The black hole information paradox
Throw a book into a black hole. When the hole evaporates, is the book's information gone forever?
Stephen Hawking, 1974–1976
Hawking's bet
Physics has a rule that sounds dull but runs deep: information is never truly destroyed. Burn a diary and, in principle, the smoke, ash and heat still contain every word, scrambled beyond any practical recovery but not erased.
In 1974 Stephen Hawking discovered that black holes aren't completely black. Thanks to quantum effects at their edge, they slowly leak radiation, now called Hawking radiation, and shrink. Given enough time, a black hole evaporates completely.
Then Hawking noticed the problem. His calculation said the radiation was perfectly random, carrying no trace of what had fallen in. So throw in a diary, wait for the black hole to evaporate, and its words would be gone. Not scrambled. Gone. That would break one of the deepest rules in physics.
Hawking bet the physicist John Preskill that information really was lost. In 2004 he changed his mind and gave Preskill a baseball encyclopedia, 'from which information can be retrieved at will'. But nobody has yet shown exactly how the information gets out.
The puzzle
Quantum physics says information can't be destroyed; Hawking's calculation says black holes destroy it.
See it for yourself
What's really going on
The paradox matters because it sits exactly where our two best theories meet: general relativity, which describes black holes, and quantum mechanics, which describes radiation and information. Whatever the answer is, it will be a clue to a theory of quantum gravity.
Most physicists now believe information is preserved, encoded in subtle patterns in the radiation. Calculations published from 2019 onwards reproduced the 'Page curve', the pattern information should follow if it escapes. But how it actually escapes, and what happens to someone falling in, is still being worked out.
Unsolved. This is one of the great unsolved problems of physics.
Think about it
A black hole the mass of the Sun would take far longer than the current age of the universe to evaporate. Does that make the paradox any less important?
More physics paradoxes
- Schrödinger's catA cat in a box is both alive and dead until someone looks. Really?
- Maxwell's demonA tiny demon opens a door and makes heat flow backwards. Free energy forever?
- The double-slit experimentFire particles one at a time through two slits. They make a wave pattern. Watch them and the pattern vanishes.