Horizia

Meet Horizia, a horizon: squeeze any mass inside her radius and not even light gets back out.

In 1916, months after Einstein published general relativity, Karl Schwarzschild found its exact solution for the spacetime around a spherical mass. It contains a special radius, r_s = 2GM/c². If all the mass lies inside that radius, it forms a black hole, and r_s marks the event horizon: from inside it, every path, even for light, leads inward.

The radius scales directly with mass. For the Sun's mass, r_s is about 2.95 kilometres; for Earth's, about 9 millimetres. Neither is a black hole, because their mass is spread far wider than that.

Astronomers have found black holes from a few to tens of solar masses, and supermassive ones in galaxy centres. The Event Horizon Telescope imaged the shadow of the black hole in galaxy M87 in 2019 and of Sagittarius A* at the centre of the Milky Way in 2022.

The Schwarzschild solution is for a non-rotating, uncharged mass. Real black holes spin, which changes the horizon's size. What happens at the centre is an open question for a quantum theory of gravity.