Hawking radiation
/ HAW-king /
A black hole is supposed to be the ultimate trap: cross its event horizon and nothing, not even light, can climb back out, so a black hole should be perfectly black and eternal. In 1974 Stephen Hawking showed that when you take quantum mechanics seriously near the horizon, this is not quite true. A black hole glows, ever so faintly, with thermal radiation, and over unimaginable spans of time it can shrink and eventually evaporate away entirely. It was the first concrete result to fuse gravity, quantum theory, and thermodynamics.
The heuristic picture is that the vacuum near the horizon is not empty but seething with quantum fluctuations, virtual particle-antiparticle pairs; occasionally one member falls in while the other escapes to infinity as real radiation, and the infalling partner carries negative energy that reduces the hole's mass. More rigorously, the curved spacetime of the black hole causes an observer far away to see the quantum vacuum as a thermal bath. Hawking's result is that a black hole radiates as a blackbody at the Hawking temperature T = hbar c^3 / (8 pi G M k_B), inversely proportional to its mass M, so smaller black holes are hotter and evaporate faster. This gives a black hole an entropy proportional to its horizon area, S = k_B c^3 A / (4 hbar G).
Hawking radiation matters enormously despite being far too faint to detect for any real astrophysical black hole (a solar-mass hole has a temperature of about 60 nanokelvin, colder than the CMB, so it absorbs more than it emits). Its importance is theoretical: it shows black holes are thermodynamic objects with temperature and entropy, and it poses the black hole information paradox, whether information that falls in is truly lost when the hole evaporates, which would violate quantum mechanics. That paradox is one of the sharpest clues we have about quantum gravity.
A black hole of one solar mass has a Hawking temperature of only about 60 nanokelvin and an evaporation time of roughly 10^67 years, far longer than the current age of the universe. A hypothetical mountain-mass primordial black hole, however, would be hot enough to be evaporating in a final burst of gamma rays today.
Smaller means hotter: Hawking temperature rises as black hole mass falls.
The escaping particle picture is a heuristic, not a literal account; the rigorous derivation is about how curved spacetime transforms the quantum vacuum. Hawking radiation is real in theory but hopelessly faint for astrophysical black holes, and the information paradox it raises is still unresolved.