[Paleopsych] BBC: Q&A: Hawking and black holes

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Q&A: Hawking and black holes
http://newsvote.bbc.co.uk/mpapps/pagetools/print/news.bbc.co.uk/1/hi/sci/tec
h/3914165.stm
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
    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 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
    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 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
    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 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.