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Information May Leak from Black Holes at Dial-Up Speeds

Why a leaky black hole is more like a mirror ball from hell



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NEW ORLEANS—A new study hints that black holes might not be as good at keeping secrets as researchers have long thought. A pair of physicists has reexamined the time it would take for information (think: your iPhone's memory) to potentially escape from inside a black hole.

They find that the 1s and 0s of your address book could be recovered as quickly as 1,000 bits per second—far faster than previously expected. "The black hole really behaves like an information mirror," says physicist John Preskill of the California Institute of Technology in Pasadena.

The finding, presented here at a meeting of the American Physical Society, marks the latest attempt to come to grips with the fate of information that has crossed the black hole event horizon, a boundary beyond which even light cannot escape. There is no doubt, of course, that the ultradense singularity at the heart of a black hole would vaporize an iPhone. The question is whether there is any imaginable way to piece together its original state.

Physicist Stephen Hawking first broached the subject in the 1970s. He postulated that black holes would gradually evaporate by radiating particles (now called Hawking radiation) that had bubbled up from the vacuum around the event horizon. The radiation would be so scrambled, he argued, that when the black hole disappeared after many trillions of years, all the information about its contents would be lost. Other researchers insisted that the data might be imprinted on Hawking's particles, and even Hawking has changed his mind.

As in previous work, Preskill and physicist Patrick Hayden of McGill University in Montreal imagined two citizens of a hyperadvanced civilization, Alice and Bob. Alice wants to destroy some bits (technically, quantum bits that are 0 and 1 simultaneously) by throwing them into a black hole. Bob aims to recover them by gathering all the Hawking radiation from the black hole. Prior research had shown that if Alice dropped her bits into a relatively young black hole, Bob would have to collect the Hawking radiation for half the life of the black hole before being able to decode a single one of Alice's bits.

In the new study, Alice holds onto her bits until after the black hole has reached the halfway mark. Before her data dump, Bob managed to prepare some bits of his own that he entangled with Alice's, meaning they were linked instantaneously across any distance. Preskill reports that Bob could reconstruct Alice's bits by mixing the next few bits of Hawking radiation following the data dump with what he'd already collected, along with his own bits. "It might be a very difficult quantum computation to do the decoding," he says, but Bob would need only about 10 percent more Hawking particles than the number of bits that Alice had thrown in. A black hole the size of the sun would emit up to 1,000 Hawking particles per second, Hayden says.

The catch is that Alice's dumped bits would have to rapidly mix with the rest of the black hole, spreading their entanglement to the outgoing Hawking radiation that Bob collects. Current theories cannot predict the speed at which entanglement would spread across a black hole. Still, researchers are impressed by the novel application of quantum information theory. "I didn't think there was much else you could say about black holes without a quantum theory of gravity," says researcher Fotini Markopoulou of the Perimeter Institute for Theoretical Physics in Waterloo, Ontario.

The effect comes perilously close to contradicting known rules of physics. After reconstructing Alice's bits, Bob could throw his copy into the black hole, where the two copies might run into one another. Such a meeting would violate the uncertainty principle of quantum mechanics because it would allow measurements of both the position and momentum of the identical quantum state. But the few-second delay between when Alice and Bob can dump their respective bits is just long enough, Preskill says, for her bits to be destroyed by the singularity.

Such a narrow escape appeals to Stanford University physicist Leonard Susskind, who explains that if Bob had to wait for half the lifetime of a black hole to extract his first bit, quantum mechanics would be safe by a curiously wide margin, in his view. "I like the idea," he says, "that the most dangerous experiment you can think of is right on the edge."

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