MACROSCOPE
Is String Theory Even Wrong?
Peter Woit
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.
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