String theory

A string theory is a physical model whose fundamental building blocks are
extended objects (strings, membranes and higher-dimensional objects) rather
than points. String theories are able to avoid problems, such as infinite
energy density, associated with the presence of mathematical points in a
physical theory.

The term 'string theory' properly refers to both the 26-dimensional bosonic
string theories and to the 10-dimensional superstring theories discovered by
adding supersymmetry to bosonic string theory. Nowadays, 'string theory'
usually refers to the supersymmetric variant while the earlier is given its
full name 'bosonic string theory'. The different superstring theories were
discovered to be different limits of an unknown 11-dimensional theory called
M-theory proposed by Horava and Witten in the 1990s.

A central consequence of string theory is that the observed physics of the
known universe can be stated as arising from two seemingly incompatible
geometries: one being 'as large as we see', and the other being smaller by
far than the Planck length. Since both geometries lead to the same observed
physics, but the small scale phenomena are beyond human investigation, there
has been considerable discussion of the impact of string theory on both the
philosophy of science (is it 'science' if no experiment can be run to
disprove it?) and the philosophy of mathematics (if two geometries lead to
the same physics, should geometry itself not reflect the same universe its
discoverers and codifiers live in, and prove that there is a single common
phenomenon that the two geometries reflect?).

On a more practical level, string theory has led to advances in the
mathematics of folding, knots and Calabi-Yau spaces. Because much of this
mathematics is new, the uncertainty has been increased somewhat, as very few
people can understand either the physics or the mathematics on which it
depends.

Problems with string theory

String theory suffers from two major problems. The first problem is that, in
the words of Wolfgang Pauli, it is not even wrong. In other words, it does
not make any predictions that may be subject to experimental verification;
thus, it can be neither proven nor disproven, which is a serious problem for
any theory of physics.

The second problem is that it assumes, as did Newtonian mechanics and
special relativity, a fixed space-time background. Ultimately, a theory
subsuming quantum mechanics is needed which is independent of any fixed
space-time and thus consistent with general relativity. M-theory has been
hypothesized to overcome the latter problem; Loop quantum gravity overcomes both.