In Depth

Soft hair: Stephen Hawking’s final theory explained

Late cosmologist’s last work tackles enduring mystery of ‘information paradox’

Stephen Hawking’s final scientific paper has been released by physicists who worked with the late cosmologist in his decades-long effort to understand what happens to information that falls into black holes.

The work - written up and posted online by his former Harvard and Cambridge university colleagues - tackles what theoretical physicists call the “information paradox”. It was completed just days before Hawking died, in March.

Malcolm Perry, Cambridge professor and co-author of the paper, titled Black Hole Entropy and Soft Hair, said the information paradox was “at the centre of Hawking’s life” for more than 40 years. Put simply, the idea that a black hole, and any information that has fallen into it, can evaporate away to nothing is at odds with the rules of quantum mechanics that demand that information is never lost.

“The difficulty is that if you throw something into a black hole it looks like it disappears,” Perry told The Guardian. “How could the information in that object ever be recovered if the black hole then disappears itself?”

At its core, the theory addresses the problematic nature of black holes, which Albert Einstein believed had only three features - mass, charge and spin.

Hawking later asserted that they also had a temperature and so obeyed the laws of thermodynamics. As such, black holes “must inevitably burn themselves out into nothing as they lose energy”, explains the London Evening Standard.

Hawking “tried to reconcile this with the competing theory that no information - meaning physical information in objects - can ever be lost”, the newspaper continues.

He reasoned that, at least in part, the information that enters a black hole goes into “soft hair” - a haze of photons at the event horizon, the outer periphery of a black hole and its point of no return.

That theory is being explored by fellow scientists including Perry, who told the Guardian: “We think it’s a pretty good step but there is a lot more work to be done. We still don’t have the technology to verify Stephen Hawking’s big ideas.” 

Among the unknowns that Perry and his colleagues must try to explain are how information “is physically stored in soft hair and how that information comes out of a black hole when it evaporates”, says news site Tech2.

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