Stephen Hawking has published a new theory on how black holes are able to store information. He proposed the theory last year, but now with colleagues Malcolm Perry and Andrew Strominger he has revealed his research. A pre-print is available on arXiv.
The paper attempts to solve the famous black hole "information loss" paradox. We know that nothing can escape from a black hole, not even light itself. Thus, any information on what a black hole has consumed should also be lost.
However, this poses somewhat of a problem, as it suggests all black holes are essentially identical; none of them would contain information on what happened in their history. So to counteract this, scientists had proposed a “hair” theory. This suggested that ripples in space-time, the so-called hairs, could store information at the event horizon, the boundary to the inner black hole.
"This paper gives an explicit description of soft hair in terms of soft gravitons or photons on the black hole horizon," the paper states.
The hairs work like this: They are individual rays of light moving away from the black hole, but they get "frozen" at the event horizon, explains Michael Byrne for Motherboard. They are deformities in space-time in the form of "super-translations" that could contain information on the black hole’s past.
"There’s this symmetry of a black hole that we all knew about in which you move uniformly forward and backward in time along all of the light rays," explained Strominger, one of Hawking’s co-authors, in an interview with Scientific American.
"But there’s another symmetry, which is the new thing in this paper (though various forms of it have been discussed elsewhere). It’s a symmetry in which the individual light rays are moved up and down. See, individual light rays can’t talk to each other – if you’re riding on a light ray, causality prevents you from talking to somebody riding on an adjacent light ray. So these light rays are not tethered together. You can slide them up and down relative to one another. That sliding is called a super-translation."
Hawking first presented the theory in Stockholm, Sweden, in August 2015. KTH
Hawking and his colleagues are particularly interested in particles with pretty much zero energy, known as “soft” particles, the soft hairs in this case. The particles lie at the event horizon, so when other particles are ejected – known as Hawking radiation – they can pick up this information on their way into the universe.
In August, though, when he first presented the theory, Hawking said this picked up information was “chaotic and useless.” But it would resolve the information loss paradox, and mean that black holes are different from each other.
The paper is very much just a theory for now, and the "hair vs no hair" debate has already begun. "Will this actually resolve anything or just leave us in an even greater state of confusion? I’m not really sure," physicist Ulf Danielsson of Sweden’s Uppsala University told Scientific American back in August.
Nonetheless, it is an interesting proposal that may explain how black holes can retain some information – and it could be a step towards solving one long-standing paradox in particle physics.