There are astrophysical black holes, and then there are mathematical ones. Astrophysical black holes sit in galactic centers, emit jets of hot plasma, and on occasion swallow stars. You have heard of those. Mathematical black holes, on the other hand, serve as the focus of physicists’ thought experiments. They are not formed from collapsing stars, they have existed forever, and they sit in an empty universe. They may be infinitely large, have perfect mirrors around them, or extend into six dimensions of space. You have heard of those too, but maybe you didn’t notice.
The story of black holes began in 1916, when Karl Schwarzschild discovered a solution to Einstein’s equations of general relativity that is able to trap light. The characteristic property of Schwarzschild’s black hole—and of all black holes discovered later—is the presence of an “event horizon,” the surface of a region from within which nothing can escape. Once you cross an event horizon, you are trapped forever.