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Physicists Euphoric but Confused about Black Hole Paradox

The recently proposed idea of “black hole firewalls” has physicists questioning some of their most cherished ideas
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“The most exciting phrase to hear in science, the one that heralds new discoveries, is not ‘eureka!’ but ‘that's funny,’” Isaac Asimov once said. Well, something seriously funny is going on in theoretical physics these days. A recent conundrum about black holes is threatening to overturn some of the most basic tenets of physics, and many scientists are nothing but thrilled.

“To me it’s the best thing that’s happened in awhile,” says University of California, Berkeley, physicist Raphael Bousso of the so-called “black hole firewall paradox,” which concerns what happens at the boundary of a black hole. “This is a 9 on the Richter earthquake scale—it’s by far the most shocking and surprising thing that has happened in my career.” The quandary prompting such jubilation is an idea first put forward in July 2012, which was extended in a paper published October 21 in Physical Review Letters. Physicists have long assumed that space is smooth at a black hole’s event horizon—the point of no return where nothing that passes through can escape. A person crossing over that line shouldn’t immediately notice anything amiss, however, and neither should a distant observer watching that person. But physicists have also assumed that information can never be destroyed. The new work says those two ideas are mutually incompatible. “It’s a paradox because several things we believed were true can’t all be true,” says Joseph Polchinski of the Kavli Institute for Theoretical Physics and U.C. Santa Barbara, one of the main architects of the firewall idea.

Polchinski and his colleagues conclude that not only is space not smooth at a black hole horizon—at that point the laws of physics completely break down. Instead of an unobtrusive boundary, the scientists argue that there must actually be a sharp division they call a firewall. “The firewall is kind of a wall of energy—it could be the end of spacetime itself,” Polchinski says. “Anything hitting it would break up into its fundamental bits and effectively dissolve.” At first, many physicists strenuously objected to the bizarre idea of firewalls. “I tried very hard to get rid of them, but I don’t think it’s likely that will happen,” Bousso says. “I’ve decided that the most promising thing for me is to assume there are firewalls, and look into why they form.” Even the main authors of the idea aren’t completely onboard. “There is a group of people, including me half the time, that thinks there must be some subtle assumption that we’ve made that’s not valid,” Polchinski says. Yet he and everyone else admit they haven’t identified a flaw in the reasoning so far.

The first argument for firewalls, put forward by Polchinski and his U.C. Santa Barbara, colleagues Ahmed Almheiri, Donald Marolf and James Sully, relied on the complex quantum mechanical concept of entanglement, where two particles can be separated over a distance but retain a profound connection. The new paper strengthens and simplifies the case for firewalls by sidestepping the issue of entanglement altogether, Marolf says. “It shows very clearly that some things you might have worried about are red herrings and not relevant to the argument.”

The new paper is far from the last word on the subject, though. In the year since the firewall idea was proposed, more than 100 papers have addressed the idea, and firewalls have been the subject of three conferences and workshops. “The last year has witnessed the kind of development we live for,” Columbia University physicist Brian Greene says. “It’s where the rubber hits the road.”

Whereas the revelation of firewalls is a breakthrough—it revealed a problem no one realized was there, and has forced scientists to rethink some of the progress that appeared to have been made in resolving a fundamental conflict in physics—the incompatibility of general relativity that describes the universe on a grand scale and quantum mechanics, which applies to the subatomic world. Both theories are needed to address tiny, yet massive black holes, but the two theories cannot currently work together.

In 1997 Juan Maldacena of the Institute for Advanced Study in Princeton, N.J., found a way to reconcile some aspects of the conflicting theories and formulate a version of quantum gravity that could describe black holes. His work was a major step forward but the firewall theory shows that Maldacena’s advance didn’t solve as many problems as was assumed at the time. “People had lulled themselves to sleep” thinking certain black hole dilemmas had been resolved, says Columbia physicist and mathematician Peter Woit. “This has shaken people out of their dogmatic slumber and they’re realizing, ‘Wait a minute, we don’t really understand this.’” Marolf agrees: “It forces us to reconsider issues that we thought were settled,” he says. “It’s definitely dismaying to see how far we still have to go.”

How to move forward now is less than clear, however. “I think it’s fair to say quantum gravity is stuck,” says Matt Strassler, a visiting physicist at Harvard University. “It’s not obvious that any big progress is being made at the moment.” Not to worry, though—there’s nothing physicists love more than a challenge. “I think we have a long way conceptually to go,” Stanford University physicist Leonard Susskind said earlier this month during a Google+ Hangout on the topic of firewalls hosted by the Kavli Institute. “I think there’s a large gap in our understanding. We’ll fill it though. Rest assured, we’ll get it.”

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