Copyright 2006 The New York Times Company
The New York Times
October 20, 2006 Friday
Late Edition - Final
Reprinted without permission.
Universe on a String
By Brian Greene.
Brian Greene, a professor of physics and mathematics at
Columbia, is the author of ''The Elegant Universe'' and ''The
Fabric of the Cosmos.''
SEVENTY-FIVE years ago this month, The New York Times reported that
Albert Einstein had completed his unified field theory - a theory of
nature's forces into a single, tightly woven mathematical
tapestry. But as had happened before and would happen again, closer
scrutiny revealed flaws that sent Einstein back to the drawing
board. Nevertheless, Einstein's belief that he'd one day complete the
unified theory rarely faltered. Even on his deathbed he scribbled
equations in the desperate but fading hope that the theory would
finally materialize. It didn't.
In the decades since, the urgency of finding a unified theory has only
increased. Scientists have realized that without such a theory,
critical questions can't be addressed, such as how the universe began
or what lies at the hart of a black hole. These unresolved issues have
inspired much progress, with the most recent advances coming from an
approach called string theory. Lately, however, string theory has come
in for conciderable criticism. And so, this is an auspicious moment to
reflect on the state of the art.
First , some context. For nearly 300 years, science has been on a path
of consolidation. In the 17th century, Isaac Newton discovered laws of
motion that apply equally to a planet moving through space and to an
apple falling earthward, revealing that the physics of the heavens
later, Michael Faraday and James Clerk produce magnetic fields, and
moving magnets can produce electric currents, establishing that these
two forces are as united as Midas' touch and gold. And in the 20th
century, Einstein's work proved that space, time and gravity are so
entwined that you can't speak sensibly about one without the others.
This striking pattern of convergence, linking concepts once thought
unrelated, inspired Einstein to dream of the next and possibly final
move: merging gravity and electromagnetism into a single, overarching
theory of nature 's forces.
In hindsight, there was almost no way he could have succeeded. He was
barely aware that there were two other forces he was neglecting - the
strong and weak forces acting within atomic nuclei. Furthermore, he
willfully ignored quantum mechanics, the new theory of the microworld
that was receiving voluminous experimental support, but whose
probabilistic framework struck him as deeply misguided. Einstein
stayed the course, but by his final years he had drifted to the fringe
of a subject he had once dominated.
After Einstein's death, the torch of unification passed to other
hands. In the 1960's, the Nobel Prize-winning works of Sheldon
Glashow, Abdus Salam and Steven Weinberg revealed that at high
energies, the electromagnetic and weak nuclear forces seamlessly
combine, much as heating a cold vat of chicken soup causes the
floating layer of fat to combine with the liquid below, yielding a
homogeneous broth. Subsequent work argued that at even higher energies
the strong nuclear force would also meld into the soup, proposed
consolidation that has yet to be confirmed experimentally, that there
is no fundamental obstacle to unifying three of nature's four forces.
For decades, however, the force of gravity stubbornly resisted joining
the fold. The problem was the very one that so troubled Einstein: the
disjunction between his own general relativity, most relevant for
extremely massive objects like stars and galaxies, and quantum
mechanics, the framework invoked by physics to deal with exceptionally
small objects like molecules and atoms and their constituents.
Time and again, attempts to merge the two theories resulted in
ill-defined mathematics, much like what happens on a calculator if you
try to divide one by zero. The display will flash an error message,
reprimanding you for misusing mathematics. The combined equations of
general relativity and quantum mechanics yield similar problems. While
the conflict rears its head only in environments that are both
extremely massive and exceptionally tiny - black holes and the Big
Bank being two primary examples - it tells of a fissure in the very
foundations of physics.
Such was the case until the mid-1980's, when a new approach, string
theory, burst onto the stage. Difficult and complex calculations by
the physicists John Schwarz and Michael Green, who had been toiling
for years in scientific obscurity, gave compelling evidence that this
new approach not only unified gravity and quantum mechanics, as well
as nature's other forces, but did so while sweeping aside previous
mathematical problems. As word of the breakthrough spread, many
physicists dropped what they were working on and joined a global
effort to realize Einstein's unified vision of the cosmos.
String theory offers a new perspective on matter's fundamental
constituents. Once viewed as point-like dots virtually no size,
particles in string theory are minuscule, vibrating, string-like
filaments. And much as different vibrations of a violin string produce
different musical notes, different vibrations of the theory's strings
produce different kinds of particles. And electron is a tiny string
vibrating in one pattern, a quark is a string vibrating in a different
pattern. Particles like the photon that convey nature's forces in the
quantum realm are strings vibrating in yet other patterns.
Crucially, the early pioneers of string theory realized that one such
vibration would produce the gravitational force, demonstrating that
string theory embraces both gravity and quantum mechanics. In sharp
contrast to previous proposals that cobbled gravity and quantum
mechanics uneasily together, their unity here emerges from the theory
While accessibility demands that I describe these developments using
familiar words, beneath them lies a bedrock of rigorous analysis. We
now have more then 20 years of painstaking research, filling tens of
thousands of published pages of calculations, which attest to string
theory's deep mathematical coherence. These calculations have given
the theory countless opportunities to suffer the fate of previous
proposals, but the fact is that every calculation that has ever been
completed within string theory is free from mathematical
Moreover, these works have also shown that many of the prized
breakthroughs in fundamental physics, discovered over the past two
centuries through arduous research using a wide range of approaches,
can be found within string theory. It 's as if one composer, working
in isolation, produced the greatest hits of Beethoven, Count Basie and
the Beatles. When you also consider that string theory has opened new
areas of mathematical research, you can easily understand why it's
captured the attention of so many leading scientists and
Nevertheless, mathematical rigor and elegance are not sufficient to
demonstrate a theory's relevance. To be judged a correct description
of the universe, a theory must make predictions that are confirmed by
experiment. And as a small but vocal group of critics of string theory
justly emphasize, string theory has yet to do so. This is a key point,
so it's worth serious scrutiny.
We understand string theory much better now than we did 20 years
ago. We've developed powerful techniques of mathematical analysis that
have improved the accuracy of its calculations and provided invaluable
insights into the theory's logical structure. Even so, researchers
worldwide are still working toward an exact and tractable formulation
of the theory's equations. And without that final formulation in hand,
the kind of detailed a definitive predictions that would subject the
theory to comprehensive experimental vettingremain beyond our reach.
There are, however, features of the theory that maybe open to
examination even with our incomplete understanding. We may be able to
test the theory's predictions of particular new particle species, of
dimensions of space beyond the three we can directly see, and even its
prediction that microscopic black holes may be produced through highly
energetic particle collisions. Without the exact quotations, our
ability to describe these attributes with precision is limited, but
the theory gives enough direction for the Large Hadron Collider, a
gigantic particle accelerator now being built in Geneva and scheduled
to begin full operation in 2008, to search for supporting evidence by
the end of the decade.
Research has also revealed a possibility that signatures of string
theory are imprinted in the radiation left over from the Big Bang, as
well as in gravitational waves rippling through space-time's
fabric. In the coming years, a variety of fabric. In the coming years,
a variety of experiments will seek such evidence with unprecedented
observational fidelity. And in a recent, particularly intriguing
development, data now emerging from the Relativistic Heavy Ion
Collider at the Brookhaven National Laboratory appear to be more
accurately described using string theory methods than with more
To be sure, no one successful experiment would establish that string
theory is right, but neither would the failure of all such experiments
prove the theory wrong. If the accelerator experiments fail to turn up
anything, it could be that we need more powerful machine; if the
astronomical observations fail to turn up anything, it could mean the
effects are too small to be seen. The bottom line is that it's hard to
test a theory that not only taxes the capacity of today's technology
but is also still very much under development.
Some critics have taken this lack of definitive predictions to mean
that string theory is a protean concept whose advocates seek to step
outside the established scientific method. Nothing could be further
from the truth. Certainly, we are feeling our way through a complex
mathematical terrain, and no doubt have much ground yet to cover. But
we will hold string theory to the usual scientific standard: to be
accepted, it must make predictions that are verified.
Other detractors have seized on recent work suggesting that one of
string theory's goals beyond unification of the forces - to provide an
explanation for the values of nature's constants, like the mass of the
electron and the strength of gravity - may be unreachable (because the
the4ory may be compatible with those constants having arrange of
values). But even if this were to prove true, realizing Einstein's
unified vision would surely be prize enough.
Finally, some have argued that if, after decades of research involving
thousands of scientists, the theory is still a work in progress, it's
time to give up. But to suggest dropping research on the most
promising approach to unification because the work has failed to meet
an arbitrary time-table for complete success is, well, silly.
I have worked on string theory for more than 20 years because I
believe it provides the most powerful framework for constructing the
long-sought unified theory. Nonetheless, should an inconsistency be
found, or should future studies reveal an insuperable barrier to
making contact with experimental data, or should new discoveries
reveal a superior approach, I'd change my research focus, and I have
little doubt that most string theorists would too.
But this hasn't happened.
String theory continues to offer profound breadth and enormous
potential. It has the capacity to complete the Einsteinian revolution
and could very well be the concluding chapter in our species' age-old
quest to understand the deepest workings of the cosmos.
Will we ever reach that goal? I don't know. But that's both the wonder
and the angst of a life in science. Exploring the unknown requires
Disclaimer: I am not the most intelligent kuron.
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