JOIN USNo escape from black holes?
A black hole has no hair.” That mysterious, koan-like statement by the theorist and legendary phrasemaker John Archibald Wheeler of Princeton has stood for half a century as one of the brute pillars of modern physics. It describes the ability of nature, according to classical gravitational equations, to obliterate most of the attributes and properties of anything that falls into a black hole, playing havoc with science’s ability to predict the future and tearing at our understanding of how the universe works.
Now it seems that the statement might be wrong. Recently Stephen Hawking wheeled across the stage in Harvard’s hoary, wood-paneled Sanders Theatre to do battle with the black hole. It is one of the most fearsome demons ever conjured by science, and one partly of his own making: a cosmic pit so deep and dense and endless that it was long thought that nothing — not even light, not even a thought — could ever escape.
But Hawking was there to tell us not to be so afraid. In a paper published recently in Physical Review Letters, Hawking and his colleagues Andrew Strominger of Harvard and Malcolm Perry of Cambridge University in England say they have found a clue pointing the way out of black holes.
‘Eternal prison’
“They are not the eternal prisons they were once thought,” Hawking said. “If you feel you are trapped in a black hole, don’t give up. There is a way out.” Black holes are the most ominous prediction of Einstein’s general theory of relativity: Too much matter or energy concentrated in one place would cause space to give way, swallowing everything inside like a magician’s cloak.
An eternal prison was the only metaphor scientists had for these monsters until 40 years ago, when Hawking turned black holes upside down — or perhaps inside out. His equations showed that black holes would not last forever. Over time, they would ‘leak’ and then explode in a fountain of radiation and particles. But his calculation violated a tenet of modern physics: that it is always possible in theory to reverse time, run the proverbial film backward and reconstruct what happened in, say, the collision of 2 cars or the collapse of a dead star into a black hole.
The universe, like a kind of supercomputer, is supposed to be able to keep track of whether one vehicle was a green pickup truck and the other was a red Porsche, or whether one was made of matter and the other antimatter. These things may be destroyed, but their ‘information’ — their essential physical attributes — should live forever. In fact, the information seemed to be lost in the black hole, according to Hawking, as if part of the universe’s memory chip had been erased. According to this theorem, only information about the mass, charge and angular momentum of what went in would survive.
Fortunately for historians, Hawking conceded defeat in the black hole information debate 10 years ago, admitting that advances in string theory, the so-called theory of everything, had left no room in the universe for information loss. At least in principle, then, he agreed, information is always preserved — even in the smoke and ashes when you, say, burn a book. With the right calculations, you should be able reconstruct the patterns of ink, the text. Hawking paid off a bet with John Preskill, a Caltech physicist, with a baseball encyclopedia, from which information can be easily retrieved.
Not bald after all
But neither Hawking nor anybody else was able to come up with a convincing explanation for how that happens and how all this ‘information’ escapes from the deadly erasing clutches of a black hole. Indeed, a group of physicists 4 years ago tried to figure it out and suggested controversially that there might be a firewall of energy just inside a black hole that stops anything from getting out or even into a black hole. The new results do not address that issue. But they do undermine the famous notion that black holes have ‘no hair’ — that they are shorn of the essential properties of the things they have consumed.
About 4 years ago, Strominger started noodling around with theoretical studies about gravity dating to the early 1960s. Interpreted in a modern light, the papers — published in 1962 by Hermann Bondi, MGJ van der Burg, AWK Metzner and Rainer Sachs, and in 1965 by Steven Weinberg, later a recipient of the Nobel Prize — suggested that gravity was not as ruthless as Wheeler had said. Looked at from the right vantage point, black holes might not be not be bald at all. The right vantage point is not from a great distance in space — the normal assumption in theoretical calculations — but from a far distance in time, the far future, technically known as ‘null infinity’.
“Null infinity is where light rays go if they are not trapped in a black hole,” Strominger tried to explain over coffee in Harvard Square recently. From this point of view, you can think of light rays on the surface of a black hole as a bundle of straws all pointing outward, trying to fly away at the speed of, of course, light. Because of the black hole’s immense gravity, they are stuck.
But the individual straws can slide inward or outward along their futile tracks, slightly advancing or falling back, under the influence of incoming material. When a particle falls into a black hole, it slides the straws of light back and forth, a process called a supertranslation. That leaves a telltale pattern on the horizon, the invisible boundary that is the point of no return of a black hole — a halo of “soft hair,” as Strominger and his colleagues put it. That pattern, like the pixels on your iPhone or the wavy grooves in a vinyl record, contains information about what has passed through the horizon and disappeared.
“One often hears that black holes have no hair,” Strominger and a postdoctoral researcher, Alexander Zhiboedov, wrote in a 2014 paper. Not true: “Black holes have a lush infinite head of supertranslation hair.” In January, Hawking, Strominger and Perry posted a paper online titled ‘Soft Hair on Black Holes’ laying out the basic principles of their idea. In the paper, they are at pains to admit that knocking the pins out from under the no-hair theorem is a far cry from solving the information paradox. But it is progress. Their work suggests that science has been missing something fundamental about how black holes evaporate, Strominger said. And now they can sharpen their questions. “I hope we have the tiger by the tail,” he said.
Whether or not soft hair is enough to resolve the information paradox, nobody really knows. Reaction from other physicists has been reserved. Juan Maldacena of the Institute for Advanced Study in Princeton, New Jersey, said of the new proposal, “Its significance for the black hole information problem remains to be seen. But it is probable that it plays some role.”