# Contents

## Idea

The theory of 11-dimensional supergravity contains a higher gauge field – the supergravity C-field – that naturally couples to higher electrically charged 2-branes, membranes (Bergshoeff-Sezgin-Townsend 87). By double dimensional reduction, these turn into the superstrings of type IIA string theory (Duff-Howe-Inami-Stelle 87). (See at duality between M-theory and type IIA string theory.)

When in (Witten95) it was argued that the 10-dimensional target space theories of the five types of superstring theories are all limiting cases of one single 11-dimensional target space theory that extends 11-dimensional supergravity (M-theory), it was natural to guess that this supergravity membrane accordingly yields a 3-dimensional sigma-model that reduces in limiting cases to the string sigma-models.

But there were two aspects that make this idea a little subtle, even at this vague level: first, there is no good theory of the quantization of the membrane sigma-model, as opposed to the well understood quantum string. Secondly, that hypothetical “theory extending 11-dimensional supergravity” (“M-theory”) has remained elusive enough that it is not clear in which sense the membrane would relate to it in a way analogous to how the string relates to its target space theories (which is fairly well understood).

Later, with the BFSS matrix model some people gained more confidence in the idea, by identifying the corresponding degrees of freedom in a special case (Nicolai-Helling 98, Dasgupta-Nicolai-Plefka 02). See also at membrane matrix model.

In a more modern perspective, the M2-brane worldvolume theory appears under AdS4-CFT3 duality as a holographic dual of a 4-dimensional Chern-Simons theory. Indeed, its Green-Schwarz action functional is entirely controled by the super-Lie algebra 4-cocycle of super Minkowski spacetime given by the brane scan. This exhibits the M2-brane worldvolume theory as a 3-dimensional higher dimensional WZW model.

## Definition

There are two different incarnations of the M2-brane. On the one hand it is defined as a Green-Schwarz sigma model with target space a spacetime that is a solution to the equations of motion of 11-dimensional supergravity. One would call this the “fundamental” M2 in analogy with the “fundamental string”, if only there were an “M2-perturbation series” which however is essentially ruled out.

On the other hand the M2 also appears as a black brane, hence as a solution to the equations of motion of 11-dimensional supergravity with singularity that looks from outside like a charged 2 dimensional object.

### As a Green-Schwarz sigma model

See at Green-Schwarz sigma model and brane scan.

### As a black brane

As a black brane solution to the equations of motion of 11-dimensional supergravity the M2 is the spacetime $\mathbb{R}^{2,1} \times (\mathbb{R}^8-\{0\})$ with pseudo-Riemannian metric being

$g = H^{-2/3} g_{\mathbb{R}^{2,1}} + H^{1/3}g_{\mathbb{R}^8-\{0\}}$

where

$H = \alpha + \frac{\beta}{r^6}$

for $(\alpha,\beta) \in \mathbb{R}^2 \setminus \{(0,0)\}$;

and the field strength of the supergravity C-field is

$F = d vol_{\mathbb{R}^{2,1}} \wedge \mathbf{d} H^{-1} \,.$

For $\alpha \beta \neq 0$ this is a 1/2 BPS state of 11d sugra.

In the above coordinates the metric is ill-defined at $r = - \beta^{1/6} \alpha$, but in fact it may be smoothly continued through this point (Duff-Gibbons-Townsend 94, section 3), which is a event horizon. An actual singularity is at $r = 0$.

The near horizon geometry of this spacetime is the Freund-Rubin compactification AdS4$\times$S7. For more on this see at AdS-CFT.

(table taken from Blumenhagen-Lüst-Theisen “Basic concepts of string theory”)

More generally, one may classify those solutions of 11-dimensional supergravity of the form $AdS_4 \times X_7$ for some closed manifold $X_7$, that are at least 1/2 BPS states. One finds (Medeiros-Figueroa 10) that all these are of the form $AdS_4 \times S^7/G_{ADE}$, where $S^7 / G_{ADE}$ is an orbifold of the 7-sphere (a spherical space form in the smooth case, see there) by a finite subgroup of SU(2) $G_{ADE} \hookrightarrow SU(2)$, i.e. a finite group in the ADE-classification

Dynkin diagram via
McKay correspondence
Platonic solidfinite subgroups of SO(3)finite subgroups of SU(2)simple Lie group
$A_n$cyclic group
$\mathbb{Z}_{n+1}$
cyclic group
$\mathbb{Z}_{n+1}$
special unitary group
$D_{n+4}$dihedron,
hosohedron
dihedral group
$D_{n+2}$
binary dihedral group
$2 D_{n+2}$
special orthogonal group
$E_6$tetrahedrontetrahedral group
$T$
binary tetrahedral group
$2T$
E6
$E_7$cube,
octahedron
octahedral group
$O$
binary octahedral group
$2O$
E7
$E_8$dodecahedron,
icosahedron
icosahedral group
$I$
binary icosahedral group
$2I$
E8

Here for $5 \leq \mathcal{N} \leq 8$-supersymmetry then the action of $G_{ADE}$ on $S^7$ is via the canonical action of $SU(2)$ as in the quaternionic Hopf fibration (Medeiros-Figueroa 10), while for $\mathcal{N} = 4$ then there is an extra twist to the action (MFFGME 09). See the table below.

## Properties

### Worldvolume theory – BLG and ABJM

The worldvolume QFT of black M2-branes is a 3d superconformal gauge field theory:

$d$$N$superconformal super Lie algebraR-symmetryblack brane worldvolume
superconformal field theory
$\phantom{A}3\phantom{A}$$\phantom{A}2k+1\phantom{A}$$\phantom{A}B(k,2) \simeq$ osp$(2k+1/4)\phantom{A}$$\phantom{A}SO(2k+1)\phantom{A}$
$\phantom{A}3\phantom{A}$$\phantom{A}2k\phantom{A}$$\phantom{A}D(k,2)\simeq$ osp$(2k/4)\phantom{A}$$\phantom{A}SO(2k)\phantom{A}$M2-brane
3d superconformal gauge field theory
$\phantom{A}4\phantom{A}$$\phantom{A}k+1\phantom{A}$$\phantom{A}A(3,k)\simeq \mathfrak{sl}(4/k+1)\phantom{A}$$\phantom{A}U(k+1)\phantom{A}$D3-brane
4d superconformal gauge field theory
$\phantom{A}5\phantom{A}$$\phantom{A}1\phantom{A}$$\phantom{A}F(4)\phantom{A}$$\phantom{A}SO(3)\phantom{A}$
$\phantom{A}6\phantom{A}$$\phantom{A}k\phantom{A}$$\phantom{A}D(4,k) \simeq$ osp$(8/2k)\phantom{A}$$\phantom{A}Sp(k)\phantom{A}$M5-brane
6d superconformal gauge field theory

(Shnider 88, also Nahm 78, see Minwalla 98, section 4.2)

Specifically, worldvolume quantum field theory of M2-branes sitting at ADE singularities (as above) is supposed to be described by ABJM theory and, for the special case of $SU(2)$ gauge group, by the BLG model. See also at gauge enhancement.

$N$ Killing spinors on
spherical space form $S^7/\widehat{G}$
$\phantom{AA}\widehat{G} =$spin-lift of subgroup of
isometry group of 7-sphere
3d superconformal gauge field theory
on back M2-branes
with near horizon geometry $AdS_4 \times S^7/\widehat{G}$
$\phantom{AA}N = 8\phantom{AA}$$\phantom{AA}\mathbb{Z}_2$cyclic group of order 2BLG model
$\phantom{AA}N = 7\phantom{AA}$
$\phantom{AA}N = 6\phantom{AA}$$\phantom{AA}\mathbb{Z}_{k\gt 2}$cyclic groupABJM model
$\phantom{AA}N = 5\phantom{AA}$$\phantom{AA}2 D_{k+2}$
$2 T$, $2 O$, $2 I$
binary dihedral group,
binary tetrahedral group,
binary octahedral group,
binary icosahedral group
$\phantom{AA}N = 4\phantom{AA}$$\phantom{A}2 D_{k+2}$
$2 O$, $2 I$
binary dihedral group,
binary octahedral group,
binary icosahedral group
(HLLLP 08b, Chen-Wu 10)

Under AdS-CFT duality the M2-brane is given by AdS4-CFT3 duality. (Maldacena 97, section 3.2, Klebanov-Torri 10).

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
$(D = 2n)$type IIA$\,$$\,$
D0-brane$\,$$\,$BFSS matrix model
D2-brane$\,$$\,$$\,$
D4-brane$\,$$\,$D=5 super Yang-Mills theory with Khovanov homology observables
D6-brane$\,$$\,$D=7 super Yang-Mills theory
D8-brane$\,$$\,$
$(D = 2n+1)$type IIB$\,$$\,$
D(-1)-brane$\,$$\,$$\,$
D1-brane$\,$$\,$2d CFT with BH entropy
D3-brane$\,$$\,$N=4 D=4 super Yang-Mills theory
D5-brane$\,$$\,$$\,$
D7-brane$\,$$\,$$\,$
D9-brane$\,$$\,$$\,$
(p,q)-string$\,$$\,$$\,$
(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$\,$
A-brane$\,$
B-brane$\,$
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
M-wave
topological M2-branetopological M-theoryC3-field on G2-manifold
topological M5-brane$\,$C6-field on G2-manifold
solitons on M5-brane6d (2,0)-superconformal QFT
self-dual stringself-dual B-field
3-brane in 6d

## References

### As a fundamental brane

The Green-Schwarz sigma-model-type formulation of the supermembrane (as in the brane scan) first appears in

and its quantization of the was first explored in

The double dimensional reduction of the M2-brane to the Green-Schwarz superstring was observed in

The interpretation of the membrane as as an object related to string theory, hence as the M2-brane was proposed in

around the time when M-theory became accepted due to

The interpretation related to the BFSS matrix model of D0-branes is discussed in some detail in

### As a black brane

The back membrane solution of 11-dimensional supergravity was found in

• Mike Duff, Kellogg Stelle, Multimembrane solutions of $D = 11$ supergravity, Phys. Lett. B 253, 113 (1991) (spire)

Its regularity throught the event horizon is due to

The Horava-Witten-orientifold of the black M2, supposedly yielding the black heterotic string is discussed in

• Zygmunt Lalak, André Lukas, Burt Ovrut, Soliton Solutions of M-theory on an Orbifold, Phys. Lett. B425 (1998) 59-70 (arXiv:hep-th/9709214)

• Ken Kashima, The M2-brane Solution of Heterotic M-theory with the Gauss-Bonnet $R^2$ terms, Prog.Theor.Phys. 105 (2001) 301-321 (arXiv:hep-th/0010286)

Meanwhile AdS-CFT duality was recognized in

where a dual description of the worldvolume theory of M2-brane appears in section 3.2. More on this is in

An account of the history as of 1999 is in

More recent review is in

• Georgios Linardopoulos, chapter 13 of Classical Strings and Membranes in the AdS/CFT Correspondence (pdf, spire)

A detailed discussion of this black brane-realization of the M2 and its relation to AdS-CFT is in

The generalization of this to $\geq 1/2$ BPS sugra solutions of the form $AdS_4 \times X_7$ is due to

Discussion of the history includes

Other recent developments are discussed in

Formulations of multiple M2-branes on top of each other are given by the BLG model and the ABJM model. See there for more pointers. The relation of these to the above is discussed in section 3 of

Discusson of boundary conditions in the ABJM model (for M2-branes ending on M5-branes) is in

A kind of double dimensional reduction of the ABJM model to something related to type II superstrings and D1-branes is discussed in

Discussion of the ABJM model in Horava-Witten theory and reducing to heterotic strings is in

Discussion of general phenomena of M-branes in higher geometry and generalized cohomology is in

Discussion from the point of view of Green-Schwarz action functional-∞-Wess-Zumino-Witten theory is in

### Dualities

The role of and the relation to duality in string theory of the membrane is discussed in the following articles.

Relation to T-duality is discussed in:

• J.G. Russo, T-duality in M-theory and supermembranes (arXiv:hep-th/9701188)

• M.P. Garcia del Moral, J.M. Pena, A. Restuccia, T-duality Invariance of the Supermembrane (arXiv:1211.2434)

Relation to U-duality is discussed in:

Discussion from the point of view of E11-U-duality and current algebra is in

Last revised on May 17, 2018 at 04:17:34. See the history of this page for a list of all contributions to it.