General Relativity

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.

When Einstein published general relativity in 1915, it was, to most of the world, an obscure piece of mathematical physics understood by only a handful of specialists, its author known mainly within the physics community for his earlier work on the photoelectric effect and special relativity. The theory made one prediction, though, that was possible to test directly with existing technology: starlight passing close to the sun should be deflected by a precise, calculable amount, twice what a simple Newtonian estimate of light bending under gravity would give. The trouble was that starlight near the sun is normally invisible, drowned out by the sun's own light, except during the brief totality of a solar eclipse.

The British astronomer Arthur Eddington, a Quaker and committed internationalist who saw testing a German scientist's theory as a way to help reunite European science after the First World War, organized two expeditions to observe the solar eclipse of May 29, 1919, one to the island of Principe off West Africa, which he led himself, and one to Sobral in Brazil. Both teams photographed stars visible near the darkened sun and compared their positions with photographs of the same stars taken months earlier, when the sun was nowhere near them. When the results were announced that November at a joint meeting of the Royal Society and the Royal Astronomical Society, the measured deflection matched Einstein's prediction rather than the smaller Newtonian figure, and the news made front pages around the world, The Times of London ran the headline Revolution in Science.

The 1919 measurements were not, in hindsight, as clean as they were presented at the time, the Principe plates were of noticeably lower quality than the Sobral ones, and Eddington's team excluded some Sobral data on technical grounds that later historians have debated as possibly, though not clearly, influenced by a wish to confirm Einstein's theory. The core result has held up regardless, far more precise tests since, using radio telescopes and, eventually, direct observation of stars near black holes, have confirmed the deflection general relativity predicts to a high degree of accuracy, so the 1919 expedition's fame has proven durable even though its data was less definitive than the headlines implied.