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| Wednesday, 5 May 2004 |
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Black holes : Hair today Just what is inside a black hole? Ever since John Wheeler coined the phrase "black hole", these complex astronomical phenomena have held a peculiar fascination for physicists and laymen alike. Physicists are interested because of the extreme conditions inside and at the edge of a black hole - a region where gravity is so strong that nothing was thought to be able to escape. These conditions test the intersection between the two theories that lie at the heart of modern physics: quantum mechanics and Einsteinian gravity (the latter known, rather confusingly, as the general theory of relativity). Both theories agree perfectly with those observations that have been made so far. But the two seem to be incompatible with each other, putting out of reach one grand, unified theory. Many physicists would like to overcome this obstacle. Laymen are probably more captivated by Dr. Wheeler's nomenclature than by the details of the physics. But black holes are not really black. In the paper that catapulted him to fame in 1974, Stephen Hawking predicted that some black holes should emit radiation (although in a manner that is still not fully understood). And now, it seems that another famous coinage by Dr. Wheeler-that "black holes have no hair" - is also false. What Dr. Wheeler meant by the hair-lessness of black holes was that they could be characterised by just three numbers; mass, angular momentum (roughly speaking, how fast a hole spins) and electric charge. To describe a star, one would have, by contrast, to say what each of the zillions of atoms inside it was doing. Once Dr. Hawking discovered that a black hole radiates, however, the lack of hair led to a paradox. Drop something-an encyclopedia, say-into a black hole, and it would be destroyed and eventually re-emitted as Hawking radiation in a random way. The information in the encyclopedia would be lost. But quantum mechanics dictates. Perhaps surprisingly, that information cannot be destroyed. If the encyclopedia were to fall into a star it would be possible (though admittedly very hard) to reconstruct it by reversing the paths of all the atoms of which it had been composed. Before Dr. Hawking's paper, that point was finessed because no one could prove that the information was not somehow preserved within the black hole. But the Hawking radiation, which is predicted by an ad hoc combination of relativity and quantum mechanics, trumps that finesse and leaves an apparent paradox. In a paper just published in Nuclear Physics B, Samir Mathur and his colleagues at Ohio State University seem to have solved the paradox using string theory which is the best available attempt to reconcile relativity and quantum mechanics. This theory, which postulates that everything in the universe is a consequence of tiny strings oscillating in ten dimensions, was thought to have observable consequences only at very small scales-as much smaller than atoms as atoms are smaller than the solar system. Dr. Mathur showed, however that at high densities of matter, such as those within a black hole, the effects attributable to strings can grow to large sizes. According to Dr. Mathur, the interior of a black hole can be thought of as a ball of strings. This ball modulates the Hawking radiation in a way that reflects the arrangement of the strings inside the hole. So, in effect, it acts as a repository of the information carried by things that have fallen into the hole. Thus, as quantum mechanics requires, no information is destroyed. Besides resolving the information paradox, this theory has the added benefit-at least in the special cases that Dr. Mathur has been able to work out exactly-of getting rid of the "singularity" that had been thought to lie at the centre of every black hole. A singularity is a mathematical anomaly where physical theories such as general relativity break down because quantities that should be finite diverge to infinity. This means that physicists are unable, even in principle, to explain what is actually happening there. It would therefore be quite a boon if Dr. Mathur is correct, and singularities do not actually exist. His result also has a bearing on wider cosmological issues. The early universe would have had a density similar to a black hole, and so the "string-ball" theory would have applied there, too. Though Dr. Mathur is cautious on the matter, his theory might supply an alternative explanation about why-when viewed on the grandest scales-the universe appears remarkably uniform. At the moment, this uniformity is put down to a phenomenon known as cosmic inflation, in which the universe is supposed to have expanded rapidly when it was very young. That expansion would have "locked in" the universe's initial uniform state by stopping local concentrations of matter from forming. Typing the early universe together with strings might provide an alternative explanation for cosmic uniformity. String theory is often criticised because it is abstract and thus hard to compare with reality. But although no one can yet see a black hole close up, and thus test Dr. Mathur's ideas for real, the fact that string theory seems able, in this case, to resolve long-standing inconsistencies between general relativity and quantum mechanics is a big point in its favour. |
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