What happens at a black hole's point of no return?

An event horizon is the edge of a black hole: a boundary in spacetime beyond which no signal can ever get back out to the rest of the universe. A black hole is so compact that its gravity stops everything, even light, from escaping, and the event horizon marks exactly where that begins. Outside it, escape is still possible. Inside it, there is no way back.

The strange part is that, according to general relativity, crossing it produces no locally detectable change. There is no wall, no flash, no sign. A traveller falling in would notice nothing special at that moment. Yet a friend watching from far away would see something very different: the traveller would appear to slow down, never quite reach the horizon, and fade into red.

Any mass could become a black hole if squeezed small enough. The size of the event horizon is called the Schwarzschild radius, and it grows with mass: about 3 kilometres for the Sun, and about 9 millimetres for Earth. It is named after Karl Schwarzschild, who worked out the maths in 1915 while serving in the German army during the First World War. In 2019, astronomers finally photographed a black hole's shadow.