[Paleopsych] NYT: String Theory, at 20, Explains It All (or Not)
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String Theory, at 20, Explains It All (or Not)
NYT December 7, 2004
By DENNIS OVERBYE
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
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
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
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
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
"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
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
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
"I was immediately convinced this was worth devoting my
life to," Dr. Schwarz recalled "It's been my life work ever
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
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
"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
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.
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
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,
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
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
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,
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
"It's a new aspect of the theory," Dr. Witten said.
"Whether we are getting closer to the deep principle, I
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
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
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
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.
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
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
"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."
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