Making connections
is the essence of scientific progress. For monumental
examples in the realm of physics, think of the amalgamation of
electricity and magnetism and light; of the recognition that heat is
atoms in motion, which brought together thermodynamics and Newtonian
mechanics; and of the realization that the chemical properties of
substances are determined by the atomic and molecular structure of
matter.

These advances, and much of our recent progress in particle physics and
cosmology, came about in large measure through the creative resonance
between experimental discovery and theoretical insight, sometimes one
taking the lead, sometimes the other. Another path to theoretical
progress focuses on foundational issues of internal logic and
self-consistency. The most recent strides—many would say leaps and
bounds—along that path have attended the development of string theory,
the subject of Brian Greene's thoughtful and important book, *The Elegant
Universe.* Greene, a professor of physics and mathematics at Columbia
University, and an adjunct professor at Cornell University, is a leading
contributor to string theory and a key agent in the growing reunion of
physics and mathematics. *The Elegant Universe* presents the ideas and
aspirations—and some of the characters—of string theory with clarity
and charm. It is both a personal story and the tale of a great
intellectual movement.

The conceptual tension that faces physics at the millennium has been
building for half a century. All of modern physics rests on two pillars.
One is Albert Einstein's general relativity, which describes the world
on the largest of scales—stars, galaxies and the immensity of the
universe itself. The other is quantum mechanics, which describes the
world on the smallest of scales—atoms, nuclei and quarks. Experiments
have confirmed the predictions of both general relativity and quantum
mechanics to a remarkable degree. But—as we currently understand these
two theories—they cannot both be right. The violent fluctuations on
ultramicroscopic scales implied by the uncertainty principle of quantum
mechanics are at odds with the smooth geometry of spacetime that is the
central feature of general relativity.

Why has this battle of titanic ideas not paralyzed physics? At most of
the distance scales that physicists contemplate, from less than a
billionth of a billionth of human scale out to the farthest reaches of the universe,
the dissonance between quantum mechanics and general relativity does not
arise. That is why many physicists are content to note the problem and
go—productively—about their business. But when we consider
stupendously short distances to analyze the earliest history of the
universe, we run up against the fundamental incompatibility between
general relativity and quantum mechanics.

Over the past three decades, an intrepid band of theoretical physicists
and mathematicians—first in tiny groups, now in a veritable army—have
concluded that the time is ripe to address this grand conflict between
the two great pillars of our understanding. The picture they are
developing is called string theory. It holds that the fundamental
constituents of the universe are not the elementary particles that we
idealize as having no size, like geometric points, but tiny strings. The
resonant patterns of vibrations of the strings are the microscopic
origin of the masses of what we perceive as particles and the strengths
we assign to the fundamental forces. Because strings have a finite,
though fantastically tiny, size, there is a limit to how finely we can
dissect nature. That limit—set by the size of strings—comes into play
before we encounter the devastating quantum fluctuations that rend
spacetime. Thus, the conflict between quantum mechanics and general
relativity is resolved

#### Searching for Symmetry

String theorists undertake their campaign to reconcile general
relativity and quantum mechanics even without detailed experimental
results, which Greene calls "the shining light of nature," to guide them
from one step to the next. Lacking experiment's guiding hand, it is
possible that one or more generations of physicists will devote their
careers to string theory without getting any experimental feedback. They
risk investing a lifetime of effort for an inconclusive result.
Why take the risk? The string theorists are not simply sticklers for
rigor. They are animated by ambition, by the hope that resolving the
conflict between general relativity and quantum mechanics will resolve
at a stroke many other issues—explaining the properties of the
elementary particles and forces, plumbing the true nature of a black
hole. "In his long search for a unified theory," Greene writes,
"Einstein reflected on whether 'God could have made the Universe in a
different way; that is, whether the necessity of logical simplicity
leaves any freedom at all.' With this remark, Einstein articulated the
nascent form of a view that is currently shared by many physicists: If
there is a final theory of nature, one of the most convincing arguments
in support of its particular form would be that the theory couldn't be
otherwise."

Early in the study of string theory, physicists learned that the theory
does not make sense if spacetime is made up of the three space
dimensions plus one time dimension of ordinary experience. String theory
also requires extra space dimensions that apparently must be curled up to a very
small size to be consistent with our never having seen them. Whether we
can probe them directly or not, these extra curled-up dimensions—six of
them, in most forms of string theory—have physical consequences. As
Greene explains it, "a tiny string can probe a tiny space. As a string
moves about, oscillating as it travels, the geometrical form of the
extra dimensions plays a critical role in determining resonant patterns
of vibration. Because the patterns of string vibrations appear to us as
the masses and charges of the elementary particles, we conclude that
these fundamental properties of the universe are determined, in large
measure, by the geometrical size and shape of the extra dimensions.
That's one of the most far-reaching insights of string theory."

String theory is still a work in progress. In fact, the equations of
string theory seem so complicated that physicists have managed to write
down only approximate versions. Within the past few years, string
theorists have realized that all the approximate formulations might be
seen as different limiting cases of an 11-dimensional theory whose
fundamental entities include two-dimensional membranes. Although it is
only partially understood, this new theory, termed M-theory, has given
unforeseen unity to the approximate versions of string theory that
previously seemed distinct and unrelated [see "The Theory Formerly Known
as Strings," by Michael J. Duff; *Scientific American,* February 1998].

If string theory arises from metaphorical travel to the realm of
infinitesimal distances and unattainably high energies, how can we hope
to test it? Edward Witten of the Institute for Advanced Study in
Princeton, N.J., urges that string theory already can claim experimental
confirmation for its prediction of gravity. As Greene explains, "Both
Newton and Einstein developed theories of gravity because their
observations of the world clearly showed them that gravity exists. On
the contrary, a physicist studying string theory … would be inexorably
led to it by the string framework."

One reason that string theory appeals so powerfully to theoretical
physicists is that it is the most symmetrical theory ever devised.
Symmetry is the concept physicists use to relate phenomena and
circumstances that seem—on first examination—to be different. James
Clerk Maxwell showed that such disparate phenomena as light, electricity
and magnetism are fundamentally intertwined; Einstein showed that all
states of motion are related. String theory encompasses these symmetries
and more. It incorporates the grandest symmetry that physicists have
imagined: supersymmetry, a quantum-mechanical extension of space and
time. Supersymmetry relates the two quantum-mechanical categories of
elementary particles, so that each of the known fundamental particles
must have a superpartner whose spin differs by half a unit. Intensive
searches for supersymmetry—or for indirect indications of the existence
of supersymmetry—preoccupy many particle physicists around the world.
Although supersymmetry can exist without string theory, the discovery of
supersymmetry would supply extremely strong encouragement for string
theory.

String theory might also be able to resolve one of the great puzzles of
cosmology. Astronomical observations have shown that Einstein's
cosmological constant, which governs the cosmic evolution of the motions
of distant galaxies, is quite small, if not exactly zero. Yet according
to our current understanding of the fundamental interactions, quantum
fluctuations throughout space tend to create a cosmological constant
that is many, many times larger than observation allows. Can string
theory show why the cosmological constant is tiny?

#### The Significance of Scale

For most of the 20th century, physicists have been living through—no,
making—a change in the way humans think about their world. Physicists
have known since the 1920s that to explain why a table is solid, or why a
metal gleams, we must explore the atomic and molecular structure of
matter. That realm is ruled not by the customs of everyday life but by
the laws of quantum mechanics. The recognition that the human scale is
not privileged, that we need to leave our surroundings the better to
understand them, has been building since the birth of quantum mechanics.
As it emerges whole, fully formed, in our unified theories and
scale-changing shifts in perspective, the notion seems to me both
profound and irresistible. I find it fully appropriate to compare this
change in perception with the shifts in viewpoint we owe to Copernicus
and Einstein. And just as the realization that we are not at the center
of the universe ultimately enlightened and empowered—not diminished and
dispirited—us humans, so, too, the recognition that our size is not the
only, or the most important, scale for comprehending nature will be a
source of insight and inspiration.
If string theory succeeds, its success will represent the culmination of
the idea that we understand the universe best when we consider it on many
scales. Here is Greene's summation: "Although we are technologically
bound to the earth and its immediate neighbors in the solar system,
through the power of thought and experiment we have probed the far
reaches of both inner and outer space. During the last hundred years in
particular, the collective effort of numerous physicists has revealed
some of nature's best-kept secrets. And once revealed, these explanatory
gems have opened vistas on a world we thought we knew, but whose splendor
we had not even come close to imagining."

Will string theory succeed? We do not know. After all, no one yet
understands completely what string theory is, and the tools needed to
extract its predictions are still being developed. It is certainly not
the only fruitful path to follow: experiments at the great accelerators
and surveys at the great observatories, together with the theoretical
developments that motivate or respond to them, will surely bring
dramatic advances in our understanding of what nature is and how it
works. But the insights that string theory has already brought
illustrate anew the power of the question "Why not?" to open our eyes to
new possibilities. String theory is a beautiful dream, beautifully told
in *The Elegant Universe.*

CHRIS QUIGG is a theoretical physicist at Fermi National Accelerator Laboratory
in Batavia, Ill. He is the author of *Gauge Theories of the Strong, Weak, and
Electromagnetic Interactions* (Perseus Books, 1997).