ARE black holes bald or hairy? On that strange and esoteric question may hang the future of the universe’s past. The present, the past and the future are all connected by physical laws, a phenomenon called “causal determinism”. With complete information about a system’s present, it ought therefore be possible to determine all its past and future states. In theory, that applies to any system, up to and including the entire universe.

In 1976, however, an up-and-coming Cambridge-based cosmologist called Stephen Hawking challenged this idea by showing that black holes (which are part of the universe, albeit a rather odd part) should evaporate over the course of time, and eventually vanish. That would cause information about anything they had swallowed (and thus a part of the universe’s past) to be lost, meaning the future could not be determined, even in principle. The naive might think this information loss had happened already, as a consequence of the swallowing. But even though it is inside the hole the information continues to exist for as long as the hole does. Causal determinism is not violated.

A black hole’s disappearance, though, really would mean the information’s loss. This, in turn, would mean the end of causal determinism—and with it the assumptions which underpin the whole of modern physics. Fortunately, 40 years on from his pushy original paper, the now-venerable Dr Hawking thinks he has found a way to permit black holes to evaporate while preserving causal determinism.

Soft particles, the magic ingredient he invokes, form ethereal hairy coats around such holes. These retain records of what has been swallowed and then imprint this information on the radiation that is carrying away the black hole’s substance and causing its evaporation. In a manner analogous to the modulation of a television or radio signal, that radiation (known as Hawking radiation in honour of the man who conceived of it) carries data about a black hole’s meals out into space even after the hole itself is defunct. Causal determinism is therefore preserved.

Soft particles are the area of expertise of Andrew Strominger of Harvard University, one of Dr Hawking’s co-authors on the causal-determinism-rescuing paper, to be published next week in Physical Review Letters. A soft particle is one that has virtually no energy. Dr Strominger showed a few years ago that the vacuum of space teems with them. The calculations he, Dr Hawking and Malcolm Perry, also at Cambridge, have made suggest that when matter falls into black holes it leaves ghostly traces in the form of two sorts of soft particle—photons (the particles of light) and gravitons (the particles, still hypothetical, that theory suggests transmit gravity)—which then hang around the event horizon, the hole’s point of no return. The number and distribution of these particles holds information that would otherwise be lost when a black hole evaporated away.

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The Hawking radiation itself is a consequence of the fact that a vacuum is not actually empty space. Rather, because of quantum uncertainty, pairs of particles (one of matter and one of antimatter) are constantly popping in and out of existence in it. These particles (which are distinct from the soft variety whose emergence Dr Strominger studied) normally annihilate each other soon after they pop up, leaving nothing behind. However, if they appear at a black hole’s event horizon one member of the pair may be pulled in while the other is not. The stranded particle is thus “forced” to become real and acquire energy, which it takes from the black hole. That reduces the hole’s mass (because energy and mass are equivalent, by E=mc2). Given enough such events, the hole will vanish. The wrinkle the new paper introduces is that the behaviour of the now-real Hawking-radiation particle is affected by the soft particles it encounters when it materialises. This causes the modulation that preserves causal determinism.

Previous attempts to explain the Hawking radiation while retaining causal determinism have involved throwing some other cherished physical axiom overboard. One jettisoned the principle of equivalence between acceleration and gravity that lies at the heart of general relativity; others violated aspects of quantum field theory, which describes the interactions of subatomic particles. The cure, in other words, was worse than the disease. The soft-particle explanation does not do this. It fits with all established laws of physics.

As yet, the new explanation is incomplete. So far, the researchers have only computed the effects caused by one property of matter falling into a black hole, its electric charge. They have not shown the effect of its mass, which would also be important. Their calculations therefore account only for part of the information that is lost. But they have established a principle that may lead to a full accounting of the matter. That would let physicists sleep easy in their beds, in the knowledge that reality is once again behaving, at least approximately, how they think it ought to.