Thursday, 10 January 2013

Death By Black Hole Firewall Incineration It Shall Be

Black holes are a perennial favorite among physics buffs, who by now have the usual facts about such objects down pat: Most black holes form when stars explode as supernovae; all that matter collapsing into a dense object from which nothing, not even light, can escape, because of the strong gravitational effects. Lurking deep within a black hole is a point that is infinitely small and dense called the singularity. While nothing can escape a black hole once it crosses the event horizon, thanks to a peculiar quirk of the quantum vacuum, it evaporates over time, emitting radiation (Hawking radiation) in the process -- and how long it takes for the black hole to evaporate depends on its size (the bigger it is, the longer it takes to evaporate).

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Oh, and if, say, an astronaut happened to accidentally cross the event horizon, he or she would technically be in freefall and thus wouldn't notice anything particularly unusual -- not at first. It's only as said astronaut approached the singularity that gravity would become so extreme, s/he would be "spaghettified."

Except now that might not be the case. There's a hypothesis currently being bandied about by theoretical physicists that, instead, the unfortunate astronaut would encounter a massive wall of fire as s/he tried to cross the event horizon and burn up before s/he got anywhere near the singularity.

Call it the 'Paradox of the Firewall.'

He'd been puzzling over a nagging suspicion that something wasn't quite right with the conventional picture of what happens at the event horizon for years, and was never quite able to put those misgivings to rest. When Polchinski and a few colleagues started playing around with toy models, essentially running the argument for Hawking radiation in reverse, it brought a few salient issues into sharp focus It started earlier this year at the Kavli Institute of Theoretical Physics in Santa Barbara, where string theorist Joe Polchinski is permanently ensconced.

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The result was a controversial paper claiming that in order to not have a firewall at the event horizon, physicists would need to sacrifice another one of their cherished assumptions. Per a Simons Science News article (by yours truly):

At the heart of this particular puzzle lies a conflict between three fundamental postulates beloved by many physicists.

The first, based on the equivalence principle of general relativity, leads to the No Drama scenario: Because Alice is in free fall as she crosses the horizon, and there is no difference between free fall and inertial motion, she shouldn’t feel extreme effects of gravity.

The second postulate is unitarity, the assumption, in keeping with a fundamental tenet of quantum mechanics, that information that falls into a black hole is not irretrievably lost.

Lastly, there is what might be best described as “normality,” namely, that physics works as expected far away from a black hole even if it breaks down at some point within the black hole — either at the singularity or at the event horizon.

Want to get rid of the firewall? It'll cost you, per Polchinski et al., and that price is either conceding that information is lost (which should make Stephen Hawking and his collaborator Kip Thorne pretty happy, since they famously embraced this view in the 1990s), or modifying quantum field theory in some significant way.

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Naturally, not everyone agrees. That paper spawned a flurry of others, many aimed at countering the controversial assertions, plus a couple of blog posts: one by Polchinski (over at Cosmic Variance) and another by Caltech physicist John Preskill. (There's also this latest take at Quantum Moxie.)

It's a complicated, multi-faceted problem, as the Simons Science News article makes clear, and much of the discussion is highly technical. But it's such an intriguing idea, it's worth sharing even just a few simple details.

Physicists are still hammering out a consensus on the solution, and many expect the firewalls argument to be disproved in the end, but it's bound to be interesting along the way, so grab the popcorn and watch the sparks fly, if you dare.

Until it gets resolved, physics fans can have a bit of fun debating the best way to die by black hole: would you choose spaghettification as you approach the singularity, or incineration by a firewall?