General Theory of Relativity
In November 1915, Albert Einstein presented the final form of his general theory of relativity to the Prussian Academy of Sciences in Berlin. The theory redefined gravity. Rather than a force pulling objects across empty space, as Isaac Newton had described, gravity became a feature of geometry: mass and energy curve the fabric of spacetime, and objects follow the straightest possible paths through that curved fabric. It remains the foundation of modern cosmology and our understanding of black holes.
The theory grew out of Einstein's special relativity of 1905, which showed that space and time are linked and that the speed of light is the same for all observers. Special relativity, however, did not include gravity, and it sat awkwardly with Newton's law, which implied instantaneous influence across distance. In 1907 Einstein had what he later called his happiest thought: a person in free fall would not feel their own weight. This suggested a deep equivalence between gravity and acceleration. Over the next eight years he worked to turn the insight into mathematics, learning the tools of differential geometry with help from his friend Marcel Grossmann and drawing on the earlier work of Bernhard Riemann and others.
The final push came in autumn 1915. Einstein had published a flawed version of his field equations in 1913 and spent considerable effort revising them. In November he delivered a series of short lectures in Berlin, culminating in the correct equations, which relate the curvature of spacetime to the distribution of matter and energy. The mathematician David Hilbert was working on closely related equations in the same weeks, and the question of priority has been discussed by historians, though Einstein is credited with the physical theory and its development. In one of those lectures he also showed that the theory correctly explained a long-standing anomaly in the orbit of Mercury, which Newtonian gravity could not account for.
Confirmation by observation made Einstein world famous. In 1919, expeditions led by Arthur Eddington and others measured the apparent shift of stars' positions near the Sun during a solar eclipse, a result widely taken to support Einstein's predicted bending of light. Later analysis raised questions about the precision of those early measurements, but subsequent experiments have confirmed the effect many times over.
The legacy is vast. General relativity predicts gravitational time dilation, which GPS satellites must account for, as well as gravitational lensing, black holes, and an expanding universe. In 2015, a century after its formulation, the LIGO observatories announced the first direct detection of gravitational waves, ripples in spacetime that the theory had predicted. It is still our best description of gravity, though physicists continue to seek a way to reconcile it with quantum mechanics.