By Tom Siegfried /The Dallas Morning News
If the rule book for the universe could be judged by its cover, it would be a
book about holograms.
In a hologram - like the credit card emblems that change appearance at different
angles - a flat two-dimensional picture creates the illusion of a 3-D image. In
a similar way, some scientists suspect, the 3-D reality of the universe can in
some sense be represented as two-dimensional information. It would be like
having all the information in a book printed on its two-dimensional cover.
So far, the only space that seems to behave this way is around the outskirts of
black holes - not a good place to be reading a book. But some scientists think
that black holes provide clues to a deep new principle that could illuminate the
fundamental laws governing the universe.
Known as the holographic principle, the new idea has inspired a flood of
calculations and speculations about the ultimate nature of space and time.
"It's a new paradigm, a new way of thinking," says physicist Willy Fischler of
the University of Texas at Austin.
At the moment, the holographic principle belongs to the arcane realm of theory
attempting to describe how matter arises out of space and why nature's particles
and forces obey the laws they do. The math works for space around black holes,
the cosmic bottomless pits that suck matter and energy into an eternal abyss.
But nobody really knows whether cosmic holography has anything to do with the
whole universe.
"We've made conceptual progress, but I think this is very, very preliminary,"
Dr. Fischler said in an interview.
Nevertheless, the holographic principle follows in the tradition of other great
principles that paved the way to advances in physics. Einstein's principle of
relativity led to understanding the equivalence of matter and energy and the
four-dimensional nature of space and time. Einstein's equivalence principle -
relating inertia to weight - led to his 1915 general theory of relativity and a
deeper understanding of gravity. In the 1920s, Werner Heisenberg's uncertainty
principle provided the foundation for understanding quantum physics and the
inner workings of atoms.
In a way, the holographic principle might help tie all the other great
principles together, suggests Stanford University physicist Leonard Susskind.
Holography seems to be closely related to superstring theory, the current top
candidate for a theory to explain all of nature's particles and forces.
In Dr. Susskind's view, superstring theory is most likely an approximation to an
ultimate "Theory X" that would encompass all the other successful theories of
physics. Constructing the true Theory X will depend on finding a guiding
principle.
"My best bet for the fundamental principle is the holographic principle," Dr.
Susskind said at a recent meeting at the Fermi National Accelerator Laboratory
in Batavia, Ill.
Holographic ideas have inspired intense interest among theoretical physicists
lately - in the last two years, more than 100 research papers related to it have
appeared on the Internet's chief site for theoretical physics.
Even to the physicists who study it, the holographic principle seems rather
bizarre.
"The picture is more or less that everything that could happen . . . can be
retrieved just by looking at the boundary of our world," says physicist Nathan
Seiberg of the Institute for Advanced Study in Princeton, N.J. "So whatever
happens in this room can be recorded just by looking at what happens near the
walls."
In other words, if the holographic principle applied to the Earth, explorers on
the surface could discover the structure of the planet's core.
For space, though, the holographic math has been successfully worked out only
for black holes, regions of space surrounding superintense gravitational fields.
Once within a black hole's grasp, matter and radiation are trapped forever.
The outer limit of a black hole's reach is marked by an invisible surface known
as an event horizon. All objects inside the horizon are hidden from an
outsider's view.
Although a black hole, like any sphere, is a three-dimensional object, its
horizon is a two-dimensional "surface," just as the surface of the Earth has two
dimensions (designated by latitude and longitude). As black holes grow in size
(by swallowing more and more matter and energy), it would seem that their
three-dimensional volume would depend on how much they had eaten. But actually,
theorists have shown, it's the two-dimensional surface area of the black hole's
outer horizon, not its volume, that is proportional to the quantity of material
consumed.
This and related puzzles about black holes have baffled physicists for years.
But in the early 1990s, Gerard 't Hooft of Utrecht University in the Netherlands
and Dr. Susskind both suggested that holographic ideas might resolve the
paradoxes. In essence, the holographic principle states that all the information
about the stuff inside a black hole can be recorded on its surface, the event
horizon. It would be a little like inscribing a record of every air molecule
blown into a beach ball on the beach ball's surface.
The idea of a surface containing as much information as the volume it encloses
is a little hard for traditional physicists to take.
"It looks surprising that you can store three-dimensional information on a
two-dimensional surface," said Dr. Seiberg. "But it seems like this is the only
way . . . to get out of these puzzles associated with black holes."
Holographic ideas got a boost two years ago when physicist Juan Maldacena, now
at Harvard University, produced a major advance in understanding the
relationship of gravity to quantum mechanics. Dr. Maldacena showed that a theory
involving only the boundary of the black hole can replace the more complicated
mathematics needed to describe the quantum nature of gravity within the black
hole.
Further work by Edward Witten of the Institute for Advanced Study has persuaded
many physicists that the holographic principle may be the key to understanding
the subatomic world.
Whether holography applies to the whole universe is another matter. The
implications for cosmology, the study of the whole universe, remain unclear.
"There are some ideas about cosmology, but I think we're fairly far from any
kind of real understanding," Dr. Susskind said in an interview. "Nobody who does
this kind of business has a real handle on cosmology yet. I would just say that
at the moment, the state-of-the-art is too primitive to understand spaces as
complicated as an expanding universe."
One problem afflicting efforts to apply holography to the universe is that so
far the math only works in an unusual spatial geometry. As Einstein's theory of
relativity showed, gravity is related to a curvature of space, causing objects
flying through that space to follow curved paths. In most circumstances
encountered in daily life, such curvature is slight. But around a black hole,
such curvature can be drastic and unusual.
Dr. Maldacena's math applies to such oddly curved space - known as anti-de
Sitter space (named for the Dutch physicist Willem de Sitter). It's not obvious
that the holographic principle would also apply to ordinary "flat" space.
Dr. Susskind points out, though, that just as the curvature of the Earth can
usually be ignored because the Earth is so big, a vast expanse of anti-de Sitter
space might also be considered "close enough" to being flat. Then a holographic
theory describing a region of flat space might work.
"If the anti-de Sitter space becomes big enough, then processes which take place
in any finite volume will reproduce what you would have in flat space," Dr.
Susskind said.
Last year, Drs. Susskind and Fischler proposed a version of the holographic
principle that might apply to the universe. In essence, they suggest that the
amount of information contained in a volume of space cannot exceed the maximum
amount of information that could be stored in the boundary surface surrounding
that space.
But at the Fermilab meeting, Stanford physicist Andrei Linde expressed
reservations about the holographic principle as applied to cosmology.
In its boldest form, he argued, the holographic principle is "obviously wrong."
"The holographic principle tells you that our universe is in a certain sense
two-dimensional - well, maybe," Dr. Linde said. For one thing, it isn't obvious
that the universe even has a boundary on which to store information about what's
inside it.
"These are subtle and touchy matters," he said.
The Susskind-Fischler proposal avoids some of the problems of the bolder forms
of holography, Dr. Linde said, but it still may have drawbacks. For one thing,
it does not seem compatible with the currently popular "inflationary" view of
the birth of the universe that Dr. Linde helped to develop. The inflation theory
proposes a rapid burst of expansion of the universe at the time of the big bang
that marked the universe's birth.
"Inflation by its nature is anti-holographic," Linde said, because the process
of inflation does not depend on the boundary of the inflating space.
Both Dr. Susskind and Fischler acknowledge that much more work is needed on
applying holography to the universe. They say their efforts are preliminary
approaches to the problem, not final solutions.
"I think I would say I am confused and I suspect most of my colleagues are,"
said Dr. Fischler. "But I have a sense that there is something very deep going
on here."