Black Holes

Karl Schwarzschild, a German astrophysicist serving on the Russian front in World War I, solved Einstein's general-relativity equations for the simplest case — the spacetime around a single non-rotating mass — and sent the solution to Einstein in January 1916. He died of a rare autoimmune disease four months later, age 42. His solution contained a feature that took four decades to take seriously: at a certain radius, the pull becomes so steep that not even light can climb back out. The phenomenon was treated as a mathematical curiosity until, in the 1960s, X-ray astronomy found binary stars that could only be explained by compact, dark, collapsed objects. Black holes are real. The 2019 Event Horizon Telescope image of M87's central black hole — a luminous ring around a darkness the size of our solar system — was the first direct visual confirmation.

A black hole is a region of space where gravity is so strong that nothing — not even light — can escape. The boundary is the event horizon: the point of no return, past which nothing can climb back out. For a non-rotating hole, this horizon sits at the Schwarzschild radius — about 3 km across for a mass equal to the Sun's, 9 mm for the Earth's. Inside it, every possible path leads inward to a central singularity, a point of formally infinite density where general relativity itself breaks down. Remarkably, a settled black hole is described completely by just three numbers — its mass, electric charge, and spin (the 'no-hair' theorem); the rotating case is the Kerr solution of 1963. Stellar-mass black holes form when the core of a massive star collapses once fusion can no longer hold it up. Supermassive ones, millions to billions of times the Sun's mass, sit at the centres of nearly all galaxies, including the Milky Way's Sgr A* (~4 million solar masses, its orbiting stars tracked by 2020 Nobel laureates Genzel and Ghez). And black holes are not perfectly black: in 1974 Stephen Hawking showed that quantum effects at the horizon make them glow faintly and, over immense spans of time, slowly evaporate — Hawking radiation, fainter the more massive the hole. This raises the information paradox: that radiation looks featureless and seems to carry nothing out, yet quantum mechanics insists information can never be destroyed — a central unsolved problem in quantum gravity. Gravitational waves from merging black holes, first caught by LIGO in 2015, are now routine, giving direct experimental access to gravity at its most extreme.