Friday, January 24, 2014

Hawking Proposes That Black Holes Lack Event Horizons

From Nature:
Now [in lieu of an event horizon, as predicted by Relativity, or a firewall, as predicted by quantum mechanics] Hawking proposes a third, tantalizingly simple, option. Quantum mechanics and general relativity remain intact, but black holes simply do not have an event horizon to catch fire. The key to his claim is that quantum effects around the black hole cause space-time to fluctuate too wildly for a sharp boundary surface to exist.

In place of the event horizon, Hawking invokes an “apparent horizon”, a surface along which light rays attempting to rush away from the black hole’s core will be suspended. In general relativity, for an unchanging black hole, these two horizons are identical, because light trying to escape from inside a black hole can reach only as far as the event horizon and will be held there, as though stuck on a treadmill. However, the two horizons can, in principle, be distinguished. If more matter gets swallowed by the black hole, its event horizon will swell and grow larger than the apparent horizon.

Conversely, in the 1970s, Hawking also showed that black holes can slowly shrink, spewing out 'Hawking radiation'. In that case, the event horizon would, in theory, become smaller than the apparent horizon. Hawking’s new suggestion is that the apparent horizon is the real boundary. “The absence of event horizons means that there are no black holes — in the sense of regimes from which light can't escape to infinity,” Hawking writes.

... Unlike the event horizon, the apparent horizon can eventually dissolve. Page notes that Hawking is opening the door to a scenario so extreme “that anything in principle can get out of a black hole”. Although Hawking does not specify in his paper exactly how an apparent horizon would disappear, Page speculates that when it has shrunk to a certain size, at which the effects of both quantum mechanics and gravity combine, it is plausible that it could vanish. At that point, whatever was once trapped within the black hole would be released (although not in good shape).

If Hawking is correct, there could even be no singularity at the core of the black hole. Instead, matter would be only temporarily held behind the apparent horizon, which would gradually move inward owing to the pull of the black hole, but would never quite crunch down to the centre. Information about this matter would not destroyed, but would be highly scrambled so that, as it is released through Hawking radiation, it would be in a vastly different form, making it almost impossible to work out what the swallowed objects once were.

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