Saturday, January 14, 2012

Physics & Math

Naked black-hole hearts live in the fifth dimension  

IT SEEMS very rude to come to someone's party and tell him that he lost a bet again," said cosmologist Luis Lehner of the Perimeter Institute in Ontario, Canada. Lehner did it anyway.

He was speaking at a meeting to celebrate Stephen Hawking's 70th birthday. The bet in question was over whether a point of infinite density and space-time curvature, known as a singularity and usually found at the centre of a black hole, can also exist in a naked form without its black hole. At a singularity, all our existing physical laws go out the window. This doesn't normally matter because the black hole that surrounds the singularity is ringed by a one-way membrane called an event horizon, which lets light and information in, but not out. That means the singularity cannot affect anything beyond the event horizon.
DIMENSIONS (3 space, 1 time)Near a naked singularity, however, things would become bewildering as we would no longer be able to predict the fate of anything in its line of sight. "It might not actually do anything nasty, but we have lost predictive power," says Lehner. "I couldn't tell you if this glass would be sitting on the table tomorrow."
In 1991, Hawking bet Kip Thorne and John Preskill of the California Institute of Technology in Pasadena that all singularities are clothed. Six years later he lost, when situations emerged where naked singularities can exist, although they are unstable and vanish or retreat behind an event horizon with the tiniest perturbation.
Lehner has now proposed another situation where naked singularities might exist: in the extra dimensions proposed by string theory. The rub is that this time, they aren't unusual.
To understand why, think of black holes as points in the four dimensions we experience - three of space and one of time. These become "black strings" when extended into a fifth dimension of space (see diagram).
Black strings are unstable and break up into smaller black holes like a stream of water splitting into droplets. Lehner showed that at the point where a smaller black hole pinches off from the stream, the black hole's radius is zero, which means its density is infinite. In other words, it is a naked singularity. Lehner showed this will happen any time you have a black string.
Thorne, the beneficiary of the bet, was in the audience and said the study was "beautiful work". Of course this comes with a big caveat: we only need to worry about these naked singularities if the universe has five dimensions - something that isn't yet clear. String theory suggests there are at least five dimensions, but cosmologists without a stake in the bet might hope that it is proved wrong. Otherwise they will have their work cut out explaining the laws of physics at a naked singularity.

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