical idea is that gravity is not a force pulling objects across space, but a feature of the shape of space and time itself. Matter and energy bend the geometry of spacetime, and that curvature is what we feel as gravity. The Sun does not reach out and tug on the Earth; instead the Sun dents the spacetime around it, and the Earth simply follows the straightest available path through that dent.

The theory grew out of a simple observation: in free fall you feel no gravity at all. An astronaut in orbit floats, and a person in a falling elevator would feel weightless. Einstein took this seriously and concluded that free fall is the natural, force-free state of motion, and that the curved paths of falling bodies reveal the curvature of spacetime rather than the pull of a force.

General relativity is the most successful theory of gravity we have. It predicted the bending of starlight by the Sun, the slow shift of Mercury's orbit, the slowing of clocks deep in gravity, black holes, and ripples in spacetime called gravitational waves. Every one of these has since been measured, often to extraordinary precision, and the theory underlies the GPS in your phone.