The Economist explains

Stephen Hawking's answer to a 40-year-old paradox about black holes

By D.J.P.

FEW topics in physics so capture the imagination as black holes. They are the sites of wholesale gravitational collapse: stellar remnants fold in on themselves, reaching a density so extreme that it pokes an incomprehensible little hole in the fabric of space itself. The resulting gravitational pull is so great, the story goes, that anything that crosses a point of no return—the event horizon—is lost, in its entirety, forever. But even that simple formulation comes with a wrinkle that has had theorists pulling their hair out for four decades. This week Stephen Hawking, a physicist known for his work on the astronomical phenomenon, put forward a possible solution to the paradox. But what is the kerfuffle all about?

Fundamentally, the discussion is about information. Every object, every particle, every packet of light energy in the universe acts as a carrier of some information; it is a storehouse of its own physical particulars. Forty years ago, Dr Hawking suggested that when any object falls into a black hole, that information becomes irretrievable to those observing outside. But he also showed that, over time, black holes radiate energy, slowly evaporating into nothingness. The information that they had gobbled up, then, was simply destroyed. To the average scientist, that was heresy: information can be transmuted or transferred or merely jumbled up, but it is never lost. Dr Hawking's suggestion touched off a crisis that Leonard Susskind, a fellow theorist, called "a clash of basic principles like no other since Einstein was young".

Two fertile decades of debate followed, giving rise along the way to entirely new conceptions of how the universe is built (black holes, it seems, are pretty fundamental components of it). As a new branch of physics called string theory found its feet, it turned out to be good at explaining the rules of order and disorder within the event horizon. And a consensus emerged that while his "Hawking radiation" story of evaporating black holes was correct, Dr Hawking's supposition about the loss of information was not. By 2004, he was forced to concede a bet on the outcome (the winner was to receive an encyclopedia, "from which information can be retrieved at will"). Information was saved. But how? It is that question that has preoccupied theorists, not least Dr Hawking himself, since then.

So his announcement at a conference in Sweden (the Hawking Radiation Conference, no less) that he may have cracked the problem was greeted with some excitement, and a predictably enthusiastic media response. His flash of inspiration came when listening to a lecture in April about what are called super-translations, a bit of the heady branch of mathematics known as group theory. Dr Hawking thinks that incoming particles shed their information like a coat as they pass into a black hole, leaving it draped on the event horizon itself. Super-translations mathematically describe how that information influx can slightly jiggle the fabric of space at the horizon, in turn shifting around when and how the black hole radiates. Something of the incoming particle's information is turned into the when-and-how of Hawking radiation: information is transmuted, not lost. But the idea remains a suggestion, rather than a solution. It must still be scrutinised by the scientific community—and, as he noted in his talk, the paper that kicked off the debate 40 years ago was rejected at first. Probably, Dr Hawking will have better luck getting published this time around.

Dig deeper:
The Economist on six of science's unsolved mysteries (August 2015)
The Large Hadron Collider is up and running again (April 2015)
A short history of Stephen Hawking—the film version (December 2014)

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