Regulars from Cosmic Variance will be well acquainted with the idea of “firewalls” around black holes, from reading Joe Polchinski’s guest post on the subject. And then you heard more about them from John Preskill’s post at Quantum Frontiers. Or maybe George Musser’s post at Scientific American. Long story short: there is a believable claim on the market that, if you believe that information is preserved in the evaporation of black holes via Hawking radiation, an infalling observer should be incinerated by a wall of high-energy radiation when they cross the event horizon, in dramatic contradiction to everything classical general relativity would lead you to believe. Important stuff, if true. (“True” might mean “the argument is valid but one of the underlying assumptions is wrong, therefore teaching us something important about quantum gravity.)
Word is now finally leaking out into the more popular press, courtesy of my lovely wife Jennifer Ouellette’s article at Simons Science News, a new initiative from the Simons Foundation. It’s a great article, which I would say even if we were not notoriously nepotistic back-scratchers.
Here’s my 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.