String theory is a developing branch of theoretical physics that combines quantum mechanics and general relativity into a quantum theory of gravity. The strings of string theory are one-dimensional oscillating lines, but they are no longer considered fundamental to the theory, which can be formulated in terms of points or surfaces too.

String theory itself comes in many different formulations, each one with a different mathematical structure, and each best describing different physical circumstances. But the principles shared by these approaches, their mutual logical consistency, and the fact that some of them easily include the standard model of particle physics, has led many physicists to believe that the theory is the correct fundamental description of nature. In particular, string theory is the first candidate for the theory of everything, a way to describe all the known natural forces (gravitational, electromagnetic, weak and strong interactions) and matter (quarks and leptons) in a mathematically complete system.

Many detractors criticise string theory because it has not yet provided quantitative experimental predictions. Like any other quantum theory of gravity, it is widely believed that testing the theory directly by experiment would require prohibitively expensive feats of engineering. Whether there are stringent indirect tests of the theory is not yet known.

String theory is of interest to many physicists because it requires new mathematical and physical ideas to mesh together its very different mathematical formulations. One of the most inclusive of these is the 11-dimensional M-theory, and in the M-theory way of thinking, string theory requires spacetime to have eleven dimensions,^[1] as opposed to the usual three space and one time. The original string theories from the 1980s describe special cases of M-theory where the eleventh dimension is a very small circle or a line, and if these formulations are considered as fundamental, then string theory requires ten dimensions. But the theory also describes universes like ours, with four observable spacetime dimensions, as well as universes with up to 10 flat space dimensions, and also cases where the position in some of the dimensions is not described by a real number, but by completely different type of mathematical quantity. So the notion of space-time dimension is not a fixed thing in string theory: it is best thought of as different in different circumstances.

String theories include objects more general than strings, called branes. The word brane, derived from “membrane”, refers to a variety of interrelated objects, such as D-branes, black p-branes and Neveu-Schwarz 5-branes. These are extended objects that are charged sources for differential form generalizations of the vector potential electromagnetic field. These objects are related to one-another by a variety of dualities. Black hole-like black p-branes are identified with D-branes, which are endpoints for strings, and this identification is called Gauge-gravity duality. Research on this equivalence has led to new insights on quantum chromodynamics, the fundamental theory of the strong nuclear forces