Every black hole is dying, yet the biggest behave like immortal infrastructure. At the core is Hawking radiation, where quantum field theory near the event horizon predicts a faint emission of particles that steals mass from the hole itself. That emission acts like a leak in a tank, but the leak rate scales with surface gravity and temperature, not with the total amount of mass stored inside.
The paradoxical twist is simple. Big ones are cold. A stellar black hole has a Hawking temperature far below any known interstellar gas, while a supermassive black hole is even colder, with effective temperature so tiny that the surrounding universe is hotter and keeps feeding it. Quantum field theory gives the temperature as inversely proportional to mass, and thermodynamics then sets the radiative power, which falls with the square of that temperature.
Longevity follows from that harsh scaling. As mass grows, evaporation power plunges, but the energy reservoir, given by Einstein’s mass–energy relation, climbs linearly. Combine the two and evaporation time grows roughly with mass cubed. A small black hole could vanish comparatively quickly in a cold universe, yet a giant at a galactic center would need a timespan beyond stellar lifecycles, still shrinking, but on a clock indifferent to galaxies.
