Editing String theory (section)

=== Overview ===
In the 20th century, two theoretical frameworks emerged for formulating the laws of physics. The first is [[Albert Einstein]]'s [[general theory of relativity]], a theory that explains the force of [[gravity]] and the structure of [[spacetime]] at the macro-level. The other is [[quantum mechanics]], a completely different formulation, which uses known [[probability]] principles to describe physical phenomena at the micro-level. By the late 1970s, these two frameworks had proven to be sufficient to explain most of the observed features of the [[universe]], from [[elementary particle]]s to [[atom]]s to the evolution of stars and the universe as a whole.<ref name="Becker, Becker 2007, p. 1">[[#Becker|Becker, Becker and Schwarz]], p. 1</ref>

In spite of these successes, there are still many problems that remain to be solved. One of the deepest problems in modern physics is the problem of [[quantum gravity]].<ref name="Becker, Becker 2007, p. 1"/> The general theory of relativity is formulated within the framework of [[classical physics]], whereas the other [[fundamental interaction|fundamental forces]] are described within the framework of quantum mechanics. A quantum theory of gravity is needed in order to reconcile general relativity with the principles of quantum mechanics, but difficulties arise when one attempts to apply the usual prescriptions of quantum theory to the force of gravity.<ref>[[#Zwiebach|Zwiebach]], p. 6</ref>

String theory is a [[mathematical theory|theoretical framework]] that attempts to address these questions.  

The starting point for string theory is the idea that the [[point particle|point-like particles]] of [[particle physics]] can also be modeled as one-dimensional objects called [[string (physics)|strings]]. String theory describes how strings propagate through space and interact with each other. In a given version of string theory, there is only one kind of string, which may look like a small loop or segment of ordinary string, and it can [[vibration|vibrate]] in different ways. On distance scales larger than the string scale, a string will look just like an ordinary particle consistent with non-string models of elementary particles, with its [[mass]], [[charge (physics)|charge]], and other properties determined by the vibrational state of the string. String theory's application as a form of quantum gravity proposes a vibrational state responsible for the [[graviton]], a yet unproven quantum particle that is theorized to carry gravitational force.<ref name="Becker, Becker 2007, pp. 2">[[#Becker|Becker, Becker and Schwarz]], pp. 2–3</ref>

One of the main developments of the past several decades in string theory was the discovery of certain 'dualities', mathematical transformations that identify one physical theory with another. Physicists studying string theory have discovered a number of these dualities between different versions of string theory, and this has led to the conjecture that all consistent versions of string theory are subsumed in a single framework known as [[M-theory]].<ref>[[#Becker|Becker, Becker and Schwarz]], pp. 9–12</ref>

Studies of string theory have also yielded a number of results on the nature of black holes and the gravitational interaction. There are certain paradoxes that arise when one attempts to understand the quantum aspects of black holes, and work on string theory has attempted to clarify these issues. In late 1997 this line of work culminated in the discovery of the [[anti-de Sitter/conformal field theory correspondence]] or AdS/CFT.<ref>[[#Becker|Becker, Becker and Schwarz]], pp. 14–15</ref> This is a theoretical result that relates string theory to other physical theories which are better understood theoretically. The AdS/CFT correspondence has implications for the study of black holes and quantum gravity, and it has been applied to other subjects, including [[nuclear physics|nuclear]]<ref name="Klebanov and Maldacena 2009"/> and [[condensed matter physics]].<ref name="Merali 2011"/><ref name=Sachdev/>

Since string theory incorporates all of the fundamental interactions, including gravity, many physicists hope that it will eventually be developed to the point where it fully describes our universe, making it a [[theory of everything]]. One of the goals of current research in string theory is to find a solution of the theory that reproduces the observed spectrum of elementary particles, with a small [[cosmological constant]], containing [[dark matter]] and a plausible mechanism for [[cosmic inflation]]. While there has been progress toward these goals, it is not known to what extent string theory describes the real world or how much freedom the theory allows in the choice of details.<ref>[[#Becker|Becker, Becker and Schwarz]], pp. 3, 15–16</ref>

One of the challenges of string theory is that the full theory does not have a satisfactory definition in all circumstances. The scattering of strings is most straightforwardly defined using the techniques of [[perturbation theory (quantum mechanics)|perturbation theory]], but it is not known in general how to define string theory [[Non-perturbative|nonperturbatively]].<ref>[[#Becker|Becker, Becker and Schwarz]], p. 8</ref> It is also not clear whether there is any principle by which string theory selects its [[vacuum state]], the physical state that determines the properties of our universe.<ref>[[#Becker|Becker, Becker and Schwarz]], pp. 13–14</ref> These problems have led some in the community to criticize these approaches to the unification of physics and question the value of continued research on these problems.<ref name="Woit 2006">[[#Woit|Woit]]</ref>