A unified theory of the fundamental interactions involving supersymmetry, in which the basic objects are one-dimensional objects (superstrings). Superstrings are thought to have a length-scale of about 10−35 m and, since very short distances are associated with very high energies, they should have energy scales of about 1019 GeV, which is far beyond the energy of any accelerator that can be envisaged.
Strings associated with bosons are only consistent as quantum theories in a 26-dimensional space–time; those associated with fermions are only consistent as quantum theories in 10-dimensional space–time. It is thought that four macroscopic dimensions arise according to Kaluza–Klein theory, with the remaining dimensions being ‘curled up’ to become very small, although other possibilities for the higher dimensions have been put forward.
One of the most attractive features of the theory of superstrings is that it leads to massless spin-2 particles, which are identified as gravitons. Thus, a superstring theory automatically contains a quantum theory of the gravitational interaction. It is thought that superstrings are free of infinities that cannot be removed by renormalization, which plague attempts to construct a quantum field theory incorporating gravity. There is some evidence that superstring theory is free of infinities but not a complete proof yet. Another appealing aspect of superstring theory is that it is in accord with the holographic hypothesis.
Although there is no direct evidence for superstrings, some features of superstrings are compatible with the experimental facts of elementary particles, such as the possibility of particles that do not respect parity, as found in the weak interactions. There has been a great deal of effort to relate superstring theory to the properties of elementary particles, but this has proved very far from straightforward.
All the viable superstring theories and also supergravity theory are related by duality, with the unified theory underpinning this being an 11-dimensional theory called M-theory. Both the entropy of black holes and Hawking radiation have been explained in terms of superstring theory. Techniques from superstring theory have been used to solve problems in other branches of physics, such as quantum chromodynamics. There is much more to superstring theory than strings alone, as the theory has membrane solutions and also solutions that are extended objects in more than two space dimensions.