Black Holes Turned ‘Inside Out’

Thursday, 22 July, 2004 – http://news.bbc.co.uk/1/hi/sci/tech/3913145.stm

Stephen Hawking has put forward a new theory that changes the way scientists view black holes, saying he was wrong about them in the past.

The physicist told a conference on gravitation in Dublin that he has revised his belief that black holes destroy everything that falls on them. He now believes that black holes may allow information to get out. His new research could even help solve the “black hole information paradox“, a crucial puzzle for modern physics. He spoke to a packed lecture hall at the 17th International Conference on General Relativity and Gravitation, giving his new views in a presentation entitled The Information Paradox For Black Holes. He is revising his 1975 ideas that are regarded as the most astonishing breakthrough in black hole studies.

A black hole is an object from which once inside it is not possible to escape. Its boundary is called its “event horizon“. But now Hawking believes that it might not be a one-way trip after all.

Gary Gibbons, a physicist at Cambridge University, said Hawking’s newly defined black holes did not have a well-delineated event horizon that hid everything in them from the outside Universe. Kip Thorne, a leading cosmologist from the California Institute of Technology said of Hawking’s new idea: “This looks to me on the face of it to be a lovely argument, but I haven’t yet seen all the details”.

Q&A >> Hawking and Black Holes

21 July, 2004 – http://news.bbc.co.uk/1/hi/sci/tech/3914165.stm

Stephen Hawking has put forward a new theory that changes the way scientists view black holes, saying he was wrong about them in the past.

The physicist told a conference on gravitation in Dublin that he has revised his belief that black holes destroy everything that falls on them.

Our science editor David Whitehouse explains black holes.

What is a black hole?

It is an object from which nothing can escape because its escape velocity is greater than the speed of light. Since nothing can travel faster than light, nothing can get out. Inside a black hole strange things are done to space and time, and at its centre could be a so-called singularity where space and time are squeezed to an infinitely small point. But some scientists are unhappy with this idea.

Have they been detected?

Probably. There are many objects that could be black holes seen orbiting stars. Often these objects pull material from their stellar companions and drag it on to themselves. It becomes hot and gives off high-energy radiation. If astronomers are lucky they can use this radiation to determine the size and mass of the compact object. It is also believed by many astronomers that there are supermassive black holes at the centres of many galaxies. There are many lines of observational evidence, such as the motion of stars in the vicinity of these mysterious objects, which lend support to this idea.

What was Hawking’s black hole theory?

In 1975, Hawking calculated that once a black hole forms, it radiates energy and starts losing mass by giving off so-called “Hawking radiation“. Scientists were astounded because this work established a connection between gravity and entropy, which is a measure of how energy changes from one form to another. Entropy has a lot to do with the information in a system. For example, a pile of bricks has more entropy than when they have been made into a house. It takes bricks and information to turn them into a house.

Can anything make a black hole?

Yes. It was said that black holes had no air, meaning that it did not matter what came together to make them. All that mattered was that a sufficiently large mass be squashed into a sufficiently small space. Before Hawking’s latest thinking it was thought that, once formed, it would be impossible to tell what went in; once something had fallen in, it was lost forever and the only information that remained was its mass and spin.

What exactly has he changed his mind about?

Hawking now believes that black holes may allow information to leak out. For several years many scientists had been unhappy with the idea that a black hole could just disappear, because it represented a loss of information from the Universe. This ran contrary to the laws of quantum physics, which are the rules to describe the behaviour of the Universe at the smallest scales. These laws say that information can never be totally lost. Whether information is or is not lost has important practical and philosophical consequences.

Is it complicated?

Certainly is. Here is the summary of Professor Hawking’s presentation.

The information paradox for black holes

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.

Black Holes and the Hawking Paradox

Thursday, 15 September 2005- http://news.bbc.co.uk/1/hi/sci/tech/4249192.stm

His popular science book A Brief History of Time was a publishing sensation, staying at the top of the best-seller lists longer than any other book in recent history. But behind the public face lies an argument that has been raging for almost 30 years.

Hawking shot to fame in the world of physics when he provided a mathematical proof for the “Big Bang” theory. This theory showed that the entire Universe exploded from a singularity – an infinitely small point with infinite density and infinite gravity. Hawking was able to come to his proof using mathematical techniques that had been developed by Roger Penrose. However, Penrose’s techniques were developed to deal not with the beginning of the Universe but with black holes.

Science had long predicted that if a sufficiently large star collapsed at the end of its life, all the matter left in the star would be crushed into an infinitely small point with infinite gravity and infinite density – a singularity.

Hawking realised that the Universe was, in effect, a black hole in reverse; instead of matter being crushed into a singularity, the Universe began when a singularity expanded to form everything we see around us today – from stars to planets to people.

Hawking realised that to come to a complete understanding of the Universe, he would have unravel the mysteries of the black hole.

Hawking Cracks Black Hole Paradox

14 July 2004 – http://www.newscientist.com/article.ns?id=dn6151

After nearly 30 years of arguing that a black hole destroys everything that falls into it, Stephen Hawking is saying he was wrong. It seems that black holes may after all allow information within them to escape. Hawking will present his latest finding at a conference in Ireland next week.The about-turn might cost Hawking, a physicist at the University of Cambridge, an encyclopaedia because of a bet he made in 1997. More importantly, it might solve one of the long-standing puzzles in modern physics, known as the black hole information paradox. It was Hawking’s own work that created the paradox. In 1976, he calculated that once a black hole forms, it starts losing mass by radiating energy. This “Hawking radiation” contains no information about the matter inside the black hole and once the black hole evaporates, all information is lost.

But this conflicts with the laws of quantum physics, which say that such information can never be completely wiped out. Hawking’s argument was that the intense gravitational fields of black holes somehow unravel the laws of quantum physics.

Other physicists have tried to chip away at this paradox. Earlier in 2004, Samir Mathur of Ohio State University in Columbus and his colleagues showed that if a black hole is modelled according to string theory – in which the universe is made of tiny, vibrating strings rather than point-like particles – then the black hole becomes a giant tangle of strings. And the Hawking radiation emitted by this “fuzzball” does contain information about the insides of a black hole (New Scientist print edition, 13 March).