Can a black hole die?

1the loophole

Everything so far says nothing escapes a black hole, so the idea that one could die sounds like a contradiction. Stephen Hawking showed in 1974 that quantum physics, the rules governing very small things, adds a loophole. The horizon is a one-way door for matter and light that fall in, but it is not perfectly sealed against everything.

Nothing can escape from inside, yet the black hole itself can still lose energy.

2particle pairs

Empty space is not perfectly empty. Quantum physics says pairs of particles constantly flicker into existence and vanish again. Right at the horizon, one partner can fall in while the other flies away. The escaping particle is real, and it carries energy out into space. Hawking's full calculation is subtler than this picture, but the picture captures the essence: the radiation comes from the horizon, not from beyond it.

Particle pairs form at the horizon; one falls in, one escapes carrying energy away.

3shrinking

Energy and mass are linked, so when the black hole pays energy out as radiation, its mass shrinks slightly. Each escaping particle makes the hole a tiny bit lighter. The black hole is slowly spending itself, like a very faint glowing ember that cannot be refueled by its own radiation.

Radiation carries away energy, so the black hole loses mass.

4too cold

The radiation is astonishingly weak. A black hole with the mass of a star glows at a temperature far colder than the faint afterglow of the Big Bang that fills all of space. Because that background is warmer, such a black hole actually absorbs more than it radiates, and for now it grows. Hawking radiation only wins once the universe cools much further.

For star-sized black holes, the radiation is far too faint to matter today.

5the far future

Smaller black holes glow hotter and leak faster, and as a black hole shrinks the leak speeds up, ending in a final burst. For a stellar black hole the timescale is around ten to the power of sixty-seven years or more, and for the giants at galaxy centers far longer still. The universe is only about fourteen billion years old, so no black hole has yet died this way.

Evaporation takes unimaginably long, and shrinking black holes leak faster and faster.

6tug of war

So a black hole's fate is a tug of war. Swallowing gas, stars, or other black holes makes it grow, and Hawking radiation makes it shrink. For now, growth wins. Only in the extremely distant future, when space is colder and nothing is left to eat, could evaporation take over and finish the job.

Growth by swallowing matter wins now; evaporation wins only in the far future.