General relativity, published by Albert Einstein in November 1915, describes gravity not as a force acting between masses but as the curvature of a four-dimensional fabric of space and time, spacetime, caused by the presence of mass and energy within it. Objects moving under the influence of gravity, in this picture, are not being pulled by any force at all, they are simply following the straightest possible path available to them through spacetime that has been bent by nearby mass, much as a straight line drawn on the curved surface of a globe will appear to bend when viewed from outside it.
The theory's central mathematical statement, the Einstein field equations, relates the geometry of spacetime at any point directly to the density of mass and energy present there, and solving them for specific situations, a single star, two orbiting black holes, the universe as a whole, remains one of the central occupations of theoretical physics a century later. The theory predicted several effects Newtonian gravity could not explain or did not predict at all: the slow rotation of Mercury's orbit that had puzzled astronomers for decades, the bending of starlight passing near a massive body, a gravitational redshift in which light loses energy climbing out of a strong gravitational field, and the existence of black holes, regions where spacetime curves so sharply that nothing, not even light, can escape.
Perhaps the theory's most dramatic confirmation came a full century after publication, in September 2015, when the LIGO detector observed gravitational waves, ripples in spacetime itself, produced by two black holes merging more than a billion light years away, a phenomenon Einstein had predicted in 1916 but doubted could ever actually be detected, given how extraordinarily small the resulting distortions in spacetime would be by the time they reached Earth.