02 Sep 2015
The 29-person audience, all VIP physicists, was eager to hear his big announcement and could hear him just fine. They knew, from a pre-announcement announcement the previous night, that Hawking was about to explain his solution to a 40-year-old mystery in physics: how information escapes from black holes.
But while his idea made big headlines, the mere nine minutes of explanation felt vague and confusing to other physicists.
The iconic image of a black hole. But, of course, we have never really seen a black hole, not even a supermassive black hole. Artist’s concept illustrates a supermassive black hole with millions to billions times the mass of our sun. Credit: NASA/JPL-Caltech
What is Hawking’s problem?
Hawking set out to resolve a problem called the “information paradox.” Understanding this snag requires a brush-up on black holes. If light or matter venture past a boundary around the black hole called the “event horizon,” they are done for: The speed required to overcome the black hole’s gravity and escape is greater than the speed of light [c]. So anything that crosses the event horizon—and any information about what it was in its previous life—stays inside the black hole. Whether you, a Snickers bar, or a whole planet fall in, they all end up the same: as anonymous extra mass piled onto the black hole itself.
Or at least that used to be the idea. Then, in 1974, Stephen Hawking showed that black holes slowly evaporate. They continuously leak radiation (later named “Hawking radiation”), dwindling away until there’s nothing left, on timescales ranging form a few billion years to much longer than the current age of the universe. But, as theoretical physicist Carlo Rovelli of Aix-Marseille University, who attended the talk, explains, “This creates a problem: Where has all the stuff gone that fell inside? Where is the information about what fell in? It cannot be anymore ‘just inside,’ because the black hole has disappeared. So, where is it? Is it really lost?”
Quantum mechanics says that information about the stuff can’t be lost. Information can neither be created nor destroyed. But black holes seem to destroy it. But they can’t. But they seem to. That’s the “paradox” part of the information paradox.
Some scientists think the escaping Hawking radiation carries the information out with it, like a set-free hostage who can tell police about the room he just spent five days in.
Ideas abound about how that radiation might spill the beans, and Hawking’s new revelation is just one contender. “The situation is not that there is a big problem, and here is the solution,” says Rovelli. “The situation is that there is a big problem, and there are a dozen solutions … none totally convincing, and now we have a new one.”
Hawking’s big idea
On August 25, as Hawking sat before the esteemed physicists, his voice played through the room’s speakers. “I propose that the information is stored not in the interior of the black hole, as one might expect, but on its boundary, the event horizon,” he said, “in the form of supertranslations of the horizon.”
Translation: If you passed over the event horizon, you would leave an imprint on it. That imprint takes the form of, essentially, a hologram called a supertranslation—a two-dimensional representation of your three-dimensional parts—etched into the black hole’s exterior geometry. When Hawking radiation bubbles up, the event horizon leaves a similar imprint on that radiation. It’s like cosmic re-gifting. The Hawking radiation then streams back out into the universe, carrying the imprint, and the encoded information, with it. That code, though, is scrambled: If you fell into a black hole, we could not create your clone from it (sorry). But, cold comfort, your informational essence wouldn’t be eternally lost.
The idea that information could be stamped onto a black hole’s event horizon was first proposed by Nobel Laureate Gerard ‘t Hooft, and supertranslations—as mathematical ideas—come from the 1960s. We don’t yet know enough about Hawking’s idea to detail how new and different it is.
The problem with Hawking’s solution
And that’s part of the problem: According to colleagues, the details of his “solution” feel fuzzy. “Two big questions are where the information from infalling stuff gets deposited, and how that information later gets transferred to stuff leaving the black hole,” says Steven Giddings, a physicist at the University of California, Santa Barbara.
Those are two big questions—the biggest, most fundamental questions. It’s great that Hawking described the what of his idea, but, in science, the how is much more important. “What we need for a more detailed understanding is a more complete description of the mathematics … to see if they’ve really nailed the answer,” says Giddings. Rovelli agrees, stating, “The picture is very preliminary for the moment.”
They weren’t the only two left scratching their heads. “In the conference, there were many world-class physicists, including Nobel Prize winners,” says Joe Polchinski, Giddings’ colleague at UC Santa Barbara. “I didn’t perceive much enthusiasm about the new idea. Everybody was interested, of course, but I couldn’t detect anybody that appeared convinced.”
Polchinski, who has previously science-battled Hawking about black hole paradoxes, also pointed out a problem beyond the idea’s fuzziness: In Hawking’s scenario, information stays on the event horizon. But the information (using the previous example, you yourself) also falls into the black hole, meaning two copies of it would exist. “In quantum mechanics, information can’t be in two places,” Polchinski says, although he points out that Hawking may have found a way to evade this problem.
Because Hawking’s black-hole revelations (as well as his proclamations about aliens and religion) receive public buzz and papers called “AdS/CFT without holography: A hidden dimension on the CFT side and implications for black-hole entropy” don’t, it may seem that his idea is totally novel. But it’s not. “From what we here understand, his suggestion builds on ideas that people have been tossing around recently,” says Giddings. Rovelli and Polchinski also point out its similarity to ’t Hooft’s 1990s ideas, although Hawking has added “some technical steps.”
Hawking claims he, and co-conspirators Andrew Strominger of Harvard University and Malcolm Perry of Cambridge University, will leak more information in a paper in late September. If that paper throws around some convincing equations—what black-hole theorists require as evidence—the result could be a big deal. Until then, scientists are waiting to reserve judgment. “For the moment the theory is far too sketchy, in the manner it has been presented,” Rovelli says. “Let me put it this way: The big news is Hawking himself: his persona, his popular fame, the wonderful manner in which he communicate to the public and transmits enthusiasm to the public. This is fantastic and is his mastership. His physics is interesting, as many others’ are.”
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