Editing String theory (section)

=== Strings ===
{{main|String (physics)}}

[[Image:World lines and world sheet.svg|left|thumb|upright=1.2|Interaction in the quantum world: [[worldline]]s of point-like [[particles]] or a [[worldsheet]] swept up by closed [[string (physics)|strings]] in string theory]]

The application of quantum mechanics to physical objects such as the [[electromagnetic field]], which are extended in space and time, is known as [[quantum field theory]]. In particle physics, quantum field theories form the basis for our understanding of elementary particles, which are modeled as excitations in the fundamental fields.<ref name="Zee 2010"/>

In quantum field theory, one typically computes the probabilities of various physical events using the techniques of [[Perturbation theory (quantum mechanics)|perturbation theory]]. Developed by [[Richard Feynman]] and others in the first half of the twentieth century, perturbative quantum field theory uses special diagrams called [[Feynman diagram]]s to organize computations. One imagines that these diagrams depict the paths of point-like particles and their interactions.<ref name="Zee 2010"/>

The starting point for string theory is the idea that the point-like particles of quantum field theory can also be modeled as one-dimensional objects called strings.<ref>[[#Becker|Becker, Becker and Schwarz]], p. 2</ref> The interaction of strings is most straightforwardly defined by generalizing the perturbation theory used in ordinary quantum field theory. At the level of Feynman diagrams, this means replacing the one-dimensional diagram representing the path of a point particle by a two-dimensional (2D) surface representing the motion of a string.<ref name="Becker, Becker 2007, p. 6">[[#Becker|Becker, Becker and Schwarz]], p. 6</ref> Unlike in quantum field theory, string theory does not have a full non-perturbative definition, so many of the theoretical questions that physicists would like to answer remain out of reach.<ref>[[#Zwiebach|Zwiebach]], p. 12</ref>

In theories of particle physics based on string theory, the characteristic length scale of strings is assumed to be on the order of the [[Planck length]], or {{math|10<sup>−35</sup>}} meters, the scale at which the effects of quantum gravity are believed to become significant.<ref name="Becker, Becker 2007, p. 6"/> On much larger length scales, such as the scales visible in physics laboratories, such objects would be indistinguishable from zero-dimensional point particles, and the vibrational state of the string would determine the type of particle. One of the vibrational states of a string corresponds to the graviton, a quantum mechanical particle that carries the gravitational force.<ref name="Becker, Becker 2007, pp. 2"/>

The original version of string theory was [[bosonic string theory]], but this version described only [[bosons]], a class of particles that transmit forces between the matter particles, or [[fermions]]. Bosonic string theory was eventually superseded by theories called [[superstring theory|superstring theories]]. These theories describe both bosons and fermions, and they incorporate a theoretical idea called [[supersymmetry]]. In theories with supersymmetry, each boson has a counterpart which is a fermion, and vice versa.<ref>[[#Becker|Becker, Becker and Schwarz]], p. 4</ref>

There are several versions of superstring theory: [[type I string|type I]], [[type IIA string|type IIA]], [[type IIB string|type IIB]], and two flavors of [[heterotic string]] theory ({{math|[[special orthogonal group|''SO''(32)]]}} and {{math|[[E8 (mathematics)|''E''<sub>8</sub>×''E''<sub>8</sub>]]}}). The different theories allow different types of strings, and the particles that arise at low energies exhibit different [[symmetry (physics)|symmetries]]. For example, the type I theory includes both open strings (which are segments with endpoints) and closed strings (which form closed loops), while types IIA, IIB and heterotic include only closed strings.<ref>[[#Zwiebach|Zwiebach]], p. 324</ref>