[Paleopsych] BBC: Q&A: Hawking and black holes
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Wed Jul 21 17:56:43 UTC 2004
Q&A: Hawking and black holes
2004/07/21 14:40:17 GMT
Stephen Hawking has put forward a new theory that changes the way
scientists view black holes, saying he was wrong about them in the
The physicist told a conference on gravitation in Dublin that he has
revised his belief that black holes destroy everything that falls on
Our science editor David Whitehouse explains what they are.
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 onto 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 its vicinity, that lend support to this.
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
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 hair, 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
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
Is it complicated?
Certainly is. Here is the summary of his presentation.
Professor Stephen Hawking (Cambridge)
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.
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