In November 1915, after eight years of dead-ends and one famous breakdown, Albert Einstein published the field equations of general relativity. The theory replaced Newton's universal gravitation — a force acting instantaneously at a distance — with something stranger and more elegant: matter and energy curve spacetime, and what we call gravity is the geometry of that curvature. Falling bodies follow straight lines through curved space; planets orbit because the Sun has bent the geometry around it. Mass tells space how to curve; space tells mass how to move. The theory was confirmed in 1919 by a British eclipse expedition and made Einstein, almost overnight, the most famous scientist in the world.
General relativity's predictions have held up over a century with extraordinary precision. Mercury's perihelion precession, anomalous under Newton, falls out of GR's equations exactly. Light bending around massive objects (predicted in 1916, observed in 1919) became the basis of gravitational lensing. Gravitational time dilation — clocks running slower in stronger gravity — is now corrected for in GPS satellites at the nanosecond level (without GR, GPS would drift by kilometres per day). Black holes, predicted by Schwarzschild in 1916 and treated as mathematical curiosities for decades, were observationally confirmed in stages culminating in the 2019 Event Horizon Telescope image of M87's central black hole. Gravitational waves, predicted in 1916, were directly detected by LIGO in 2015 from a binary black hole merger 1.3 billion light-years away. The theory's only known failure is at singularities (black hole centres, the Big Bang), where general relativity and quantum mechanics give incompatible answers — the quantum gravity problem is unsolved a century later.
Modern cosmology is essentially applied general relativity: the expansion of the universe, the cosmic microwave background, dark matter, dark energy, the inflationary epoch, the geometry and fate of the universe. Black hole physics is a thriving observational science. Gravitational-wave astronomy opened a new window in 2015 and now detects events at a rate of dozens per year. The unresolved quantum gravity programme — string theory, loop quantum gravity, asymptotic safety — is one of the most ambitious projects in contemporary theoretical physics, and a unified theory remains the field's holy grail.