Definition
General relativity is Einstein's 1915 theory of gravity: gravity is not a force acting at a distance but the geometric curvature of four-dimensional spacetime caused by the presence of mass and energy.
Massive bodies tell spacetime how to curve; the curvature tells freely-falling bodies how to move. The mathematical heart of the theory is the Einstein field equations, which relate the geometry of spacetime to its matter and energy content.
Why it matters
How it works
The seed idea is the equivalence principle: a person in a sealed elevator cannot tell, by any local experiment, whether they are floating in free space or falling freely in a gravitational field. Gravity, in other words, can be transformed away locally — which means it cannot be a force in the Newtonian sense. Einstein spent eight years (1907–1915) turning this insight into a mathematical theory of curved spacetime, learning differential geometry from Marcel Grossmann along the way.
The Einstein field equations, written compactly, are G_μν + Λg_μν = (8πG/c⁴) T_μν. The left-hand side is geometry (curvature, expressed through the Einstein tensor and the cosmological constant Λ); the right-hand side is matter and energy (the stress-energy tensor). The two sides together encode "spacetime tells matter how to move; matter tells spacetime how to curve."
The classical tests followed quickly. The 43 arcsecond-per-century anomaly in Mercury's perihelion — known since 1859 and unexplained — fell out as a clean prediction. The 1919 solar eclipse measurement of starlight deflection by the Sun confirmed light bending. Gravitational redshift was measured in the Pound–Rebka experiment (1959), gravitational time dilation in Hafele–Keating (1971), frame-dragging by Gravity Probe B (2011), and gravitational waves by LIGO (2015). Every test to date has agreed with the theory to the limits of measurement precision.