![]() ![]() This is much earlier than physicists assumed. Page calculated that this reversal would have to occur roughly halfway through the process, at a moment now known as the Page time. Illustration: Samuel Velasco/Quanta Magazine Maybe, thought Page, information can come out of the black hole in a similarly encrypted form. But together they unlock the information. ![]() The password, if you have chosen a good one, is meaningless too. The data without the password is gibberish. It’s like encrypting your data with a password. If you measure either the radiation or the black hole on its own, it looks random, but if you consider them jointly, they exhibit a pattern. The emitted radiation maintains a quantum mechanical link to its place of origin. He considered an aspect of the process that had been relatively neglected: quantum entanglement. The puzzle wasn’t just what happens at the end of the black hole’s life, but also what leads up to it. On his Pasadena vacation, Page realized that both groups had missed an important point. “But particle physicists tended to agree with me.” “Most general relativists I talked to agreed with Hawking,” said Page. In 1980 he broke with his former adviser and argued that black holes must release or at least preserve information. But Page was perturbed, because irreversibility would violate the fundamental symmetry of time. Hawking and most other theorists at the time accepted that conclusion-if irreversibility flouted the laws of physics as they were then understood, so much the worse for those laws. That makes black hole formation and evaporation an irreversible process, which appears to defy the laws of quantum mechanics. It would be impossible to recover whatever fell in. All that’s left is a big amorphous cloud of particles zipping here and there at random. That’s a problem, because at some point the black hole emits its last ounce and ceases to be. Nothing about the radiation reveals whether it came from an astronaut or a lump of lead. Yet when the hole emits the equivalent of 100 kilograms in radiation, that radiation is completely unstructured. If a 100-kilogram astronaut falls in, the hole grows in mass by 100 kilograms. The particles it sheds appear to carry no information about the interior contents. Photograph: John Ulan/University of Alberta The revised semiclassical theory has yet to explain how exactly the information gets out, but such has been the pace of discovery in the past two years that theorists already have hints of the escape mechanism.ĭon Page at the University of Alberta in 2017. Not only does information spill out, anything new that falls in is regurgitated almost immediately. The hole transforms from a hermit kingdom to a vigorously open system. Muted at first, these effects come to dominate when the black hole gets to be extremely old. ![]() They have found additional semiclassical effects-new gravitational configurations that Einstein’s theory permits, but that Hawking did not include. That is what the authors of the new studies dispute. Any further progress would have to treat gravity, too, as quantum. Physicists figured that Hawking had nailed the semiclassical calculation. Hawking and others sought to describe matter in and around black holes using quantum theory, but they continued to describe gravity using Einstein’s classical theory-a hybrid approach that physicists call “semiclassical.” Although the approach predicted new effects at the perimeter of the hole, the interior remained strictly sealed off. The more sophisticated understanding of black holes developed by Stephen Hawking and his colleagues in the 1970s did not question this principle. According to Einstein’s general theory of relativity, the gravity of a black hole is so intense that nothing can escape it. This is a peculiar role reversal for gravity. Original story reprinted with permission from Quanta Magazine, an editorially independent publication of the Simons Foundation whose mission is to enhance public understanding of science by covering research developments and trends in mathematics and the physical and life sciences. ![]()
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