For nearly 18 years now, most advanced mathematical work in
theoretical particle physics has centered on something known as
string theory. This theory is built on the idea that elementary
particles are not pointlike objects but are the vibration modes
of one-dimensional "stringlike" entities. This
formulation hopes to do away with certain lingering problems
in fundamental particle physics and to offer the possibility
of soon explaining *all* physical
phenomenaýeverything from neutrinos to black
holesýwith a single theory. Fifteen years ago Edward
Witten of the Institute for Advanced Study made the widely
quoted claim that "string theory is a part of
21st-century physics that fell by chance into the 20th
century,"** **so perhaps it is now
time to begin judging the success or failure of this new way
of thinking about particle physics.

The strongest scientific argument in favor of string theory
is that it appears to contain a theory of gravity embedded
within it and thus may provide a solution to the thorny
problem of reconciling Einstein's general relativity with
quantum mechanics and the rest of particle physics. There
are, however, two fundamental problems, which are hard to
get around.

First, string theory predicts that the world
has 10 space-time dimensions, in serious disagreement with
all the evidence of one's senses. Matching string theory
with reality requires that one postulate six unobserved
spatial dimensions of very small size wrapped up in one way
or another. All the predictions of the theory depend on how
you do this, but there are an infinite number of possible
choices, and no one has any idea how to determine which is
correct.

The second concern is that even the part of
string theory that is understood is internally inconsistent.
This aspect of the theory relies on a series expansion, an
infinite number of terms that one is supposed to sum
together to get a result. Whereas each of the terms in the
series is probably finite, their sum is almost certainly
infinite. String theorists actually consider this
inconsistency to be a virtue, because otherwise they would have
an infinite number of consistent theories of gravity on
their hands (one for each way of wrapping up six
dimensions), with no principle for choosing among them.