nLab string


This entry is about the notion in particle physics/quantum gravity. For the notion in computer science see at string (computer science).


String theory


physics, mathematical physics, philosophy of physics

Surveys, textbooks and lecture notes

theory (physics), model (physics)

experiment, measurement, computable physics



A string or 1-brane is a brane of dimension one higher than an ordinary particle:

where a 1-dimensional sigma-model may be thought of a describing the dynamics of particles propagating of a target space XX, a 2-dimensional sigma-model is said to described the dynamics of a string on some target space.

Much of traditional quantum field theory on XX may be understood in terms of second quantization of 1-dimensional sigma-models with target space XX. What is called string theory is the corresponding study of what happens to this situation as the 1-dimensional σ\sigma-model is replaced by a 2-dimensional one (for more on this see at worldline formalism).

A key motivation/application for strings is their identification with flux tubes in confined Yang-Mills theory/QCD [Polyakov (1998), Polyakov (1999)], for more on this see at AdS-CFT correspondence and AdS-QCD correspondence.

There is

(also called the F1-brane)


Table of branes appearing in supergravity/string theory (for classification see at brane scan).

branein supergravitycharged under gauge fieldhas worldvolume theory
black branesupergravityhigher gauge fieldSCFT
D-branetype IIRR-fieldsuper Yang-Mills theory
(D=2n)(D = 2n)type IIA\,\,
D0-brane\,\,BFSS matrix model
D4-brane\,\,D=5 super Yang-Mills theory with Khovanov homology observables
D6-brane\,\,D=7 super Yang-Mills theory
(D=2n+1)(D = 2n+1)type IIB\,\,
D1-brane\,\,2d CFT with BH entropy
D3-brane\,\,N=4 D=4 super Yang-Mills theory
(D25-brane)(bosonic string theory)
NS-branetype I, II, heteroticcircle n-connection\,
string\,B2-field2d SCFT
NS5-brane\,B6-fieldlittle string theory
D-brane for topological string\,
M-brane11D SuGra/M-theorycircle n-connection\,
M2-brane\,C3-fieldABJM theory, BLG model
M5-brane\,C6-field6d (2,0)-superconformal QFT
M9-brane/O9-planeheterotic string theory
topological M2-branetopological M-theoryC3-field on G2-manifold
topological M5-brane\,C6-field on G2-manifold
membrane instanton
M5-brane instanton
D3-brane instanton
solitons on M5-brane6d (2,0)-superconformal QFT
self-dual stringself-dual B-field
3-brane in 6d



See references at string theory, bosonic string and superstring.

Polyakov gauge/string duality

Key ideas underlying what is now known as the holographic duality in string theory and specifically holographic QCD (see notably also at holographic light front QCD) were preconceived by Alexander Polyakov in efforts to understand confined QCD (the mass gap problem) by regarding color-flux tubes between quarks as dynamical strings:

Early suggestion that confined QCD is described by regarding the color-flux tubes as string-like dynamical degrees of freedoms:

[[old personal page]]: My main interests this year [[1993?]] were directed towards string theory of quark confinement. The problem is to find the string Lagrangian for the Faraday’s “lines of force”,which would reproduce perturbative corrections from the Yang-Mills theory to the Coulomb law at small distances and would give permanent confinement of quarks at large distances.

Early suggestion (due to the Liouville field see in the quantization of the bosonic string via the Polyakov action) that such flux tubes regarded as confinig strings are to be thought of a probing higher dimensional spacetime, exhibiting a holographic principle in which actual spacetime appears as a brane:

eventually culminating in the formulation of the dictionary for the AdS-CFT correspondence:

Relations between gauge fields and strings present an old, fascinating and unanswered question. The full answer to this question is of great importance for theoretical physics. It will provide us with a theory of quark confinement by explaining the dynamics of color-electric fluxes.

and the suggestion of finding the string-QCD correspondence:

in the strong coupling limit of a lattice gauge theory the elementary excitations are represented by closed strings formed by the color-electric fluxes. In the presence of quarks these strings open up and end on the quarks, thus guaranteeing quark confinement. Moreover, in the SU(N)SU(N) gauge theory the strings interaction is weak at large NN. This fact makes it reasonable to expect that also in the physically interesting continuous limit (not accessible by the strong coupling approximation) the best description of the theory should involve the flux lines (strings) and not fields, thus returning us from Maxwell to Faraday. In other words it is natural to expect an exact duality between gauge fields and strings. The challenge is to build a precise theory on the string side of this duality.

Historical review:

Already in 1974, in his famous large NN paper, ‘t Hooft already tried to find the string-gauge connections. His idea was that the lines of Feynman’s diagrams become dense in a certain sense and could be described as a 2d surface. This is, however, very different from the picture of strings as flux lines. Interestingly, even now people often don’t distinguish between these approaches. In fact, for the usual amplitudes Feynman’s diagrams don’t become dense and the flux lines picture is an appropriate one. However there are cases in which t’Hooft’s mechanism is really working.

Last revised on December 21, 2022 at 11:14:18. See the history of this page for a list of all contributions to it.