MACROSCOPE

# Is String Theory Even Wrong?

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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