What if something leaks out of a black hole after all?

That time, that place

By definition nothing escapes a black hole — not even light. So a black hole should be black, and should stay as it is.

But there was a problem. Whatever it swallows, only mass, spin and charge remain; the rest of the information is gone. That would decrease entropy, which collides with the second law of thermodynamics.

Why this question

Hawking calculated the vacuum just outside the boundary.

In quantum mechanics the vacuum is not empty. Particle-antiparticle pairs appear and vanish constantly, normally meeting again at once.

But right beside the boundary, one of the pair can fall in while the other stays out. The one left behind has lost its partner and cannot vanish, so it flies away.

From far off it looks as though something came out of the black hole.

What was found

So a black hole glows very faintly and loses mass. The radiation has a temperature, and smaller holes are hotter.

Having a temperature means having entropy, and the clash with thermodynamics dissolves.

A new problem appeared instead: if a black hole evaporates completely, where did the information that fell in go? That is the information paradox, argued over for fifty years.

Hawking radiation has not been observed. A stellar-mass black hole is colder than the cosmic microwave background, so for now it takes in more than it gives out.

Terms on this card

Singularity