Non-critical string theory: Difference between revisions

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Revision as of 06:02, 24 February 2013

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of string theory in 4-dimensional Minkowski space-time.

There are several applications of the non-critical string. Through the [[AdS/CFT correspondence]] it provides a holographic description of gauge theories which are asymptotically free.{{Fact|date=February 2007}} It may then have applications to the study of the [[Quantum chromodynamics|QCD]], the theory of strong interactions between [[quarks]]. Another area of much research is two-dimensional string theory which provides simple models[[toy model]]s of [[string theory]]. There also exists a [[string duality|duality]] to the 3-dimensional [[Ising model]].{{Fact|date=February 2007}}

== The Criticalcritical Dimensiondimension and Centralcentral Chargecharge ==

{{main|Critical dimension}}

In order for a [[string theory]] to be consistent, the [[worldsheet]] theory must be conformally invariant. The obstruction to [[conformal symmetry]] is known as the [[Weyl anomaly]] and is proportional to the [[central charge]] of the worldsheet theory. In order to preserve conformal symmetry the Weyl anomaly, and thus the central charge, must vanish. For the [[bosonic string]] this can be accomplished by a worldsheet theory consisting of 26 free [[bosons]]. Since each boson is interpreted as a flat spacetime dimension, the critical dimension of the bosonic string is 26. A similar logic for the [[superstring]] results in 10 free bosons (and 10 free [[fermions]] as required by worldsheet [[supersymmetry]]). The bosons are again interpreted as spacetime dimensions and so the critical dimension for the superstring is 10. A string theory which is formulated in the critical dimension is called a '''critical string'''.

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The non-critical string is not formulated with the critical dimension, but nonetheless has vanishing Weyl anomaly. A worldsheet theory with the correct central charge can be constructed by introducing a non-trivial target space, commonly by giving an [[expectation value]] to the [[dilaton]] which varies linearly along some spacetime direction. For this reason non-critical string theory is sometimes called the linear dilaton theory. Since the dilaton is related to the string [[coupling constant]], this theory contains a region where the coupling is weak (and so perturbation theory is valid) and another region where the theory is strongly coupled. For dilaton varying along a [[spacelike]] direction, the dimension of the theory is less than the critical dimension and so the theory is termed '''subcritical'''. For dilaton varying along a [[timelike]] direction, the dimension is greater than the critical dimension and the theory is termed '''supercritical'''. The dilaton can also vary along a [[lightlike]] direction, in which case the dimension is equal to the critical dimension and the theory is a critical string theory.

== Two-dimensional Stringstring Theorytheory ==

Perhaps the most studied example of non-critical string theory is that with two-dimensional target space. While clearly not of phenomenological interest, string theories in two dimensions serve as important toy models. They allow one to probe interesting concepts which would be computationally intractable in a more realistic scenario.

These models often have fully non-perturbative descriptions in the form of the quantum mechanics of large matrices. Such a description known as the c=1 matrix model captures the dynamics of [[bosonic string theory]] in two dimensions. Of much recent interest are matrix models of the two-dimensional [[Type 0 string theory|Type 0 string theories]]. These "matrix models" are understood as describing the dynamics of [[open string]]s lying on [[D-branes]] in these theories. Degrees of freedom associated with [[closed string]]s, and [[spacetime]] itself, appear as emergent phenomena, providing an important example of open string [[tachyon condensation]] in string theory.