By DENNIS OVERBYE
Published: December 7, 2004
From top,
Rick Friedman; Laura Pedrick;
Emilio Flores, all for The
New York Times
Scientists
around the country leading the
study of string theory include
Dr. Andrew Strominger and Dr.
Cumrun Vafa, photo at top; Dr.
Edward Witten, middle; and Dr.
Joe Polchinski, above. |
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ASPEN,
Colo. - They all laughed 20 years ago.
It was then that a physicist named
John Schwarz jumped up on the stage during a cabaret at the
physics center here and began babbling about having discovered
a theory that could explain everything. By prearrangement men
in white suits swooped in and carried away Dr. Schwarz, then a
little-known researcher at the California Institute of
Technology.
Only a few of the laughing audience
members knew that Dr. Schwarz was not entirely joking. He and
his collaborator, Dr. Michael Green, now at Cambridge
University, had just finished a calculation that would change
the way physics was done. They had shown that it was possible
for the first time to write down a single equation that could
explain all the laws of physics, all the forces of nature -
the proverbial "theory of everything" that could be
written on a T-shirt.
And so emerged into the limelight a
strange new concept of nature, called string theory, so named
because it depicts the basic constituents of the universe as
tiny wriggling strings, not point particles.
"That was our first public
announcement," Dr. Schwarz said recently.
By uniting all the forces, string
theory had the potential of achieving the goal that Einstein
sought without success for half his life and that has embodied
the dreams of every physicist since then. If true, it could be
used like a searchlight to illuminate some of the deepest
mysteries physicists can imagine, like the origin of space and
time in the Big Bang and the putative death of space and time
at the infinitely dense centers of black holes.
In the last 20 years, string theory
has become a major branch of physics. Physicists and
mathematicians conversant in strings are courted and recruited
like star quarterbacks by universities eager to establish
their research credentials. String theory has been celebrated
and explained in best-selling books like "The Elegant
Universe," by Dr. Brian Greene, a physicist at Columbia
University, and even on popular television shows.
Last summer in Aspen, Dr. Schwarz and
Dr. Green (of Cambridge) cut a cake decorated with "20th
Anniversary of the First Revolution Started in Aspen," as
they and other theorists celebrated the anniversary of their
big breakthrough. But even as they ate cake and drank wine,
the string theorists admitted that after 20 years, they still
did not know how to test string theory, or even what it meant.
As a result, the goal of explaining
all the features of the modern world is as far away as ever,
they say. And some physicists outside the string theory camp
are growing restive. At another meeting, at the Aspen
Institute for Humanities, only a few days before the string
commemoration, Dr. Lawrence Krauss, a cosmologist at Case
Western Reserve University in
Cleveland, called string theory "a colossal
failure."
String theorists agree that it has
been a long, strange trip, but they still have faith that they
will complete the journey.
"Twenty years ago no one would
have correctly predicted how string theory has since
developed," said Dr. Andrew Strominger of Harvard.
"There is disappointment that despite all our efforts,
experimental verification or disproof still seems far away. On
the other hand, the depth and beauty of the subject, and the
way it has reached out, influenced and connected other areas
of physics and mathematics, is beyond the wildest imaginations
of 20 years ago."
In a way, the story of string theory
and of the physicists who have followed its siren song for two
decades is like a novel that begins with the classic
"what if?"
What if the basic constituents of
nature and matter were not little points, as had been presumed
since the time of the Greeks? What if the seeds of reality
were rather teeny tiny wiggly little bits of string? And what
appear to be different particles like electrons and quarks
merely correspond to different ways for the strings to
vibrate, different notes on God's guitar?
It sounds simple, but that small
change led physicists into a mathematical labyrinth, in which
they describe themselves as wandering, "exploring almost
like experimentalists," in the words of Dr. David Gross
of the Kavli Institute for Theoretical Physics in Santa
Barbara, Calif.
String theory, the Italian physicist
Dr. Daniele Amati once said, was a piece of 21st-century
physics that had fallen by accident into the 20th century.
And, so the joke went, would require
22nd-century mathematics to solve.
Dr. Edward Witten of the Institute for Advanced Study in
Princeton, N.J., described it this way: "String theory is
not like anything else ever discovered. It is an incredible
panoply of ideas about math and physics, so vast, so rich you
could say almost anything about it."
The string revolution had its roots in a quixotic effort in
the 1970's to understand the so-called "strong"
force that binds quarks into particles like protons and
neutrons. Why were individual quarks never seen in nature?
Perhaps because they were on the ends of strings, said
physicists, following up on work by Dr. Gabriele Veneziano of
CERN, the European research consortium.
That would explain why you cannot have a single quark - you
cannot have a string with only one end. Strings seduced many
physicists with their mathematical elegance, but they had some
problems, like requiring 26 dimensions and a plethora of
mysterious particles that did not seem to have anything to do
with quarks or the strong force.
When accelerator experiments supported an alternative
theory of quark behavior known as quantum chromodynamics, most
physicists consigned strings to the dustbin of history.
But some theorists thought the mathematics of strings was
too beautiful to die.
In 1974 Dr. Schwarz and Dr. Joel Scherk from the École
Normale Supérieure in
France noticed that one of the mysterious particles predicted
by string theory had the properties predicted for the
graviton, the particle that would be responsible for
transmitting gravity in a quantum theory of gravity, if such a
theory existed.
Without even trying, they realized, string theory had
crossed the biggest gulf in physics. Physicists had been stuck
for decades trying to reconcile the quirky rules known as
quantum mechanics, which govern atomic behavior, with
Einstein's general theory of relativity, which describes how
gravity shapes the cosmos.
That meant that if string theory was right, it was not just
a theory of the strong force; it was a theory of all forces.
"I was immediately convinced this was worth devoting
my life to," Dr. Schwarz recalled "It's been my life
work ever since."
It was another 10 years before Dr. Schwarz and Dr. Green
(Dr. Scherk died in 1980) finally hit pay dirt. They showed
that it was possible to write down a string theory of
everything that was not only mathematically consistent but
also free of certain absurdities, like the violation of cause
and effect, that had plagued earlier quantum gravity
calculations.
In the summer and fall of 1984, as word of the achievement
spread, physicists around the world left what they were doing
and stormed their blackboards, visions of the Einsteinian
grail of a unified theory dancing in their heads.
"Although much work remains to be done there seem to
be no insuperable obstacles to deriving all of known
physics," one set of physicists, known as the Princeton
string quartet, wrote about a particularly promising model
known as heterotic strings. (The quartet consisted of Dr.
Gross; Dr. Jeffrey Harvey and Dr. Emil Martinec, both at the
University of Chicago; and Dr. Ryan M. Rohm, now at the
University of North Carolina.)
The Music of Strings
String theory is certainly one of the most musical
explanations ever offered for nature, but it is not for the
untrained ear. For one thing, the modern version of the theory
decreed that there are 10 dimensions of space and time.
To explain to ordinary mortals why the world appears to
have only four dimensions - one of time and three of space
-string theorists adopted a notion first bruited by the German
mathematicians Theodor Kaluza and Oskar Klein in 1926. The
extra six dimensions, they said, go around in
sub-submicroscopic loops, so tiny that people cannot see them
or store old National Geographics in them.
But that was only the beginning. In 1995, Dr. Witten showed
that what had been five different versions of string theory
seemed to be related. He argued that they were all different
manifestations of a shadowy, as-yet-undefined entity he called
"M theory," with "M" standing for mother,
matrix, magic, mystery, membrane or even murky.
In M-theory, the universe has 11 dimensions - 10 of space
and one of time, and it consists not just of strings but also
of more extended membranes of various dimension, known
generically as "branes."
This new theory has liberated the imaginations of
cosmologists. Our own universe, some theorists suggest, may be
a four-dimensional brane floating in some higher-dimensional
space, like a bubble in a fish tank, perhaps with other branes
- parallel universes - nearby. Collisions or other
interactions between the branes might have touched off the Big
Bang that started our own cosmic clock ticking or could
produce the dark energy that now seems to be accelerating the
expansion of the universe, they say.
Toting Up the Scorecard
One of string theory's biggest triumphs has come in the
study of black holes. In Einstein's general relativity, these
objects are bottomless pits in space-time, voraciously
swallowing everything, even light, that gets too close, but in
string theory they are a dense tangle of strings and
membranes.
In a prodigious calculation in 1995, Dr. Strominger and Dr.
Cumrun Vafa, both of Harvard, were able to calculate the
information content of a black hole, matching a famous result
obtained by Dr. Stephen Hawking of Cambridge University using
more indirect means in 1973. Their calculation is viewed by
many people as the most important result yet in string theory,
Dr. Greene said.
Another success, Dr. Greene and others said, was the
discovery that the shape, or topology, of space, is not fixed
but can change, according to string theory. Space can even rip
and tear.
But the scorecard is mixed when it comes to other areas of
physics. So far, for example, string theory has had little to
say about what might have happened at the instant of the Big
Bang..
Moreover, the theory seems to have too many solutions. One
of the biggest dreams that physicists had for the so-called
theory of everything was that it would specify a unique
prescription of nature, one in which God had no choice, as
Einstein once put it, about details like the number of
dimensions or the relative masses of elementary particles.
But recently theorists have estimated that there could be
at least 10100 different solutions to the string
equations, corresponding to different ways of folding up the
extra dimensions and filling them with fields - gazillions of
different possible universes.
Some theorists, including Dr. Witten, hold fast to the
Einsteinian dream, hoping that a unique answer to the string
equations will emerge when they finally figure out what all
this 21st-century physics is trying to tell them about the
world.
But that day is still far away.
"We don't know what the deep principle in string
theory is," Dr. Witten said.
For most of the 20th century, progress in particle physics
was driven by the search for symmetries - patterns or
relationships that remain the same when we swap left for
right, travel across the galaxy or imagine running time in
reverse.
For years physicists have looked for the origins of string
theory in some sort of deep and esoteric symmetry, but string
theory has turned out to be weirder than that.
Recently it has painted a picture of nature as a kind of
hologram. In the holographic images often seen on bank cards,
the illusion of three dimensions is created on a
two-dimensional surface. Likewise string theory suggests that
in nature all the information about what is happening inside
some volume of space is somehow encoded on its outer boundary,
according to work by several theorists, including Dr. Juan
Maldacena of the Institute for Advanced Study and Dr. Raphael
Bousso of the University of California, Berkeley.
Just how and why a three-dimensional reality can spring
from just two dimensions, or four dimensions can unfold from
three, is as baffling to people like Dr. Witten as it probably
is to someone reading about it in a newspaper.
In effect, as Dr. Witten put it, an extra dimension of
space can mysteriously appear out of "nothing."
The lesson, he said, may be that time and space are only
illusions or approximations, emerging somehow from something
more primitive and fundamental about nature, the way protons
and neutrons are built of quarks.
The real secret of string theory, he said, will probably
not be new symmetries, but rather a novel prescription for
constructing space-time.
"It's a new aspect of the theory," Dr. Witten
said. "Whether we are getting closer to the deep
principle, I don't know."
As he put it in a talk in October, "It's plausible
that we will someday understand string theory."
Tangled in Strings
Critics of string theory, meanwhile, have been keeping
their own scorecard. The most glaring omission is the lack of
any experimental evidence for strings or even a single
experimental prediction that could prove string theory wrong -
the acid test of the scientific process.
Strings are generally presumed to be so small that
"stringy" effects should show up only when particles
are smashed together at prohibitive energies, roughly 1019
billion electron volts. That is orders of magnitude beyond the
capability of any particle accelerator that will ever be built
on earth. Dr. Harvey of
Chicago said he sometimes woke up thinking, What am I doing
spending my whole career on something that can't be tested
experimentally?
This disparity between theoretical speculation and testable
reality has led some critics to suggest that string theory is
as much philosophy as science, and that it has diverted the
attention and energy of a generation of physicists from other
perhaps more worthy pursuits. Others say the theory itself is
still too vague and that some promising ideas have not been
proved rigorously enough yet.
Dr. Krauss said, "We bemoan the fact that Einstein
spent the last 30 years of his life on a fruitless quest, but
we think it's fine if a thousand theorists spend 30 years of
their prime on the same quest."
The Other Quantum Gravity
String theory's biggest triumph is still its first one,
unifying Einstein's lordly gravity that curves the cosmos and
the quantum pinball game of chance that lives inside it.
"Whatever else it is or is not," Dr. Harvey said
in Aspen, "string theory is a theory of quantum gravity
that gives sensible answers."
That is no small success, but it may not be unique.
String theory has a host of lesser known rivals for the
mantle of quantum gravity, in particular a concept called,
loop quantum gravity, which arose from work by Dr. Abhay
Ashtekar of Penn State and has been carried forward by Dr.
Carlo Rovelli of the University of Marseille and Dr. Lee
Smolin of the Perimeter Institute for Theoretical Physics in
Waterloo,
Ontario, among others.
Unlike string theory, loop gravity makes no pretensions
toward being a theory of everything. It is only a theory of
gravity, space and time, arising from the applications of
quantum principles to the equations of Einstein's general
relativity. The adherents of string theory and of loop gravity
have a kind of Microsoft-Apple kind of rivalry, with the
former garnering a vast majority of university jobs and
publicity.
Dr. Witten said that string theory had a tendency to absorb
the ideas of its critics and rivals. This could happen with
loop gravity. Dr. Vafa; his Harvard colleagues, Dr. Sergei
Gukov and Dr. Andrew Neitzke; and Dr. Robbert Dijkgraaf of the
University of
Amsterdam report in a recent paper that they have found a
connection between simplified versions of string and loop
gravity.
"If it exists," Dr. Vafa said of loop gravity,
"it should be part of string theory."
Looking for a Cosmic Connection
Some theorists have bent their energies recently toward
investigating models in which strings could make an observable
mark on the sky or in experiments in particle accelerators.
"They all require us to be lucky," said Dr. Joe
Polchinski of the Kavli Institute.
For example the thrashing about of strings in the early
moments of time could leave fine lumps in a haze of radio
waves filling the sky and thought to be the remains of the Big
Bang. These might be detectable by the Planck satellite being
built by the European Space Agency for a 2007 launching date,
said Dr. Greene.
According to some models, Dr. Polchinski has suggested,
some strings could be stretched from their normal
submicroscopic lengths to become as big as galaxies or more
during a brief cosmic spurt known as inflation, thought to
have happened a fraction of a second after the universe was
born.
If everything works out, he said, there will be loops of
string in the sky as big as galaxies. Other strings could
stretch all the way across the observable universe. The
strings, under enormous tension and moving near the speed of
light, would wiggle and snap, rippling space-time like a
tablecloth with gravitational waves.
"It would be like a whip hundreds of light-years
long," Dr. Polchinski said.
The signal from these snapping strings, if they exist,
should be detectable by the Laser Interferometer Gravitational
Wave Observatory, which began science observations two years
ago, operated by a multinational collaboration led by Caltech
and the Massachusetts Institute of Technology.
Another chance for a clue will come in 2007 when the Large
Hadron Collider is turned on at CERN in
Geneva and starts colliding protons with seven trillion volts
of energy apiece. In one version of the theory - admittedly a
long shot - such collisions could create black holes or
particles disappearing into the hidden dimensions.
Everybody's favorite candidate for what the collider will
find is a phenomenon called supersymmetry, which is crucial to
string theory. It posits the existence of a whole set of
ghostlike elementary particles yet to be discovered. Theorists
say they have reason to believe that the lightest of these
particles, which have fanciful names like photinos, squarks
and selectrons, should have a mass-energy within the range of
the collider.
String theory naturally incorporates supersymmetry, but so
do many other theories. Its discovery would not clinch the
case for strings, but even Dr. Krauss of Case Western admits
that the existence of supersymmetry would be a boon for string
theory.
And what if supersymmetric particles are not discovered at
the new collider? Their absence would strain the faith, a bit,
but few theorists say they would give up.
"It would certainly be a big blow to our chances of
understanding string theory in the near future," Dr.
Witten said.
Beginnings and Endings
At the end of the Aspen celebration talk turned to the
prospect of verification of string theory. Summing up the long
march toward acceptance of the theory, Dr. Stephen Shenker, a
pioneer string theorist at Stanford, quoted Winston Churchill:
"This is not the end, not even the beginning of the
end, but perhaps it is the end of the beginning."
Dr. Shenker said it would be great to find out that string
theory was right.
From the audience Dr. Greene piped up, "Wouldn't it be
great either way?"
"Are you kidding me, Brian?" Dr. Shenker
responded. "How many years have you sweated on
this?"
But if string theory is wrong, Dr. Greene argued, wouldn't
it be good to know so physics could move on? "Don't you
want to know?" he asked.
Dr. Shenker amended his remarks. "It would be great to
have an answer," he said, adding, "It would be even
better if it's the right one."
A simple example, the story goes, is a garden hose. Seen
from afar, it is a simple line across the grass, but up close
it has a circular cross section. An ant on the hose can go
around it as well as travel along its length. To envision the
world as seen by string theory, one only has to imagine a
tiny, tiny six-dimensional ball at every point in space-time.
