String theory

String theory is a model of fundamental physics whose building blocks are one-dimensional extended objects called strings, rather than the zero-dimensional point particles that form the basis for the Standard Model of particle physics. The phrase is often used as shorthand for Superstring theory, as well as related theories such as M-theory. String theorists are attempting to adjust the Standard Model by removing the assumption in quantum mechanics that particles are point-like. By removing this assumption and replacing the point-like particles with strings, a sensible quantum theory of gravity seems to naturally emerge. Moreover, string theory attempts to "unify" the known natural forces (gravitational, electromagnetic, weak nuclear and strong nuclear) by describing them with the same set of equations. (See Theory of everything)

For a scientific theory to be valid it must be verified experimentally. Few avenues for such contact with experiment have been claimed. With the construction of the Large Hadron Collider in CERN some scientists hope to produce relevant data. It is generally expected though that any theory of quantum gravity would require much higher energies to probe.

Another potential problem is that it is not a theory that is tightly constrained. There are different versions of string theory, depending on factors such as whether or not supersymmetry is incorporated into the formulation. These versions are thought to be related to each other as different limits of one theory, coined M-theory. There is a huge number of possible solutions to string theory as it is currently understood. Thus it has been claimed by some scientists that string theory may not be falsifiable and may have no predictive power.

Studies of string theory have revealed that it predicts higher-dimensional objects called branes. String theory strongly suggests the existence of ten or eleven (in M-theory) spacetime dimensions, as opposed to the usual four (three spatial and one temporal) used in relativity theory; however the theory can describe universes with four effective (observable) spacetime dimensions by a variety of methods.

An important branch of the field is dealing with a conjectured duality between string theory as a theory of gravity and gauge theory. It is hoped that research in this direction will lead to new insights on quantum chromodynamics, the fundamental theory of strong nuclear force. This direction of research has better hopes to make contact with experiment, compared to string theory as a quantum theory of gravity, though currently the alternative, Lattice QCD, is doing a much better job and has already made contact with experiments at various fields with good results , though the computations are numerical rather than analytic.