Today’s Top Science Headline: Stephen Hawking’s “Last Riddle” –‘Could Lead to the Theory of Quantum Gravity’




The renowned British physicist, who died at 76, left behind a riddle that could eventually lead his successors to the theory of quantum gravity.

Hawking was something of a betting man, regularly entering into friendly wagers with his colleagues over key questions in theoretical physics. “I sensed when Stephen and I first met that he would enjoy being treated irreverently,” wrote John Preskill, a physicist at the California Institute of Technology, earlier today on Twitter. “So in the middle of a scientific discussion I could interject, ‘What makes you so sure of that, Mr. Know-It-All?’ knowing that Stephen would respond with his eyes twinkling: ‘Wanna bet?’”


And bet they did, writes Jennifer Ouellette in Quanta. In 1991, Hawking and Kip Thorne bet Preskill that information that falls into a black hole gets destroyed and can never be retrieved. Called the black hole information paradox, this prospect follows from Hawking’s landmark 1974 discovery about black holes — regions of inescapable gravity, where space-time curves steeply toward a central point known as the singularity. Hawking had shown that black holes are not truly black. Quantum uncertainty causes them to radiate a small amount of heat, dubbed “Hawking radiation.” They lose mass in the process and ultimately evaporate away. This evaporation leads to a paradox: Anything that falls into a black hole will seemingly be lost forever, violating “unitarity” — a central principle of quantum mechanics that says the present always preserves information about the past.

Hawking and Thorne argued that the radiation emitted by a black hole would be too hopelessly scrambled to retrieve any useful information about what fell into it, even in principle. Preskill bet that information somehow escapes black holes, even though physicists would presumably need a complete theory of quantum gravity to understand the mechanism behind how this could happen.

Physicists thought they resolved the paradox in 2004 with the notion of black hole complementarity. According to this proposal, information that crosses the event horizon of a black hole both reflects back out and passes inside, never to escape. Because no single observer can ever be both inside and outside the black hole’s horizon, no one can witness both situations simultaneously, and no contradiction arises. The argument was sufficient to convince Hawking to concede the bet. During a July 2004 talk in Dublin, Ireland, he presented Preskill with the eighth edition of Total Baseball: The Ultimate Baseball Encyclopedia, “from which information can be retrieved at will.”

Thorne, however refused to concede, and it seems he was right to do so. In 2012, a new twist on the paradox emerged. Nobody had explained precisely how information would get out of a black hole, and that lack of a specific mechanism inspired Joseph Polchinski and three colleagues to revisit the problem. Conventional wisdom had long held that once someone passed the event horizon, they would slowly be pulled apart by the extreme gravity as they fell toward the singularity. Polchinski and his co-authors argued that instead, in-falling observers would encounter a literal wall of fire at the event horizon, burning up before ever getting near the singularity.

Image credit: BBC and Getty Images

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