The event horizon is the point of no return around a black hole. Cross it, and nothing can escape, not even light. It is not a physical surface. There is no wall, no membrane, no sign. It is a boundary in spacetime defined by gravity.

The size of the event horizon depends on the mass of the black hole. The Schwarzschild radius, named after Karl Schwarzschild, who solved Einstein's equations for a non-rotating black hole in 1916, is the radius of the event horizon. For a black hole the mass of the Sun, it is about 3 kilometers. For the supermassive black hole at the center of the Milky Way, it is about 12 million kilometers.

At the event horizon, escape velocity equals the speed of light. Anything crossing it is pulled inexorably inward. From the outside, an object falling in appears to slow down and freeze at the horizon, its light redshifted into oblivion. From the object's perspective, it crosses the horizon in finite time and continues to the singularity.

Event horizons are not just theoretical. The Event Horizon Telescope has imaged the shadow of the black hole in M87, and LIGO has detected the mergers of black holes. Both confirm that event horizons exist.