Nothing escapes a black hole's event horizon, not even light

Nothing that crosses a black hole’s event horizon can get back out, not even light, because at that boundary the escape velocity reaches the speed of light, and nothing can travel faster.

Escape velocity is the speed needed to break free of an object’s gravity: about 11.2 km/s for a rocket leaving Earth. Squeeze the same mass into a smaller ball and the figure climbs. The radius at which it hits light speed is the Schwarzschild radius: about 3 km for an object with the Sun’s mass, and just 9 mm for Earth’s. Anything smaller than its Schwarzschild radius is a black hole.

General relativity gives a second way to see it. Inside the horizon, space is curved so severely that every path light could take points back inward; there is simply no route labelled “out”. The horizon itself is not a solid surface but a boundary enclosing the black hole’s matter.

What the one-way door doesn’t mean

Black holes are not cosmic vacuum cleaners. From far enough away their gravity acts like that of any object of the same mass: replace the Sun with a black hole of equal mass and the solar system would get much colder, but the planets would stay in their orbits. Danger starts only close in.

Because no light leaves, black holes are found indirectly, through the matter and stars around them. Tracking stars orbiting an unseen mass at the Milky Way’s centre, Sagittarius A*, about 4 million times the Sun’s mass, proved a supermassive black hole sits there and won the 2020 Nobel Prize.

There is one theoretical loophole. Stephen Hawking proposed that black holes slowly leak radiation and shrink, eventually evaporating. Theory predicts the smallest primordial black holes should already have vanished, while ones with roughly Earth’s mass could have survived to today.