[Paleopsych] New Scientist: Black holes, but not as we know them

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Fri Jan 21 14:59:22 UTC 2005

Black holes, but not as we know them

    THEY are the most fearsome objects in the universe. They swallow and
    destroy everything that crosses their path. Everyone knows that
    falling into a black hole spells doom. Or does it? In the past few
    years, cracks have started to appear in the conventional picture.
    Researchers on the quest for a more complete understanding of our
    universe are finding that black holes are not so black, and perhaps
    not holes either. Furious debates are raging over what black holes
    contain and even whether they deserve the name.

    The term "black hole" was coined in the 1960s by physicist John
    Wheeler to describe what happens when matter is piled into an
    infinitely dense point in space-time. When a star runs out of nuclear
    fuel, for example, the waste that remains collapses in on itself, fast
    and hard. The gravitational attraction of this matter can overwhelm
    its natural tendency to repel itself. If the star is big enough, the
    result will be a singularity. Around the singularity lies an event
    horizon, a point of no return. Light cannot escape once it passes
    beyond this boundary, and the eventual fate of everything within it is
    to be crushed into the singularity.

    But this picture always contained the seeds of its own destruction. In
    1975 Stephen Hawking at the University of Cambridge calculated that
    black holes would slowly but inexorably evaporate. According to the
    laws of quantum mechanics, pairs of "virtual" particles and
    antiparticles continually bubble up in empty space. Hawking showed
    that the gravitational energy of the black hole could be lent to
    virtual particles near the event horizon. These could then become
    real, and escape, carrying away positive energy in the form of
    "Hawking radiation". Over time, the black hole will bleed away into
    outer space.

    This led to a problem dubbed the information paradox. While relativity
    seems to suggest that information about matter falling into a black
    hole would be lost, quantum mechanics seemed to be suggesting it would
    eventually escape. Hawking claimed the random nature of Hawking
    radiation meant that while energy could escape, information could not.
    But last summer, he changed his mind (New Scientist, 17 July 2004, p
    11). His reversal was just one part of a larger movement to rethink
    the rules that govern black holes.

    Much of the impetus for this rethink comes from string theory, our
    best attempt to unify general relativity and quantum mechanics. Now 20
    years old, string theory posits that space-time, and everything in it,
    is composed of vibrating strings so small we will be lucky ever to get
    evidence of their existence. Its big appeal is the promise that it
    could unite general relativity and quantum mechanics, because one type
    of string carries the force of gravity, while the vibration of the
    strings is random, as predicted by quantum mechanics.

    String theory was first applied to black holes in the mid-1990s.
    Andrew Strominger and Cumrun Vafa of Harvard University began to work
    on the information paradox by imagining what the inside of a black
    hole might be like. The researchers found that string theory would
    allow them to build highly dense little structures from strings and
    other objects in string theory, some of which had more than three
    dimensions. These structures worked just like black holes: their
    gravitational pull prevents light escaping from them.

    Strominger and Vafa counted how many ways the strings in these black
    holes could be arranged, and found this was amazingly large.

    The calculation was heralded as a huge validation for string theory.
    In the 1970s, Hawking and Jacob Bekenstein, then at Princeton
    University, had calculated the entropy of a black hole using quantum
    mechanics. The entropy of an object is roughly a measure of the amount
    of information it can contain. In particular, it measures the number
    of different ways the parts making up an object can be arranged. It
    just so happened that the number of ways that Strominger and Vafa
    calculated that strings could be arranged in a black hole exactly
    matched the entropy calculated by Hawking and Bekenstein.


    But this did not tell physicists how those strings were arranged. Over
    the past year, Samir Mathur of Ohio State University and his
    colleagues have begun to look at what string configurations there
    could be in black holes. They found that the strings would always
    connect together to form a large, very floppy string, which would be
    much larger than a point-size singularity.

    Mathur's group calculated the total physical sizes of several stringy
    black holes, which they prefer to call "fuzzballs" or "stringy stars".
    To his surprise, they found they were the same size as the event
    horizon is in traditional theory. "It is changing our picture of the
    black hole interior," says Mathur. "It would really mean the picture
    of the round hole with a black dot in the centre is wrong."

    Mathur's fuzzball does away with the idea of the event horizon as a
    sharp boundary. In the traditional view, the event horizon is a
    well-defined limit. Objects passing particular points in space at
    particular moments in time are guaranteed to end up being pulverised
    at the black hole singularity. In the fuzzball picture, the event
    horizon is a frothing mass of strings, not a sharp boundary.

    The fuzzball picture also challenges the idea that a black hole
    destroys information. In Mathur's description, there is no
    singularity. The mass of strings reaches all the way to the fuzzy
    event horizon. This means information can be stored in the strings and
    imprinted on outgoing Hawking radiation.

    So what happens to the information that falls into a black hole?
    Imagine pouring cream into black coffee. Drop the coffee and cream
    into the old-style black hole and they will go to the singularity and
    be lost. You will never see the results of the mixture. But drop your
    coffee and cream onto a Mathur fuzzball and information about the
    cream-coffee mixture will be encoded into string vibrations. Hawking
    radiation that comes out can carry detailed information about what
    happened to each particle of cream and every particle of coffee.
    "There's no information problem. It's like any other ball of cotton,"
    says Mathur. This picture is very preliminary, cautions Vafa. Mathur
    has not yet calculated exactly how his model applies to large black
    holes or understood how a black hole evolves over time.

    Gary Horowitz of the University of California, Santa Barbara, and Juan
    Maldacena of the Institute for Advanced Study in Princeton, New
    Jersey, also recently proposed that information can get out of a black
    hole. But unlike Mathur, they believe that black holes do contain a
    singularity at their heart. They suggested that information might
    escape by means of quantum teleportation. This allows the state of one
    particle to be instantly teleported to another. So Horowitz and
    Maldacena suggested that information could pass from matter hitting
    the singularity to outgoing Hawking radiation.

    But to make their calculation work, they had to assume that infalling
    matter and outgoing radiation would not collide with each other. If
    they did, this could disrupt the teleportation process. Quantum
    information theorists Daniel Gottesman of the Perimeter Institute for
    Theoretical Physics in Waterloo, Canada, and John Preskill of the
    California Institute of Technology in Pasadena say such disruption
    could occur very easily.

    That seems to raise a problem for Horowitz and Maldacena. But last
    summer, Seth Lloyd of the Massachusetts Institute of Technology worked
    out that all such disruptions would actually cancel each other out.
    Then Lloyd calculated that the most information a black hole would
    possibly retain permanently was just half a bit - everything else will
    eventually escape. This applies to all black holes, whether they are
    supermassive ones at the heart of a galaxy (see "Giants of the
    universe") or mini black holes created in a particle accelerator (see
    "Baby black holes").

    But Gottesman and Preskill have a second criticism that might be more
    fatal to the teleportation picture. They showed that the effect could
    allow faster-than-light communication, which is taboo in relativity.
    The teleportation calculation relies on the assumption that every
    piece of matter inside a black hole has the same quantum state.
    Although quantum mechanics allows one particle to have an
    instantaneous effect on the quantum state of another, this cannot be
    used to communicate. For example, if one person, Alice, measures the
    quantum state of a particle that is linked with a particle held by her
    friend, Bob, the effect of this measurement will be instantaneously
    communicated to Bob's, but there is no way to use this to communicate
    faster than light, because Alice needs to tell Bob what kind of
    measurement process she carried out on her particle, before he can
    decode the meaning of the change he sees. That information has to
    travel to Bob in the normal way.

Black hole communication

    However, if Alice throws her particle into a black hole, the
    researchers found the measurement will be immediately constrained to
    the quantum state of the black hole. This would have an effect on
    Bob's particle that he could determine without needing the extra
    information from Alice. Gottesman concludes that the teleportation
    idea cannot work very well. Indeed, he wonders if the framing of the
    information paradox is wrong in a way that is not yet understood. "My
    own guess is somehow we're asking a stupid question," he says.

    The scenario also bothers quantum-gravity theorist Ted Jacobson of the
    University of Maryland, who still believes that the information that
    falls into black holes is lost forever to those outside the black
    hole. He finds the teleportation picture particularly unconvincing. "I
    put it in the category of desperate attempts to make information come
    out," he says. And even the researchers themselves aren't sure they
    are right. "We suggested one possibility," says Horowitz, but he
    admits it doesn't have a good basis in string theory yet. "So I can't
    say we are confident this is the right picture."

    Jacobson argues that the connection between the outside and inside of
    a black hole is so complicated in string theory that no one can be
    sure they have ruled out the possibility of information leaking out of
    our space-time. People may be simply assuming the conclusion that they
    want for their own reasons. "I see no problem with letting the darn
    stuff fall down the drain. Why are people so afraid of the

    The problem, says Vafa, is that the concept of information could be
    very subtle in string theory, and not yet well-defined. "Information
    loss is a critical question, but our understanding of black holes is
    too primitive."

    So whether information can escape from black holes or is destroyed
    remains a topic of intense debate. But there might turn out to be a
    third option. One competing theory to string theory is called loop
    quantum gravity, pioneered by, among others, Lee Smolin of the
    Perimeter Institute in Waterloo, Canada. It proposes that space-time
    is constructed of loops even smaller than strings. Joining loops
    together creates a mesh of nodes and branches called a spin network.
    The advantage of this model is that space-time itself can be built out
    of these networks instead of having to be assumed, as it is in string

    Abhay Ashtekar of Pennsylvania State University in Pittsburgh and
    Martin Bojowald of the Max Planck Institute for Gravitational Physics
    in Golm, Germany, have studied a model of a black hole created using
    spin networks. They found the equations that describe space-time
    continue to apply in an orderly way even at the singularity itself.
    This is very different to the conventional picture, in which the
    equations of physics break down when space-time collapses. It means
    that information that reaches the singularity could survive there,
    encoded in the spin networks. As far as Ashtekar and Bojowald can
    tell, the information trapped in a black hole will be unable to escape
    via Hawking radiation. Wait long enough, however, and it will survive,
    eventually rejoining the rest of the universe when the black hole

    So whatever the theory that eventually supersedes relativity, it seems
    a good possibility that black holes may be just a little less dramatic
    than we thought. After all, who's afraid of a big ball of string?

Giants of the universe

    While debate rages over what black holes really are, the astronomical
    evidence that every galaxy is built around a supermassive black hole
    is stronger than ever.

    Observations made with the Hubble Space Telescope have found that
    every galaxy has a mass at its core millions of times as massive as
    our sun. The bigger this mass, the larger the size of the "galactic
    bulge" - the number of stars clustered around the galactic centre.

    The speed with which stars orbit the centre of a galaxy reveals the
    mass of the object they are orbiting, and very careful measurements
    can reveal its size too. For a handful of galaxies, including the
    Milky Way, the central mass is known to be crammed into a space just a
    few times as wide as the distance between the Earth and the sun,
    indicating that what lies within is so dense, it must be a black hole.

    Some young galaxies emit copious amounts of high-energy radio and
    X-ray radiation. Lines in X-ray spectra taken from these objects are
    shifted as if the rays had struggled to escape from the strong
    gravitational field of a supermassive black hole.

    The closest object to the centre of our galaxy is a bright, compact
    source of radiation known as Sagittarius A*. X-ray flares coming from
    it, and picked up by the Chandra X-ray telescope, are thought to be
    the dying gasps of matter falling into a supermassive black hole.

Baby black holes

    You don't have to go to space to find a black hole: mini versions
    could be created to order, right here on Earth. That's what some
    physicists claim will be possible using the world's most powerful
    particle accelerator, due to turn on in 2007.

    Currently under construction at the CERN laboratory in Geneva, the
    Large Hadron Collider will smash protons together with a collision
    energy of 14,000 billion electronvolts. This might just be enough to
    create several black holes every second, provided some strange ideas
    about unknown physics turn out to be right. Each mini wonder would
    weigh no more than a few micrograms and be smaller than a speck of

    A black hole is thought to form when the core of a massive star
    collapses under its own weight and is crushed to a point. Vast amounts
    of matter weighing more than a few suns are needed to produce gravity
    strong enough for this to happen.

    Yet the special theory of relativity gives a clue to making black
    holes in the laboratory. Einstein used the theory to show that energy
    is equivalent to matter. So black holes should also pop into existence
    when vast amounts of energy are concentrated into a point, and that's
    exactly what happens when particles smash together at extreme

    But there's a snag. According to our existing knowledge of particles
    and the forces that operate between them, the minimum energy needed to
    make a black hole this way is 10 million billion times more than LHC
    can produce. And the chances of ever building a particle accelerator
    that can reach such energies are virtually nil.

    In the past few years though, the prospects for making black holes in
    the lab have improved. This is down to a theory that says gravity is
    actually much stronger than we think. Huge masses are needed for the
    force of gravity to become important in everyday life, and this
    feebleness puzzles physicists. Some suggest that it can be explained
    if space has extra, invisible dimensions that only gravity can reach.
    The gravitational force leaks away into them, while our universe and
    the particles spewing out of accelerators are trapped in three
    dimensions, rather like specks of dust on the surface of a soap

    If the idea is right, gravity could be much stronger when it applies
    over distances so small that there is no chance of leakage into other
    dimensions. Pack enough energy into a 10-20-metre space and it could
    be enough to create a black hole.

    These mini curiosities will evaporate within 10-26 seconds, losing
    most of their mass by radiating energy, as predicted by Stephen
    Hawking. A group led by Roberto Emparan at the University of the
    Basque Country in Bilbao, Spain, calculated that most of this Hawking
    radiation should appear as particles that can be spotted by detectors.
    If Emparan is right, the LHC could provide the first evidence for
    Hawking radiation from a black hole.

    A computer simulation devised by Bryan Webber at the University of
    Cambridge and others creates mini black holes from LHC-style
    collisions. The simulation shows that the structures should decay into
    a large number of high-energy particles, which would be sprayed all
    over the detector. If the theory is right, researchers expect to see
    many more of these striking events than they might otherwise. By
    measuring the energy and momentum of the particles radiated, they hope
    to measure the mass of the mini marvels.

    Valerie Jamieson

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