Down the black hole

astrophysics

Einstein’s theory on black holes has been challenged for quite sometime now. Scientists have found that following the known laws of physics leads to a contradiction, the “firewall paradox”. However, a recent study has drawn a lot of attention for saying Einstein was right after all, writes Dennis Overbye.

This time, they say, Einstein might really be wrong. A high-octane debate has broken out among the world’s physicists about what would happen if you jumped into a black hole, a fearsome gravitational monster that can swallow matter, energy and even light. You would die, of course, but how? Crushed smaller than a dust mote by monstrous gravity, as astronomers and science fiction writers have been telling us for decades? Or flash-fried by a firewall of energy, as an alarming new calculation seems to indicate?

This dire-sounding debate has spawned a profusion of papers, blog posts and workshops over the last year. At stake is not Einstein’s reputation, which is after all secure, or even the efficacy of our iPhones, but perhaps the basis of his general theory of relativity, the theory of gravity, on which our understanding of the universe is based. Or some other fundamental long-established principle of nature might have to be abandoned, but physicists don’t agree on which one, and they have been flip-flopping and changing positions almost weekly, with no resolution in sight.

Raphael Bousso, a theorist at the University of California, Berkeley, said, “I’ve never been so surprised. I don’t know what to expect.” You might wonder who cares, especially if encountering a black hole is not on your calendar. But some of the basic tenets of modern science and of Einstein’s theory are at stake in the “firewall paradox,” as it is known. “It points to something missing in our understanding of gravity,” said Joseph Polchinski, of the Kavli Institute for Theoretical Physics in Santa Barbara, Calif., one of the theorists who set off this confusion. Down this rabbit hole are many of the jazzy magical mysteries of modern physics: Black holes. The shortcuts through space and time called wormholes.

Quantum entanglement, also known as spooky action at a distance, in which particles separated by light-years can still instantaneously appear to remain connected. The reward for going down this hole could be a new understanding of why we think we live in a universe with space and time at all, with suitably unpredictable consequences. Black holes are the most extreme predictions of Einstein’s theory, which describes how matter and energy warp the geometry of space and time the way a heavy sleeper causes a mattress to sag. Too much matter and energy in one place could cause space to sag so far that the matter inside it would disappear, collapsing endlessly to apoint of infinite density known as a singularity.

Einstein thought the idea was ridiculous when it was pointed out to him at the time, in 1916, but now astronomers agree that the universe is speckled with such dark monsters and many resulted from the collapse of dead stars. General relativity is based on what Einstein later called his “happiest thought,” that a freely falling person would not feel his weight. It says that empty space looks the same everywhere and to everyone.

Things got more interesting, however, in 1974 when Stephen Hawking, the British cosmologist, stunned the world by showing that when the paradoxical quantum laws that describe subatomic behaviour were taken into account, black holes would leak particles and radiation, and in fact eventually explode, although for a hole with the mass of a star it would take longer than the age of the universe. Hawking concluded that the radiation coming from a black hole would be completely random, conveying no information about what had fallen into it. When the black hole finally exploded, all that information would be erased from the universe forever.

Particle physicists cried foul, saying that this violated a basic tenet of modern science and of quantum theory, that information is always preserved. It was front-page news in 2004 when Hawking finally said that he had been wrong.

Now, however, some physicists say that Hawking might have conceded too soon.   Four researchers based in Santa Barbara — Ahmed Almheiri, Donald Marolf and James Sully, all from the University of California, Santa Barbara, and Polchinski of the Kavli Institute have found that following the known laws of physics would lead to a contradiction, the firewall paradox.

Their calculations showed that having information flowing out of a black hole was incompatible with having an otherwise smooth Einsteinian space-time at its boundary, the event horizon. In its place would be a discontinuity in the vacuum that would manifest itself as energetic particles — a “firewall” — lurking just inside the black hole. The existence of a firewall would mean that the horizon, which according to general relativity is just empty space, is a special place, pulling the rug out from under Einstein’s principle, his theory of gravity, and modern cosmology.

The firewall argument hinges on one of the weirder aspects of quantum physics, the action called entanglement. Quantum theory predicts that a pair of particles can be connected in such a way that measuring a property of one — its direction of spin, say — will immediately affect the results of measuring the other one, even if it is light-years away. Einstein used this “spooky action at a distance” to suggest the absurdity of quantum mechanics.

Consider two particles (let’s call them Bob and Alice) that have been radiated by a black hole. Bob left it eons ago, as it began leaking radiation; quantum entanglement theory dictates that in order for the black hole to keep track of what information it has been transmitting, Bob out there has to be entangled with Alice, who just left. But that scenario competes with another kind of entanglement, between particles on either side of the event horizon, the black hole’s boundary.

If space is indeed smooth, as Einstein postulated, Alice must be entangled with another particle, Ted, who is just inside the black hole. But quantum theory forbids promiscuous entanglements. In the language of quantum information, Alice can marry either Bob or Ted, but not both. Alice should have a consistent explanation of the universe, Polchinski explained, “just as we ourselves must, even though we are inside the cosmic horizon.” Recently a new way of solving the firewall conundrum has attracted a lot of attention. In 1935 Einstein and Nathan Rosen had found that, mathematically, black holes could come in pairs connected by shortcuts through space known as wormholes. Physicists have proposed that Bob might be light years away from the event horizon, but he would still be connected to the interior of the black hole thus coming to the conclusion that, “Ted and Bob are the same.” In that case, then, there is no firewall, no contradiction in the laws of physics. And Einstein survives to fight another day.

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