Hawking proposes solution to black hole problem

Light sliding along the outside of a black hole is the key to understanding what’s inside, Stephen Hawking says.

The proposal from the world’s most famous living physicist, presented August 25 at a conference in Stockholm, is the latest attempt to explain what happens to information that falls into the abyss of a black hole. Losing that information would violate a key principle of quantum mechanics, leading to what’s known as the information paradox.

Hawking and two collaborators claim that the contents of a black hole are inventoried on a hologram on its boundary, the event horizon. Unlike previous descriptions of this hologram, the researchers say, their proposal lays out a specific mechanism for storing information that applies to every black hole in the universe. “This resolves the information paradox,” Hawking said in his presentation at the Hawking Radiation conference at the KTH Royal Institute of Technology.

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Harvard theorist Andrew Strominger, one of Hawking’s collaborators, is more cautious, noting that the research is still in progress. Other physicists say the idea sounds interesting but is only as compelling as the supporting mathematical evidence, which is not yet complete. But Strominger is confident the research will “completely reformulate the way we think about black holes.” Ultimately, physicists hope that resolving the information paradox will help them understand how gravity operates at the tiny scales governed by quantum mechanics.

First things first, though: If you fell into a black hole, you’d die. Black holes are regions of extreme gravity from which not even light can escape. The question that has puzzled physicists for decades is whether your “ashes” — some sort of physical evidence that you existed — would ever come out. In the 1970s, Hawking showed that black holes ultimately disappear due to energy leaking away in the form of Hawking radiation. Information about everything inside the black hole, including dead astronauts, would presumably disappear, too, a startling conclusion that flouts the rules of quantum mechanics. Physicists have devised workarounds but no airtight explanation for how one could recover information from a black hole’s interior (SN: 9/25/04, p. 202).

Now Hawking says that he (along with Strominger and University of Cambridge physicist Malcolm Perry) has solved the problem of his own making. The solution borrows an idea from string theory that the event horizon contains a three-dimensional hologram (two spatial dimensions and one time dimension) that perfectly depicts the four-dimensional spacetime inside a black hole (SN: 5/31/14, p. 16).

The hologram, Strominger says, is made of light that is stuck on the event horizon; the light moves away from the black hole yet makes no progress, as if it were rowing upstream at the same speed as a river’s current. Hawking and colleagues argue that a proton, atom or unlucky person that drifts into the black hole causes some of this light to shift along the event horizon. Since every entrant leaves behind a signature rearrangement, or supertranslation, of the light, the hologram encodes all the stuff inside the black hole. Eventually Hawking radiation emanates from the event horizon, carrying the hologram’s information away from the black hole bit by bit.

Strominger says the challenge is proving that supertranslations have the enormous storage capacity required to preserve all, not just some, of the information about a black hole’s contents. “This might not be the only kind of storage device that the hologram uses,” he says. But he says that applying supertranslations to black holes is an important step forward. And it doesn’t rely on unproven tenets of string theory to be correct.

Several physicists say it’s hard to make judgments without reviewing a paper. “Stephen whet our appetite but didn’t really flesh out the ideas,” says Michael Duff, a theoretical physicist from Imperial College London who attended Hawking’s talk. The suspense will continue until at least next month, when Hawking said he plans to post a paper online. Strominger says to expect a longer wait.