It has been said that Newton gave us answers; Stephen Hawking gave us questions. A trio of scientists are one step closer to resolving the black-hole information paradox, one of the most intriguing physics mysteries of our time.
“Spacetime seems to fall apart at a black hole, implying that space-time is not the root level of reality as suggested by the famous paradox that Stephen Hawking first described five decades ago, but emerges from something deeper,” observes George Musser, author of Spooky Action at a Distance, for Quanta about Hawking’s radical theory that in a fiery marriage of relativity and quantum physics says that when a black hole forms and then subsequently evaporates away completely by emitting radiation, the information that went into the black hole cannot come back out and is inevitably lost, violating the laws of physics that insist unequivocally that information can never get totally lost.
Enter Einstein–The Dissolution of Spacetime
In 2003, Hawking found a way that information might escape during the hole’s evaporation, but he did not prove that the information escapes, so the paradox continued, until now. “They are not the eternal prisons they were once thought,” Hawking said. “Things can get out of a black hole both on the outside and possibly to another universe.”
“Although Einstein conceived of gravity as the curved geometry of space-time, his theory also entails the dissolution of space-time, which is ultimately why information can escape its gravitational prison,” adds Musser, summarizing a landmark series of calculations by three physicists that show that information does escape a black hole through the workings of ordinary gravity with a single layer of quantum effects, which seems impossible by definition based on new gravitational calculations that Einstein’s theory permits, but that Hawking did not include.
“The Most Exciting Thing Since Hawking”
“That is the most exciting thing that has happened in this subject, I think, since Hawking,” said one of the co-authors, Donald Marolf of the University of California, Santa Barbara.
“It’s from that mysterious area — where relativity and quantum mechanics don’t quite mesh, that the question of what happens to information in a black hole emerges,” says ” says researcher Henry Maxfield at the University of California, Santa Barbara in calculating the quantum information content of a black hole and its radiation.
The Big Question
Maxfield was co-author of a paper, co-written with physicists Ahmed Almheiri at the Institute for Advanced Study and MIT’s Netta Engelhardt and Marolf UC Santa Barbara in 2019, that takes us one step closer, says Maxfield, “ to resolving the black hole information paradox. “The hope was, if we could answer this question — if we could see the information coming out — in order to do that we would have had to learn about the microscopic theory,” said Geoff Penington of the University of California, Berkeley, alluding to a fully quantum theory of gravity.
“Black Holes Gently Glow and Radiate”
“It goes back to this problem in the 1970s that Stephen Hawking discovered,” Maxfield explained. Black holes — those extremely dense, high-gravity voids in space-time — aren’t completely “black.” “They gently glow and radiate,” he said. And as they do that, the black holes evaporate. But one element of Hawking’s calculations, Maxfield continued, is that this state of “Hawking radiation” destroys information about the original quantum state of the material drawn into the hole.
“This is very different from what quantum mechanics does,” Maxfield said. “In principle, the laws of physics are completely reversible.” In other words, information about the material’s original quantum state should exist in some form. “So there was this conflict that quantum mechanics behaves one way and gravity seems to behave another way.”
Tip of the Iceberg
“We were interested in something closely related, which was trying to identify where the information is located,” Maxfield said about the non-linear path to their calculation as “a modification to Hawking’s calculation” — broadening it to include a method for quantifying the information.
“So there’s that early radiation when the black hole is still young that doesn’t really carry any information,” Maxfield said about their calculation about how much information is stored in a black hole as it evaporates, and the finding that the amount of information indeed decreases over time.. “But once the black hole has shrunk away to half its size — it takes a very long time — the quantum information starts coming out. This is what you’d expect from quantum mechanics.”
The calculation that Maxfield, Englehardt, Almheiri and Geoff Penington (who was concurrently doing very similar work at Stanford) made, reports UC Santa Barbara, is but a tip of the iceberg.
The Biggest Clue We’ve Had
“It doesn’t mean that we’ve completely understood everything,” Maxfield said. “But it is the biggest clue we’ve had for a really long time as to how this tension gets resolved.”
“They found that the information is coming out, even if they didn’t have all the reasons why it comes out,” Marolf commented. “But the idea is that this is a first step. If you have a way of performing that calculation, you should be able to open that calculation up and figure out what the physical mechanism is. This calculation is something we expect is going to give us insight into quantum processes in black holes and how information comes out of them.”
“I’m very resistant to people who come in and say, ‘I’ve got a solution in just quantum mechanics and gravity,’” said a skeptical Nick Warner of the University of Southern California. “Because it’s taken us around in circles before.”