CLASE DEL 70: Here Sean Carroll's* attempt to squeeze the firewall argument down to its essence, for people who know a little quantum mechanics.

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.