Objects may be able to fall through black holes into an alternate universe, Stephen Hawking suggested as he prepared to publish an eye-catching study in 2016.
The renowned physicist, who died aged 76 in the early hours of Wednesday morning, laid out a hypothesis that suggested the holes were not quite as black as previously thought.
Rather than destroying everything that goes near them, we might not need to be so afraid of black holes, he said in a paper published in Physical Review Letters, written with colleagues Andrew Strominger, a professor of physics at Harvard University, and Malcolm Perry, a professor of theoretical physics at Cambridge University.
If the work is correct – and the paper's publication suggested the claims had received approval from other experts – then it could help solve a central paradox of black holes.
Professor Hawking’s paper addressed a fundamental assumption about black holes – that they have “no hair”. It had previously been assumed that anything that fell into a black hole would be destroyed and lost forever.
That caused problems because the information about the object has to be preserved, even if the object itself is entirely swallowed up, and it had remained unclear how those two things could both happen. The universe is meant to keep a kind of log of what it contains – even if some objects fall into a black hole and are destroyed – and until now the information in that log was thought to be lost inside the hole along with the objects themselves.
But Prof Hawking had been implying since 2015 that anything that falls into a black hole shouldn’t give up hope of coming back out – somewhere. The paradox was solved because the information is stored on the boundary, or event horizon, of the hole, so it doesn’t come back out of the pit so much as stay away from its most terrifying part.
“Black holes are not the eternal prisons they were once thought”, Prof Hawking said in a speech in 2015. “If you feel you are trapped in a black hole, don’t give up. There is a way out.”
That way out wouldn’t take people back to where they’d come from, he has said. Instead, they would reappear, but somewhere else – perhaps even in an alternative universe.
“The existence of alternative histories with black holes suggests this might be possible”, Prof Hawking said. “The hole would need to be large and if it was rotating it might have a passage to another universe. But you couldn’t come back to our universe.
“So, although I’m keen on space flight, I’m not going to try that.”
Prof Hawking’s suggestions helped keep some of the most central parts of our assumptions about the universe intact.
If it is possible to destroy information, for instance, then it is possible to speculate that the past might not exist at all. Black holes would be able to delete parts of the past – and, Prof Hawking said, “It’s the past that tells us who we are. Without it we lose our identity."
Previously, scientists were not sure how exactly that information could be preserved when it dropped into the dark pit of the black hole. They do that through the “hair” that was previously rejected.
That means that if you were able to look at a black hole in the right way – essentially standing at a distance far ahead in time – you might indeed see those hairs hovering on the edge. The pattern that you would be able to see would be made up of the “soft hairs” that serve as a log of what has been lost inside the black hole.
Those hairs, the workings of which were laid out in the paper, still did not fully solve the information paradox about where exactly it could go. But they could provide some of the fundamental parts that are needed to try to help us understand exactly how they work.
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