Definition
Hawking radiation is the thermal radiation that quantum field theory predicts black holes must emit from just outside their event horizon, causing them to slowly lose mass over time and eventually evaporate.
The temperature of the radiation is inversely proportional to the black hole's mass: small black holes are hot, large black holes are cold. A solar-mass black hole has a Hawking temperature of about 60 nanokelvin — colder than the cosmic microwave background by many orders of magnitude.
Why it matters
How it works
In quantum field theory, the vacuum is not truly empty: virtual particle-antiparticle pairs are constantly being created and annihilated. Hawking realized that near a black hole's event horizon this picture changes drastically. One member of a pair can fall inside the horizon while the other escapes. From far away, the escaping particle looks like a real emission, while the infalling partner carries negative energy that reduces the black hole's mass.
A more rigorous calculation tracks quantum field modes as they propagate near the horizon. The mode that an observer falling into the black hole would call vacuum looks, to a far-away observer, like a thermal bath of particles at the Hawking temperature T = ℏc³ / (8πGMk). For a Schwarzschild black hole the spectrum is precisely thermal — the same as any blackbody emitter at that temperature.
As a black hole radiates it loses mass; losing mass makes it hotter; getting hotter makes it radiate faster. The process accelerates and ends in a final flare of high-energy radiation. The lifetime scales as M³, which is why stellar-mass black holes are effectively eternal but a primordial black hole of mass ~10¹² kg should be evaporating today, producing a detectable gamma-ray burst at the end. No such bursts have been observed yet, placing limits on primordial black-hole abundance.