If information escapes from a black hole,
the radiation emitted at late times
must share quantum entanglement
with radiation that escaped at early times,
in order to describe a pure quantum state
(from which the black hole presumably formed).
At the same time, to an observer near the event horizon,
the local conditions are supposed to look almost like empty space
— the quantum vacuum. But within that vacuum are virtual particles,
some of which will eventually escape in the form of radiation
and some of which will eventually fall into the black hole.
In order for the state near the horizon to look like the vacuum,
that outgoing radiation and the ingoing radiation must also be entangled.
Therefore, it appears that the outgoing radiation
is both entangled with the ingoing radiation,
and with the radiation that escaped at earlier times.
But that’s impossible; quantum mechanics
won’t let degrees of freedom be separately (maximally) entangled
with two different other sets of degrees of freedom.
Entanglement is monogamous.
A simple — but unpalatable — way out
is to suggest that the state near the horizon
is not a quiet state of maximal entanglement,
but a noisy thermal state of high-energy radiation — a firewall.
It’s a slightly tricky business, as you expect it to be
when we’re mixing up quantum mechanics
with things happening in spacetime,
in the absence of a once-and-for-all
theory of quantum gravity.
Probably most people who have thought about the issue
don’t believe firewalls really exist (although some do),
but in that either there’s a secret flaw in the argument,
or one of our fundamental assumptions is out of whack.
Maybe information is not conserved,
or maybe it’s transferred faster than light,
or maybe quantum mechanics
doesn’t work quite the way we thought.
The story should continue
to be interesting no matter what happens.
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