Parallel Worlds (35 page)

(The idea that
our universe might be a black hole is not as farfetched as it seems. We have
the intuitive notion that a black hole must be extremely dense, with an
enormous, crushing gravitational field, but this is not always the case. The
size of a black hole's event horizon is proportional to its mass. The more
massive a black hole is, the larger its event horizon. But a larger event
horizon means that matter is spread out over a larger volume; as a result, the
density actually decreases as the mass increases. In fact, if a black hole
were to weigh as much as our universe, its size would be approximately the size
of our universe, and its density would be quite low, comparable to the density
of our universe.)

Some
astrophysicists, however, are not impressed with the application of string
theory and M-theory to cosmology. Joel Primack of the University of California
at Santa Cruz is less charitable than others: "I think it's silly to make
much of a production about this stuff . . . The ideas in these papers are
essentially untestable." Only time will tell if Primack is right, but
because the pace of string theory has been accelerating, we may find a
resolution of this problem soon, and it may come from our space satellites. As
we see in chapter 9, a new generation of gravity wave detectors to be sent
into outer space by 2020, like LISA, may give us the ability to rule out or
verify some of these theories. If the inflation theory is correct, for example,
LISA should detect violent gravity waves created by the original inflationary
process. The ekpyrotic universe, however, predicts a slow collision between
universes and hence much milder gravity waves. LISA should be able to rule out
one of these theories experimentally. In other words, encoded within gravity
waves created by the original big bang are the data necessary to determine
which scenario is correct. LISA may be able, for the first time, to give solid
experimental results concerning inflation, string theory, and M-theory.

MINI-BLACK HOLES

Since string
theory is really a theory of the entire universe, to test it directly requires
creating a universe in the laboratory (see chapter 9). Normally, we expect
quantum effects from gravity to occur at the Planck energy, which is a
quadrillion times more powerful than our most powerful particle accelerator,
making direct tests of string theory impossible. But if there really is a
parallel universe that exists less than a millimeter from ours, then the energy
at which unification and quantum effects occur may be quite low, within reach
of the next generation of particle accelerators, such as the Large Hadron
Collider (LHC). This, in turn, has sparked an avalanche of interest in black
hole physics, the most exciting being the "mini-black hole."
Mini-black holes, which act as if they are subatomic particles, are a
"laboratory" in which one can test some of the predictions of string
theory. Physicists are excited about the possibility of creating them with the
LHC. (Mini-black holes are so small, comparable to an electron in size, that
there is no threat that they will swallow up Earth. Cosmic rays routinely hit
Earth with energies exceeding these mini-black holes, with no ill effect on the
planet.)

As revolutionary
as it may seem, a black hole masquerading as a subatomic particle is actually
an old idea, first introduced by

Einstein in
1935. In Einstein's view, there must be a unified field theory in which
matter, made of subatomic particles, could be viewed as some sort of distortion
in the fabric of space-time. To him, subatomic particles like the electron were
actually "kinks" or wormholes in curved space that, from a distance,
looked like a particle. Einstein, with Nathan Rosen, toyed with the idea that
an electron may actually be a mini-black hole in disguise. In his way, he
tried to incorporate matter into this unified field theory, which would reduce
subatomic particles to pure geometry.

Mini-black holes
were introduced again by Stephen Hawking, who proved that black holes must
evaporate and emit a faint glow of energy. Over many eons, a black hole would
emit so much energy that it would gradually shrink, eventually becoming the
size of a subatomic particle.

String theory is
now reintroducing the concept of mini-black holes. Recall that black holes form
when a large amount of matter is compressed to within its Schwarzschild radius.
Because mass and energy can be converted into each other, black holes can also
be created by compressing energy. There is considerable interest in whether the
LHC may be able to produce mini-black holes among the debris created by
smashing two protons together at 14 trillion electron volts of energy. These
black holes would be very tiny, weighing perhaps only a thousand times the mass
of an electron, and last for only 10
^-23
seconds. But they would be
clearly visible among the tracks of subatomic particles created by the LHC.

Physicists also
hope that cosmic rays from outer space may contain mini-black holes. The
Pierre Auger Cosmic Ray Observatory in Argentina is so sensitive that it can
detect some of the largest bursts of cosmic rays ever recorded by science. The
hope is that mini-black holes may be found naturally among cosmic rays, which
would create a characteristic shower of radiation when they hit Earth's upper
atmosphere. One calculation shows that the Auger Cosmic Ray detector might be
able to see up to ten cosmic ray showers per year triggered by a mini-black
hole.

The detection of
a mini-black hole either at the LHC in Switzerland or the Auger Cosmic Ray
detector in Argentina, perhaps within this decade, would provide perhaps good
evidence for the existence of parallel universes. Although it would not
conclusively prove the correctness of string theory, it would convince the
entire physics community that string theory is consistent with all experimental
results and is in the right direction.

BLACK HOLES AND THE INFORMATION PARADOX

String theory
may also shed light on some of the deepest paradoxes of black hole physics,
such as the information paradox. As you will recall, black holes are not
perfectly black but emit small amounts of radiation via tunneling. Because of
the quantum theory, there is always the small chance that radiation can escape
the viselike grip of a black hole's gravity. This leads to a slow leakage of
radiation from a black hole, called Hawking radiation.

This radiation,
in turn, has a temperature associated with it (which is proportional to the
surface area of the black hole's event horizon). Hawking gave a general
derivation of this equation that involved a lot of hand-waving. However, a
rigorous derivation of this result would require using the full power of statistical
mechanics (based on counting the quantum states of a black hole). Usually, statistical
mechanical calculations are done by counting the number of states that an atom
or molecule can occupy. But how do you count the quantum states of a black
hole? In Einstein's theory, black holes are perfectly smooth, so counting their
quantum states was problematic.

String theorists
were anxious to close this gap, so Andrew Strominger and Cumrum Vafa of Harvard
decided to analyze a black hole using M-theory. Since the black hole itself was
too difficult to work with, they took a different approach and asked a clever
question: what is the dual to a black hole? (We recall that an electron is
dual to a magnetic monopole, such as a single north pole. Hence, by examining
an electron in a weak electric field, which is easy to do, we can analyze a
much more difficult experiment: a monopole placed in a very large magnetic
field.) The hope was that the dual of the black hole would be easier to analyze
than the black hole itself, although they would ultimately have the same final
result. By a series of mathematical manipulations, Strominger and Vafa were
able to show that the black hole was dual to a collection of one-branes and
five-branes. This was a tremendous relief, since counting the quantum states
of these branes was known. When Strominger and Vafa then calculated the number
of quantum states, they found that the answer precisely reproduced Hawking's
result.

This was a piece
of welcome news. String theory, which is sometimes ridiculed for not
connecting with the real world, gave perhaps the most elegant solution for
black hole thermodynamics.

Now, string
theorists are trying to tackle the most difficult problem in black hole
physics, the "information paradox." Hawking has argued that if you
throw something into a black hole, the information it carries is lost forever,
never to return again. (This would be a clever way to commit the perfect crime.
A criminal could use a black hole to destroy all incriminating evidence.) From
a distance, the only parameters that we can measure for a black hole are its
mass, spin, and charge. No matter what you throw into a black hole, you lose
all its information. (This goes by the statement that "black holes have no
hair"—that is, they have lost all information, all hair, except for these
three parameters.)

The loss of
information from our universe seems to be an inevitable consequence of
Einstein's theory, but this violates the principles of quantum mechanics,
which state that information can never really be lost. Somewhere, the
information must be floating in our universe, even if the original object was
sent down the throat of a black hole.

"Most
physicists want to believe that information is not lost," Hawking has
written, "as this would make the world safe and predictable. But I
believe that if one takes Einstein's general relativity seriously, one must
allow for the possibility that spacetime ties itself in knots and that
information gets lost in the folds. Determining whether or not information
actually does get lost is one of the major questions in theoretical physics
today."

This paradox,
which pits Hawking against most string theorists, still has not been resolved.
But the betting among string theorists is that we will eventually find where
the missing information went. (For example, if you throw a book into a black
hole, it is conceivable that the information contained in the book will gently
seep back out into our universe in the form of tiny vibrations contained within
the Hawking radiation of an evaporating black hole. Or perhaps it reemerges
from a white hole on the other side of the black hole.) That is why I
personally feel that when someone finally calculates what happens to information
when it disappears into a black hole in string theory, he or she will find that
information is not really lost but subtly reappears somewhere else.

In 2004, in a
stunning reversal, Hawking made the front page of the
New York Times
when he announced before TV cameras that he was wrong about
the information problem. (Thirty years ago, he bet other physicists that
information could never leak out of a black hole. The loser of the bet was to
give the winner an encyclopedia, from which information can be easily
retrieved.) Redoing some of his earlier calculations, he concluded that if an
object such as a book fell into a black hole, it might disturb the radiation
field it emits, allowing information to leak back into the universe. The
information contained within the book would be encoded in the radiation slowly
seeping out of the black hole, but in mangled form.

On one hand,
this put Hawking in line with the majority of quantum physicists, who believe
that information cannot be lost. But it also raised the question: can
information pass to a parallel universe? On the surface, his result seemed to
cast doubt on the idea that information may pass through a wormhole into a
parallel universe. However, no one believes that this is the last word on the
subject. Until string theory is fully developed, or a complete quantum gravitational
calculation is made, no one will believe that the information paradox is fully
resolved.

THE HOLOGRAPHIC UNIVERSE

Last, there is a rather mysterious prediction of M-theory that
is still not understood but may have deep physical and philosophical consequences.
This result forces us to ask the question: is the universe a hologram? Is there
a "shadow universe" in which our bodies exist in a compressed
two-dimensional form? This also raises another, equally disturbing question: is
the universe a computer program? Can the universe be placed on a CD, to be
played at our leisure?

Holograms are
now found on credit cards, in children's museums, and in amusement parks. They
are remarkable because they can capture a complete three-dimensional image on
a two-dimensional surface. Normally, if you glance at a photograph and then
move your head, the image on the photograph does not change. But a hologram is
different. When you glance at a holographic picture and then move your head,
you find the picture changing, as if you were looking at the image through a
window or a keyhole. (Holograms may eventually lead to three-dimensional TV and
movies. In the future, perhaps we will relax in our living room and glance at a
wall screen that gives us the complete three-dimensional image of distant locations,
as if the TV wall screen were actually a window peering out over a new
landscape. Furthermore, if the wall screen were shaped like a large cylinder
with our living room placed in the center, it would appear as if we were
transported to a new world. Everywhere we looked, we would see the
three-dimensional image of a new reality, indistinguishable from the real
thing.)