drnickbone comments on The Fabric of Real Things - Less Wrong

16 Post author: Eliezer_Yudkowsky 12 October 2012 02:11AM

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Comment author: drnickbone 28 February 2013 01:09:22PM 2 points [-]

On the one hand, we don't want information to disappear beyond the event horizon, so the information must be absorbed into the event horizon itself, theoretically readable off the structure of the horizon (or off Hawking radiation). On the other hand, from the perspective of an infalling observer, nothing special happens as she passes through the horizon. It certainly doesn't seem to her as if the information she carries has been smeared over the event horizon. Susskind's response is essentially that both of these things happen.

An update. I didn't realise this a couple of months ago, but it seems there has been a big controversy brewing recently about black hole complementarity, and whether it is consistent. There was a key paper by Polchinski and three others in August Complementarity or Firewalls?; see also his guest post in Discover.

The basic argument is that there is a new black hole paradox: quantum states on the edge of a horizon have to be fully entangled BOTH with Hawking radiation that has already emerged from the black hole AND with neighbouring states that are just inside the black hole. And that is not possible, because there is a "monogamy" of quantum entanglement. Further, complementarity doesn't help, because an observer could in principle collect the entangled radiation that had already emerged from the black hole, distill it, and then bring it into the black hole to meet its duplicate entangled state inside, which would lead to quantum cloning. Oops. Instead Polchinksi et al propose that there is a "firewall" at the black hole event horizon which would destroy the second entanglement, and also destroy any observers going into the black hole.

There seems to have been a big fight on the high energy physics archive, with lots of authors drafting papers in an attempt to refute Polchinski et al, then withdrawing them or heavily editing them. Bousso also had a go, claiming that they had misunderstood complementarity, then retracted; his latest version argues that they've found a genuine paradox after all.

To be fair, not many of these physicists/cosmologists agree with the firewall solution, probably because it can leads to observers suddenly disappearing into flame without warning (it is possible to reach an event horizon around a very large black hole in otherwise normal space, with no outward-sign that is coming, then smash into the firewall and die). That violates the same sorts of physical intuitions that Eliezer raises in the main article (and which I challenged). It's also not clear exactly when a firewall forms (if it does) or if there are firewalls at cosmological horizons.

Worth watching for more developments.