The Day Feynman Worked Out Black-Hole Radiation on My Blackboard

Feynman said that what Bill and Saul were describing sounded like simulated emission, and so there should be a spontaneous emission process analogous to it. Feynman went to the blackboard and began working out the equations for spontaneous emission from black holes. But Feynman had postulated, after listening to Bill and Saul talk at lunch, that if a spinning black hole can emit with light coming in, it can also emit energy with nothing coming in, if you take into account quantum mechanics.

Source: nautil.us