Stephen Hawking

Stephen William Hawking, CH, CBE, FRS (born January 8 1942, in Oxford, England) is one of the world's leading theoretical physicists. Hawking is Lucasian professor of mathematics at the University of Cambridge (a post once held by Sir Isaac Newton), and a fellow of Gonville and Caius College. The fact that he holds this post while being almost completely incapacitated with severe amyotrophic lateral sclerosis has made him a worldwide celebrity.

Losing an old bet

Hawking was in the news in July 2004 for presenting a new theory about black holes which goes against his own long-held belief about their behaviour, thus losing a bet he and Kip Thorne made with John Preskill, a particle physicist. Classically, it can be shown that information crossing the event horizon of a black hole is lost to our universe, and that as a consequence all black holes are identical, beyond their mass, electrical charge and angular velocity (the "no hair theorem"). The problem with this theorem is that it implies the black hole will emit the same radiation regardless of what goes into the black hole, and as a consequence that if a pure quantum state is thrown into a black hole, an "ordinary" mixed state will be returned. This runs counter to the rules of quantum mechanics and is known as the black hole information paradox.

Related Topics:
2004 - Black hole - Bet - Kip Thorne - John Preskill - Particle physicist - Event horizon - No hair theorem - Radiation - Quantum mechanics - Black hole information paradox

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Hawking had earlier speculated that the singularity at the centre of a black hole could form a bridge to a "baby universe" into which the lost information could pass; such theories have been very popular in science fiction. But according to Hawking's new idea, presented at the 17th International Conference on General Relativity and Gravitation, on 21 July, 2004 in Dublin, Ireland, black holes eventually transmit, in a garbled form, information about all matter they swallow.

Related Topics:
Singularity - General Relativity - Gravitation - 21 July - 2004 - Dublin - Ireland

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

:The Euclidean path integral over all topologically trivial metrics can be done by time slicing and so is unitary when analytically continued to the Lorentzian. On the other hand, the path integral over all topologically non-trivial metrics is asymptotically independent of the initial state. Thus the total path integral is unitary and information is not lost in the formation and evaporation of black holes. The way the information gets out seems to be that a true event horizon never forms, just an apparent horizon.

Related Topics:
Euclidean - Topologically - Time slicing - Analytically continued - Lorentzian - Asymptotically - Integral - Evaporation - Black holes - Event horizon

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

~ ~ ~ ~ ~ ~ ~ ~ ~ ~