[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
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
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.
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