Black hole information paradox


Black hole information paradox

The black hole information paradox results from the combination of quantum mechanics and general relativity. It suggests that physical information could "disappear" in a black hole. It is a contentious subject since it violates a commonly assumed tenet of science—that information cannot be destroyed. [cite video | people = Stephen Hawking | title = The Hawking Paradox | publisher = Discovery, Inc. | location = Discovery Channel, The | date = 2006]

Hawking radiation

In 1975, Stephen Hawking and Jacob Bekenstein showed that black holes should slowly radiate away energy, which poses a problem. From the no hair theorem one would expect the Hawking radiation to be completely independent of the material entering the black hole. Nevertheless, if the material entering the black hole were a pure quantum state, the transformation of that state into the mixed state of Hawking radiation would destroy information about the original quantum state. This violates Liouville's theorem and presents a physical paradox.

More precisely, if there is an entangled pure state, and one part of the entangled system is thrown into the black hole while keeping the other part outside, the result is a mixed state after the partial trace is taken over the interior of the black hole. But since everything within the interior of the black hole will hit the singularity within a fixed time, the part which is traced over partially "might" "disappear", never to appear again. Of course, it is not really known what goes on at singularities once quantum effects are taken into account, which is why this theory is conjectural and controversial.

Hawking was convinced, however, because of the simple elegance of the resulting equation which "unified" thermodynamics, relativity, gravity, and Hawking's own work on the Big Bang. This annoyed many physicists, notably John Preskill, who in 1997 bet Hawking and Kip Thorne that information was not lost in black holes.

There are various ideas about how the paradox is solved. Since the 1997 proposal of the AdS/CFT correspondence, the predominant belief among physicists is that information is preserved and that Hawking radiation is not precisely thermal but receives quantum corrections. Other possibilities include the information being contained in a Planckian remnant left over at the end of Hawking radiation or a modification of the laws of quantum mechanics to allow for non-unitary time evolution.

In July 2005, Stephen Hawking published a paper and announced a theory that quantum perturbations of the event horizon could allow information to escape from a black hole, which would resolve the information paradox. His argument assumes the unitarity of the AdS/CFT correspondence which implies that an AdS black hole that is dual to a thermal conformal field theory, is unitary. When announcing his result, Hawking also conceded the 1997 bet, paying Preskill with a baseball encyclopedia (ISBN 1-894963-27-X) 'from which information can be retrieved at will'. However, Thorne remains unconvinced of Hawking's proof and declined to contribute to the award (see Thorne-Hawking-Preskill bet).

On October 28, 2006, The Discovery Channel aired a show called "The Hawking Paradox”. The show explained Hawking's conclusion that one must look at the multiverse as a whole, and that information lost in black holes is saved in parallel universes where no black holes exist.

The equation

The entropy of a black hole is given by the equation:

S = frac{c^{3}kA}{4 hbar G}

where S is the entropy, c is the speed of light, k is Boltzmann's constant, A is the surface area of the event horizon, ħ ("h-bar") is the reduced Planck's Constant (or Dirac's Constant) and G is the gravitational constant.

ee also

*Cosmic censorship hypothesis
*Maxwell's Demon (in which information cannot be destroyed)

References

External links

* [http://math.ucr.edu/home/baez/physics/Relativity/BlackHoles/info_loss.html Black Hole Information Loss Problem] , a USENET physics FAQ page
* " [http://xxx.lanl.gov/abs/hep-th/9209058 Do black holes destroy information?] ", John Preskill (1992), [http://xxx.lanl.gov/abs/hep-th/9209058 hep-th/9209058] . Discusses methods of attack on the problem, and their apparent shortcomings.
* [http://www.newscientist.com/news/news.jsp?id=ns99996193 Report] on Hawking's 2004 theory at New Scientist
* [http://www.nature.com/news/2004/040712/full/040712-12.html Report] on Hawking's 2004 theory at Nature
* Hawking, S. W. (July 2005), Information Loss in Black Holes, [http://arxiv.org/abs/hep-th/0507171 arxiv:hep-th/0507171] . Stephen Hawking's purported solution to the black hole unitarity paradox.
* [http://motls.blogspot.com/2005/07/hawking-and-unitarity.html Hawking and unitarity] : an up-to-date discussion of the information loss paradox and Stephen Hawking's role in it
* [http://video.google.com/videoplay?docid=8205975042597389275 The Hawking Paradox - BBC Horizon documentary (2005)]


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