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NATURE|Vol 438|22/29 December 2005 ESSAY
1085E
SSAY
Unravelling string theoryString theory may provide the best
clues yet about how to obtain a unified theory that describes
all the laws of nature, but do we even understand what string
theory is?
Edward Witten
Albert Einstein famously devoted the laterpart of his life to
seeking a theory that wouldoffer, at least in principle, a
comprehensivedescription of the laws of nature. This uni-fied field
theory, Einstein believed, wouldendow all of natures laws with the
beauty ofgeneral relativity. Ultimately, Einstein left uswith
plenty of inspiration, but not manyideas about how to proceed.
In fact, there are ample reasons why onemight doubt whether
Einsteins vision isachievable, or at least achievable in the
fore-seeable future. Crucial clues may be hope-
lessly out of reach. When looking back atEinsteins own work,
most physicistswould say that many of the mostimportant clues for a
unified fieldtheory involving strong andweak nuclear interactions,
the roleof gauge theory and the world ofelementary particles were
simplynot known in Einsteins day.
Moreover, even if we couldsomehow find the unified fieldtheory,
it is not at all clear whetherwe could determine that it is
right.From a simple combination of
Plancks constant, the speed of light,and Newtons gravitational
constant,one can construct a natural unit oflength the Planck
length. Firstdefined by Max Planck a centuryago, this length is so
fantasticallysmall that if it, or something close to it,
isfundamental in physics, then some of themost important phenomena
may be per-manently beyond our experimental reach.
I remember vividly how impressed I wasin 1981 when the
distinguished experi-mentalist Norman Ramsey (who later wonthe
Nobel prize for his work on magnetic
resonance) forecast that within 50 yearsthere would be a clear
outline of a unifiedfield theory, with all the forces fitting
intogether, even if not perfectly. I certainlydid not see any
useful way to work on sucha thing, and doubted that I would see
it.
Meanwhile, I was only dimly aware of thework that was being done
by MichaelGreen, John Schwarz, Lars Brink and
very few others to revive string theory.Originally a candidate
theory of the nuclearforce, string theory had been developed
anddiscarded a decade earlier. Its revival wasmotivated by the hope
that it would give the
basis for a unified field theory.By 1982 or 1983, I began to
notice this
work, which had advanced to the point that
it was possible to formulate fairly convinc-ing theories of
quantum gravity unifiedwith matter. There was, however, whatseemed
like an obvious flaw. String theoryseemed incompatible with parity
violationin weak interactions (such as nuclear betadecay). Parity
violation the fact that thelaws of nature are not invariant under a
mir-ror reflection is oneof themost
important findings ever madeabout elementary particles.In 1984,
this flaw was abruptly
overcome when Green and Schwarzdiscovered an elegant new
mechanism ofanomaly cancellation. Not only could theweak
interactions violate parity but, espe-cially after the invention of
the heteroticstring, it soon became possible to
derivesemi-realistic models of elementary part-icles with all their
known forces, includinggravity. At this point, it really did seem
rea-
sonable to work on unified field theory.I suppose that there are
three basic rea-sons why string theory has attracted somuch
interest in the past 20 years. One isthat it is there. String
theory is the onlyknown generalization of relativistic quan-tum
field theory that makes sense. Theframework of special relativity
plus quan-tum mechanics is so rigid that it practicallyforces
quantum field theory upon us. Thetightness of the modern framework
is oneof the main reasons why physicists wereable to discover what
has become the stan-dard model of elementary particles. A big
idea like a consistent generalization ofquantum field theory
comes along only
every now and then. So we are duty-bound
to take it seriously.A second reason has to do with what
physicists have learned in developing stringtheory. String
theory forces general rela-tivity upon us, whereas standard
quantumfield theory apparently makes it impossibleto incorporate
general relativity. And stringtheory leads in a remarkably simple
way toa reasonable rough draft of particle physicsunified with
gravity.
And finally, string theory has proved tobe remarkably rich, more
so than even theenthusiasts tend to realize. It has led
topenetrating insights on topics from quark
confinement to quantum mechanics ofblack holes, to numerous
problems
in pure geometry. All this sug-gests that string theory is on
theright track; otherwise, whywould it generate so many unex-pected
ideas? And where critics
have had good ideas, they havetended to be absorbed as part
of
string theory, whether it was black-hole entropy, the
holographicprinciple of quantum gravity,noncommutative geometry,
ortwistor theory.
But what is string theory? It maywell be the only way to
reconcile
gravity and quantum mechanics, but whatis the core idea behind
it? Einstein under-stood the central concepts of general
rela-tivity years before he developed the detailedequations. By
contrast, string theory hasbeen discovered in bits and pieces over
aperiod that has stretched for nearly fourdecades without anyone
really under-standing what is behind it. As a result, everybit that
is unearthed comes as a surprise. Westill dont know where all these
ideas arecoming from or heading to.
One day we may understand what stringtheory really is. But even
if we do, and thetheory is on the right track, will we be ableto
learn how it works in nature? I certainlyhope so. Realistically, it
all depends onmany unknowns, including the nature ofthe answer, how
clever we will be, and theclues we can get from experiment. Edward
Witten is at the Institute for
Advanced Study, School of Natural Sciences,
Princeton, New Jersey 08540, USA.
FURTHERREADING
Ramsey, N. F. Phys. Today 34,2634 (November 1981).
Zwiebach, B. A First Course in String Theory (Cambridge
Univ. Press, 2004).Greene, B. The Elegant Universe (W. W.
Norton, 1999).
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