Quantum string theory
Quantum String theory is aimed at solving the core problem of 20th-century theoretical physics: The mathematical incompatibility of Quantum mechanics and the General Theory of Relativity (General Relativity). String theory has emerged as a promising candidate for a microscopic theory of gravity. And it is much more ambitious than that: it attempts to provide a complete, unified, and consistent description of the fundamental structure of our universe. (For this reason it is sometimes, quite arrogantly, called a 'Theory of everything').
General relativity describes the force of gravity and is usually applied to large objects. Quantum mechanics is most relevant in describing the smallest structures in the universe such as atoms, electrons and quarks.Any calculation which simultaneously uses General Relativity and Quantum Mechanics yields nonsensical answers. In order to solve this problem, the basic idea is to replace point-like elementary particles with fundamental strings, which are line-like objects of very small length, based on the (deceptively simple) premise that at Planckian scales, where the quantum effects of gravity are strong, particles are actually one-dimensional extended objects.
In contrast with particle theories, string theory is highly constrained in the choice of interactions, supersymmetries and gauge groups. In fact, all the usual particles emerge as excitations of the string and the interactions are simply given by the geometric splitting and joining of these strings. Among the particles arising as vibrations of the string, we find some which are very similar to electrons, muons, neutrinos and quarks -- the known matter particles. There are others similar to photons, W and Z bosons and gluons -- the known force carriers. And there is one particle similar to the graviton, the elusive fourth force carrier.
Different string theories:
| Type | Spacetime Dimensions |
Details |
|---|---|---|
| Bosonic | 26 | Only bosons, no fermions means only forces, no matter, with both open and closed strings. Major flaw: a particle with imaginary mass, called the tachyon |
| I | 10 | Supersymmetry between forces and matter, with both open and closed strings, no tachyon, group symmetry is SO(32) |
| IIA | 10 | Supersymmetry between forces and matter, with closed strings only, no tachyon, massless fermions spin both ways (nonchiral) |
| IIB | 10 | Supersymmetry between forces and matter, with closed strings only, no tachyon, massless fermions only spin one way (chiral) |
| HO | 10 | Supersymmetry between forces and matter, with closed strings only, no tachyon, heterotic, meaning right moving and left moving strings differ, group symmetry is SO(32) |
| HE | 10 | Supersymmetry between forces and matter, with closed strings only, no tachyon, heterotic, meaning right moving and left moving strings differ, group symmetry is E8 x E8 |
The mathematical apparatus of string theory is very involved and is based on ideas from conformal field theory, infinite-dimensional algebras and Einstein's General Relativity. Although a complete picture of string theory is not yet available, there are indications that string theory is at this moment a promising candidate theory for a unified description of the fundamental particles and forces in nature including gravity.
In the past years, however, strings have been subsumed by M-theory.