Stuff that falls into a black hole is gone forever, right? Not so, says Stephen Hawking.
“If you feel you are in a black hole, don’t give up,” he told an audience at a public lecture in Stockholm, Sweden, yesterday. He was speaking in advance of a scientific talk today at the Hawking Radiation Conference being held at the KTH Royal Institute of Technology in Stockholm. “There’s a way out.”
You probably know that black holes are stars that have collapsed under their own gravity, producing gravitational forces so strong that even light can’t escape. Anything that falls inside is thought to be ripped apart by the massive gravity, never to been seen or heard from again.
What you may not know is that physicists have been arguing for 40 years about what happens to the information about the physical state of those objects once they fall in. Quantum mechanics says that this information cannot be destroyed, but general relativity says it must be – that’s why this argument is known as the information paradox.
Now Hawking says this information never makes it inside the black hole in the first place. “I propose that the information is stored not in the interior of the black hole as one might expect, but on its boundary, the event horizon,” he said today.
“Black holes ain’t as black as they are painted”
The event horizon is the sphere around a black hole from inside which nothing can escape its clutches. Hawking is suggesting that the information about particles passing through is translated into a kind of hologram – a 2D description of a 3D object – that sits on the surface of the event horizon. “The idea is the super translations are a hologram of the ingoing particles,” he said. “Thus they contain all the information that would otherwise be lost.”
So how does that help something escape from the black hole? In the 1970s Hawking introduced the concept of Hawking radiation – photons emitted by black holes due to quantum fluctuations. Originally he said that this radiation carried no information from inside the black hole, but in 2004 changed his mind and said it could be possible for information to get out.
Just how that works is still a mystery, but Hawking now thinks he’s cracked it. His new theory is that Hawking radiation can pick up some of the information stored on the event horizon as it is emitted, providing a way for it to get out. But don’t expect to get a message from within, he said. “The information about ingoing particles is returned, but in a chaotic and useless form. This resolves the information paradox. For all practical purposes, the information is lost.”
Last year Hawking made headlines for saying “there are no black holes” – although what he actually meant was a little more complicated, as he proposed replacing the event horizon with a related concept, an apparent horizon. This new idea is compatible with his previous one, which wasn’t really news to theoretical physicists, says Sabine Hossenfelder of the Nordic Institute for Theoretical Physics in Stockholm, who attending Hawking’s lecture.
“He is saying that the information is there twice already from the very beginning, so it’s never destroyed in the black hole to begin with,” she says. “At least that’s what I understood.”
More details are expected later today when one of Hawking’s collaborators Malcom Perry expands on the idea, and Hawking and his colleagues say they will publish a paper on the work next month, but it’s clear he is gunning for the idea that black holes are inescapable. It’s even possible information could get out into parallel universes, he told the audience yesterday.
“The message of this lecture is that black holes ain’t as black as they are painted. They are not the eternal prisons they were once thought,” he said. “Things can get out of a black hole both on the outside and possibly come out in another universe.”
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