# Learning M-theory: Gauge theory of membranes, brane intersections, and the self-dual string

I’ve been learning a lot about M-theory. It’s such a broad topic that, when people ask me ‘what is M-theory?’, I continue to struggle to know where to start. Right now, much of my learning is textbook and I have more questions than answers. I naturally take the approach of first wanting as broad and general of a picture as possible. In some sense, it is like starting with the general and working toward the particular. Or, in another way, it’s like when being introduced to a new landscape and wanting, at the outset, a broad orientation to its general geographical features, except in this case we are speaking in conceptual and quantitative terms. I may not ever be smart enough to grasp M-theory in its entirety, but what is certain is that I am working my hardest.

In surveying its geographical features and charting my own map, if I may continue the analogy, obtaining a better sense of the fundamental objects of M-theory is a particular task; but my main research interest has increasingly narrowed to the study and application of gauge theory and higher gauge theory. This can be sliced down further in that I am very interested in the relationship between string and gauge theory, and furthermore in studying the higher dimensional generalisation of gauge theory. This interest naturally follows from the importance of gauge theory in contemporary physics, and then how we may understand it from the generalisation of point particle theory to string theory and then to other higher dimensional extended objects (i.e., branes). We’ve talked a bit in the past about how the dynamics on the D-brane worldvolume is described by a gauge theory. We’ve also touched on categorical descriptions, and how in p-brane language when we study the quantum theory the resemblance of the photon can be seen as a p-dimensional version of the electromagnetic field (by the way, we’re going to start talking about p-branes in my next string note). That is to say, we obtain a p-dimensional analogue of Maxwell’s equations. More advanced perspectives from the gauge theory view, or in this case higher gauge theory view in M-theory, illuminate the existence of new objects like self-dual strings.

There is so much here to write about and explore, I look forward to sharing more as I progress through my own studies and thinking. In this post, though, I want to share some notebook reflections on things I’ve been learning more generally in the context of M-theory: some stuff about membranes, 11-dimensional supergravity, and the self-dual string. This post is not very technical; it’s just me thinking out loud.

## 11-dimensional supergravity

The field content of 11-dimensional supergravity consists of the metric $g_{\mu \nu}$, with 44 degrees of freedom; a rank 3 anti-symmetric tensor field $C_{\mu \nu \rho}$, with 84 degrees of freedom; and these are paired off with a 32 component Majorana gravitino $\Psi_{\alpha \mu}$, with 128 degrees of freedom. Although much has progressed since originally conceived, the Lagrangian for the bosonic sector is similar to as it was originally written [3]

$S_{SUGRA} = \frac{1}{2k_{11}^2} \int_{M_{11}} \sqrt{g} \ (R - \frac{1}{48}F^{2}_{4}) - \frac{1}{6} F_{4} \wedge F_{4} \wedge C_3. \ \ (1)$

The field strength is $F_4 = dC_3$ and $k_{11}$ is the 11-dimensional coupling constant. The field strength is defined conventionally,

$\mid F_n \mid^2 = \frac{1}{n !} G^{M_1 N_1} G^{M_2 N_2} ... G^{M_n N_n}F_{M_{1}M_{2} ... M_{n}}F_{N_1 N_2 ... N_n}. \ \ (2)$

The 11-dimensional frame field in the metric combination is $G_{MN} = \eta_{AB}E^{A}_{M}E^{B}_{N}$, where we have the elfbeins $E^{B}_{N}$, $M,N$ are indices for curved base-space vectors, and $A,B$ are indices for tangent space vectors. The last term in (2) is the Cherns-Simons structure. This is a topological dependent term independent of the metric. We see this structure in a lot of different contexts.

Although, from what I presently understand, the total degrees of freedom of M-theory are not yet completely nailed down, we can of course begin to trace a picture in parameter space. As we’ve discussed before on this blog, it can be seen how 10-dimensional type IIA theory in the strong coupling regime behaves as an 11-dimensional theory whose low-energy limit is captured by 11-dimensional supergravity. Reversely, compactify 11-dimensional supergravity on a circle of fixed radius in the $x^{10} = z$ direction, from the 11-dimensional metric we then obtain the 10-dimensional metric, a vector field and the dilaton. The 3-form potential leads to both a 3-form and a 2-form in 10-dimensions. The mysterious 11-dimensional theory can also be seen to give further clue at its parental status given how supergravity compactified on unit interval ${\mathbb{I} = [0,1]}$, for example, leads to the low-energy limit of $E8 \times E8$ heterotic theory.

## Non-renomoralisability of 11-dimensional SUGRA

One thing that I’ve known about for sometime but I have not yet studied in significant detail concerns precisely how 11-dimensional supergravity is non-renormalisable [4,5,6]. Looking at the maths, what I understand is that above two-loops the graviton-graviton scattering is divergent. Moreover, as I still have some questions about this, what I find curious is that in the derivative expansion in 11-dimensional flat spacetime (using a 1PI/quantum effective Lagrangian approach) the generating functional for the graviton S-matrix is non-local. But due to supersymmetry, low order terms in the derivative expansion can be separated into local terms, such as $t_8 t_8 R^4$, and non-local (or global) terms that correspond to loop amplitudes. But what happens is that, at 2-loops, a logarithmic divergence that is cut off at the Planck scale mixes with a local term of the schematic form $D^{12}R^4$, where $R^4$ is the supersymmetrised vertex. In the literature, one will find a lot of discussion about this $R^4$ vertex. But like I said, I really need more time looking at this.

In short, the important mechanism in string theory that allows us to avoid UV divergences is absent, or appears absent, in maximal supergravity. What could the UV regulator be? As in any supergravity, from what I understand, it is not clear that a Lagrangian description is sufficient at the Planck scale.

The facts of 11-dimensional supergravity and how it relates to 10-dimensional string theory are textbook and well-known. Going beyond dualities relating different string theories, an obvious question concerns what M-theory actually constitutes. One thing that is known is that M-theory reduces to 11-dimensional SUGRA at low-energies, as we touched on, and it is known that fundamental degrees of freedom are 2-dimensional and 5-dimensional objects, known as M2-branes and M5-branes. Study of these non-perturbative states offer several intriguing hints. There are also solutions to classical supergravity known as F1 – the fundamental string – and its magnetic dual, the NS5-brane. As it relates to the story of the five string theories, the M-branes realize all D-branes, and this is why D-branes are considered consistent objects in quantum gravity.

The way that M-theory sees D-branes is via the net of dualities. All of the D-branes and the NS5 brane are solutions to type II theories, both A and B. So, when you reduce M-theory on a circle, in that you get back to Type IIA, the M2-branes and M5-branes reduce to the various D-branes such that under S-duality from the D5-brane you get the NS5.

The worldvolume theory of the M5-brane is always strongly coupled, which can be seen in moduli space (its parameters are simply a point). So there is no Lagrangian for this theory, and it suggests something deep is needed or is missing. It is expected that its worldvolume theory will be a 6-dimensional superconformal field theory, typically known as the 6d(2,0) theory. The worldvolume theory for M2-branes (on an orbifold) has been found to be a 3-dimensional superconformal Chern-Simons theory with classical $\mathcal{N} = 6$ supersymmetry.

If one considers a single M5-brane, a theory can be formulated in terms of an Abelian (2,0)-tensor multiplet, consisting of a self-dual 2-form gauge field, 5 scalars, and 8 fermions, but it is not known how to generalise the construction to describe multiple M5-branes. To give an example, using AdS/CFT [7] it is described how the worldvolume theory for a stack of $N$ M5-branes is dual to M-theory on $AdS7 \times S4$ with $N$ units of flux through the 4-sphere, which reduces to 11-dimensional SUGRA on this background in the limit large $N$ limit.

## Brane intersections and stacks

The existence of branes is one of the most fascinating things about quantum gravity. There is a lot to unpack when learning about D1-branes, D3-branes, D5-branes, M2-branes, and M5-branes, as well as how they may intersect and what sort of consistent solutions have already been found [8,9, 10, 11, 12].

For example, an M2-brane, or a stack of coincident M2-branes, can end on a D5-brane. This is similar to the more simplified story of how D-branes, coincident D-branes, can intersect in string theory. Typically, D1-D3 systems in Type IIB string theory are studied because this system relates to the M2-M5 system by dimensional reduction and T-duality.

## Self-dual string

For a membrane to end on a D5-brane, the membrane boundary must carry the charge of the self-dual field $B$ on the five-brane worldvolume. There are different solutions to the field equations of $B$. For instance, a BPS solution was found [10] by looking at the supersymmetry transformation.

The linearised supersymmetry equation is

$\delta_{\epsilon} \Omega^{j}_{\beta} = \epsilon^{\alpha i}(\frac{1}{2} (\gamma^{a})_{\alpha \beta}(\gamma_{b^{\prime}})^{j}_{i}\partial_a X^{b^{\prime}} - \frac{1}{6}(\gamma^{abc})_{\alpha \beta}\delta^{j}_i h_{abc}) = 0. \ \ (3)$

Here $b^{\prime}$ labels transverse scalars, a indices label worldvolume directions, $\alpha, \beta$ denote spinor indices of spin(1,5), and i,j are spinor indices of $USp(4)$. The solution balances the contribution of the 3-form field strength h with a contribution from the scalars. Additionally, the worldvolume of the string soliton can be taken to be in the 0,1 directions with all fields independent of $x^0$ and $x^1$. An illustration of the solution is given below, showing an M2-brane ending on an M5-brane with a cross section $S^3 \times \mathbb{R}$.

As I am still trying to understand the calculation, I am currently looking at the following string solution

$H_{01m} = \pm \frac{1}{4} \partial_m \phi,$

$H_{mnp} = \pm \frac{1}{4} \epsilon_{emnpq}\delta^{qr}\partial_r \phi,$

$\phi = \phi_0 + \frac{2Q}{\mid x - x_0 \mid^2}, \ \ (4)$

where $\phi$ may be replaced by a more general superposition of solutions. We denote $\pm Q$ as the magnetic and electric charge. There is a conformal factor in the full equations of motion which guarantees that they are satisfied even at $x = x_0$, which means the solution is solitonic. This string soliton is said to possess its own anomalies that require cancellation (I assume Weyl, Lorentz). What is neat is that this string can be dimensionally reduced to get various T-duality configurations, which is something that would be fun to look into at some point down the road.

References

[1] D. Fiorenza, H. Sati, and U. Schreiber, The rational higher structure of m-theory. Fortschritte der Physik, 67(8-9):1910017, May 2019. [arXiv:1903.02834 [hep-th]].

[2] E. Witten, String theory dynamics in various dimensions. Nuclear PhysicsB, 443(1):85 – 126, 1995.

[3] E. Cremmer, B. Julia, and J. Scherk, Supergravity Theory in 11-dimensions. Phys. Lett. B76, No. 4, (409-412) 19 June 1978.

[4] S. Chester, S. Pufu, and X Yin, The M-Theory S-Matrix from ABJM: Beyond 11D supergravity. (2019). [arXiv:1804.00949v3 [hep-th]].

[5] A. Tseytlin, R4 terms in 11 dimensions and conformal anomaly of (2,0) theory. (2005). [arXiv:hep-th/0005072v4 [hep-th]].

[6] G. Russo, and A. Tseytlin, One-loop four-graviton amplitude in eleven-dimensional supergravity. (1997). [arXiv:hep-th/9707134v3 [hep-th]].

[7] P. Heslop, and A. Lipstein, M-theory Beyond The Supergravity Approximation. (2017). [arXiv:1712.08570 [hep-th]].

[8] P.K. Townsend, D-branes from M-branes. (1995). [arXiv:hep-th/9512062 [hep-th]].

[9] A. Strominger, \textit{Open p-branes}. Phys. Lett. B 383 (1996) 44. [arXiv:hep-th/9512059 [hep-th]].

[10] P.S. Howe, N.D. Lambert, and P.C. West, The self-dual string soliton. Nucl. Phys. B 515 (1998) 203. [arXiv:hep-th/9709014 [hep-th]].

[11] M. Perry and J.H. Schwarz, Interacting chiral gauge fields in six dimensions and Born-Infeld theory. Nucl. Phys. B 489 (1997) 47. [arXiv:hep-th/9611065 [hep-th]].

[12] D.S. Berman, Aspects of M-5 brane world volume dynamics. Phys. Lett. B 572 (2003) 101. [arXiv:hep-th/0307040 [hep-th]].

[13] J. Huerta, H. Sati, and U. Schreiber, Real ADE-equivariant (co)homotopy and Super M-branes. (2018). [arXiv:1805.05987 [hep-th]].

[14] N. Copland, Aspects of M-Theory Brane Interactions and String Theory Symmetries. [https://arxiv.org/abs/0707.1317].

[15] S. Palmer, Higher gauge theory and M-theory. [https://arxiv.org/abs/1407.0298].

# Stringification as categorisation

In quantum field theory one is typically taught to use perturbation theory when the equations of motion for the fields are nonlinear and weakly interacting. For example, in $\phi^4$ theory one can use a formal series as described by Rosly and Selivanov [1]. Perturbative theory is about mastering series expansions. The basic idea, upon constructing some correlation function in the full nonlinear model, is to expand in powers of $\alpha$, namely the interaction strength. In the language of perturbative physics, Feynman diagrams give a representation of each term in the expansion such that we use them to illustrate linear operators. This ultimately enables us to obtain a good approximation to the exact solution. Needless to say, there is a real power and usefulness about perturbative methods and the sum of Feynman diagrams.

When computing amplitudes with Feynman diagrams, the amplitudes depend on various topological properties (i.e., vertices, loops, and so on). Although not always made explicit in the perturbative view, from the Fenynman diagrams of 0-dimensional points with 1-dimensional graphs (to use the language of p-branes, which we’ll get to in a moment), we have topologies that describe linear operators: i.e., what Feynman diagrams start to make explicit is the deeper role of topology in physics [2]. This was summarised wonderfully in a lovely article by Atiyah, Dijkgraaf, and Hitchin [3]. Mathematically, and from the perspective of geometry, the main idea is that a linear operator behaves very much like an n-dimensional manifold going between manifolds of one dimension less, which we may define as a cobordism (i.e., think of a stringy ‘trousers’ diagram) [2,4].

Now, consider the story of p-branes, in particular the perspective as we pass from standard quantum field theory to string theory. The language of p-branes as first described by Duff et al [5] may be reviewed in any introductory string theory textbook. We can, from first-principles, motivate string theory thusly: in a special, if not unique way, we may generalise the point-like 0-dimensional particle to the 1-dimensional string, which is made explicit when we generalise the action for a relativistic particle to the Nambu-Goto action for the relativistic string. In the language of p-branes, which are p-dimensional objects moving through a $D(D \geq p)$ dimensional space-time, a 0-brane is a (0-dimensional) point particle that that traces out a (0+1)-dimensional worldline. The generalisation of the point particle action $S_0 = -m \int ds$ to a p-brane action in a $D(\geq p)$-dimensional space-time background is given by $S_p = -T_p \int d\mu_p$. Here $T_p$ is the p-brane tension with units mass/vol, and $d\mu_p$ is the (p + 1)-dimensional volume element. For the special case where $p=1$such that we have 1-brane, we obtain the string action which sweeps out a (1+1)-dimensional surface that is the string worldsheet propagating through space-time. We can also go on to speak of higher-dimensional objects, such as those that govern M-theory. For instance, a 2-brane is a membrane. Historically, these were considered as 2-dimensional particles. There are also 3-branes, 4-branes, and so on.

This generalising process, if we can describe it that way, is what I like to think of as stringification. For the case where $p=1$, Feynman diagrams of ordinary quantum field theory with 2-dimensional cobordisms represent world-sheets traced out by strings. The generalising picture, or stringification, show these 2-dimensional cobordisms equipped with extra structure give a powerful mathematical language (describing the relation between physics and topology, as string diagrams enable us to sum over the various topologies and provide a valuable mathematical tool for thinking about composition). But of course this picture can still be extended. Not only does the important analogy between operators and cobordisms come directly into focus, it is also, in some sense, where stringification meets categorification. That is, from the maths side, we arrive at the logic of higher-dimensional algebra and the arrows of monoidal and higher categories. In each, physical processes are describe by morphisms or functors (functors are like morphisms between categories). This generalising picture toward higher geometry, higher algebra, and, indeed, higher structures is called ‘categorifying’ or ‘homotopifying’ (my notes on which I have started to upload to this blog). In this post, I want to think a bit about this idea of stringification as categorification.

***

There is a view of M-theory, and I suppose of fundamental physics as whole, that I find fascinating and compelling: stringification as the categorisation of physics. The notion of stringification is not formal, but captures if nothing else an intuition about a certain generalising process or abstract story, or at least that is how I presently see it. It is a term I have picked up that used to float around in different contexts a couple of decades ago. As described through the language of p-branes, the story begins with the generalisation or stringification of point particle theory (and all that it implies) toward the existence of the string and eventually other extended objects in fundamental physics. Meanwhile, the notion of categorification is certainly formal, signalling, at its origin, the process of finding category-theoretic analogs of set-theoretic concepts by replacing sets with categories. This process, when iterated, gives definition to the notion of n-category theory, where we also replace functions with functors, and equations between functions by natural isomorphisms between functors [6]. As Schreiber pointed out in 2004, there is a sort of harmony between these two processes – stringification and categorification – which has certainly started to clarify over the last decade or more.

As one example, the observation that Schreiber describes in the linked post refers to boundaries of membranes attached to stacks of 5-branes, which conceptually appear as a higher-dimensional generalisation of how boundaries of strings appear.

To understand this think, firstly, of the simple example of the existence of D-branes (Dirichlet membranes) and how the endpoints of open strings can end on these extended objects. In fact, an introductory string textbook will guide one to see why the equations of motion of string theory require that the endpoints of an open string satisfy one of two types of boundary conditions (Dirichlet or Neumann) ending on a brane. If the endpoint is confined to the condition that it may move within some p-dimensional hyperplane, one then obtains a first description of Dp-branes. (I think this was one of the first things I calculated when learning strings!). For the sake of saving space I won’t go into the arrangement of D-branes or other related topics. The main point that I am driving at, the technicalities of which we could review in another post, is how these branes are dynamic and as such they may influence the dynamics of a string (i.e., how an open string might move and vibrate). Thus, the arrangement of branes (e.g., we can have parallel branes or ‘stacks’) will also impact or control the types of particles in our theory. It is truly a beautiful picture.

In p-brane language, if you take the Nambu–Goto action and for the quantum theory study the spectrum of particles, you will see that it exhibits what we may describe as the photon, which of course is the fundamental quantum of the electromagnetic field. Now, what is nice about this is that, the resemblance of the photon is actually a p-dimensional version of the electromagnetic field, so it is in fact a p-dimensional analogue of Maxwell’s equations.

What Schreiber is highlighting in his post is not just that in string theory, the points of the string ending on a Dp-brane give rise to ordinary gauge theory. (One could even take the view that string theory predicts electromagnitism such that string theory predicts the existence of D-branes. It is by their nature that these extended objects all carry an electromagnetic field on their volume, i.e., what we call the brane volume). The point made is that, given there is reason to extend the picture further – the picture of stringification so to speak – to higher-dimensional generalisations, we can then replace strings with membranes, and so on. From the maths side, it was realised that from the perspective of categories, something analogous is happening: replacing points with arrows (i.e., morphisms) one finds the gauge string may be described by the structure of nonabelian gerbes (a gerbe is just a generalised analogue of a fibre bundle), and so on.

When I first learned strings, the picture of stringification was in my mind but I didn’t yet have a word for it. I also didn’t possess category theoretic language at the time; it was really only a vague sense of a picture, perhaps emphasised in the way I learned string theory. So when I discovered and read last year about the idea of stringification as categorisation [7] in Schreiber’s thesis, I was excited.

A nice illustration comes from the first pages of this work. Take some ordinary point-particle, which traces out a worldline over time $t$. The thrust of the idea is that, given some charge, there is a connection in some bundle (yet unspecified) such that, locally, a group element $g \in G$ is associated to the path. Diagrammatically this may be represented as,

Now consider some time $t^{\prime}$, where $t^{\prime} > t$. The particle has travelled a bit further,

We can of course compose these paths. The composition is associative and the operation is multiplication. In fact, what we’re doing is multiplying the group elements. We can also define an inverse $g^{-1}$. The punchline is that, from the theory of fibre bundles with connection, we can consider how this local picture may fit globally. If $g$ is an element in a non-abelian group, the particle we are generalising is non-abelian. Generalise from a point-particle to a string, and the diagrammatic representation of the world-sheet takes the form

Ultimately, we can continue to play this game and develop the theory of non-abelian strings (and on to higher-dimensional branes), which, it turns out, corresponds with a 2-category theory [7,8]. Sparing details, in n-category theory a 2-category is a special type of category wherein, besides morphisms between objects, it possesses morphisms between morphisms. What is interesting about this example is how we can go on to show the idea of SUSY quantum mechanics on loop space relates to ideas in higher gauge theory, particularly in the sense of categorifying standard gauge theory. For example, John Baez’s paper on higher Yang-Mills [9]. But even before all of that, from the view of perturbative string theory being the categorification of supersymmetric quantum mechanics, we can play the same game such that the generalisation of the membranes of M-theory are a categorification of the supersymmetric string, and so on. The intriguing and, perhaps, grand idea, is that this process of stringification as categorification can be utilised to describe the whole of physics, or, so, it is suspected.

***

I’ve been thinking about this picture quite a bit recently, perhaps spurred by all of my ongoing studies in M-theory. The view to be encircled, as the notion of categorisation enters the stringy picture, also marks for me the beginning of the story about higher structures in fundamental physics (in terms of the view of category theory and higher category theory). In a sense, as much as I currently understand it (as I am very much in the process of studying and forming my thoughts on the matter) we are encircling not much more than an abstract story; but it is one in which many tantalising hints exist about a potentially foundational view.

The history of this higher structure view is rich with examples [10, 11], and, for many reasons, it leads us directly to a study of the plausible existence of M-theory. From the use of braided monoidal categories in the context of string diagrams through to knot theory (See Witten’s many famous lectures); the notion of quantum groups; Segal’s famous work on the axioms of conformal field theory (described in terms of monoidal functors and the category $2Cob_{\mathbb{C}}$ whose morphisms are string world-sheets such that we can compose the morphisms, and so on); and of course the work of Atiyah in topological quantum field theory (TQFT) followed by Dijkgraaf’s thesis on 2d TQFTs in terms of Frobenius algebras – the list is far to big to summarise in a single paragraph. All of this indicates, in some general sense, a very abstract story from basic quantum mechanics through to string theory and, I would say, as a natural consequence M-theory.

It is a fascinating perspective. There is so much to be said about this developing view, including why higher geometry and algebra seem to hold the important clues of M-theory as a fundamental theory of physics. What is also interesting, as I am beginning to understand, is that in the higher structure picture, a striking consequence from a geometric persective is that the geometry of fundamental physics (higher geometry and supergeometry) may not be described by spaces with sets of points. And, in fact, we start to see this for each value of $p$. Instead of a traditional notion space associated with the definition of topological spaces or differentiable manifolds, the geometric observation is that what we’re dealing with is functorial geometry of the sort described by Grothendieck, or synthetic differential geometry of the sort described by Lawvere, or a variation of them both.

Anyway, this is just a short note of me thinking aloud.

References

[1] Rosly, A.A., and Selivanov, K.G., On amplitudes in self-dual sector of Yang-Mills theory. [arXiv:9611101 [hep-th]].

[2] Baez, J., and Stay, M., Physics, Topology, Logic and Computation: A Rosetta Stone. [arXiv:0903.0340 [quant-ph]].

[3] Atiyah, M., Dijkgraaf, R., and Hitchin, N., Geometry and physics. Phil. Trans. R. Soc., (2010), A.368, 913–926. [http://doi.org/10.1098/rsta.2009.0227].

[4] Baez, J., and Lauda, A., A Prehistory of n-Categorical Physics. [https://math.ucr.edu/home/baez/history.pdf.].

[5] M. J. Duff, T. Inami, C. N. Pope, E. Sezgin [de], and K. S. Stelle, Semiclassical quantization of the supermembrane. Nucl. Phys. B297 (1988), 515.

[6] Baez, J., and Dolan, J., Categorification. (1998). [arXiv: 9802029 [math.QA]].

[7] Schreiber, U., From Loop Space Mechanics to Nonabelian Strings [thesis]. (2005). [hep-th/0509163].

[8] Baez, J. et al., Categorified Symplectic Geometry and the Classical String. (2008). [math-ph/0808.0246v1].

[9] Baez, J., \textit{Higher Yang–Mills theory}. (2002). [hep-th/0206130].

[10] Baez, J., and Lauda, A., A Prehistory of n-Categorical Physics. [https://math.ucr.edu/home/baez/history.pdf.]

[11] Jurco, B. et al., \textit{Higher structures in M-theory}. (2019). [arXiv:1903.02807v2].

# Mathematical physics and M-theory: The study of higher structures

In recent posts we’ve begun to discuss some ideas at the foundation of the duality symmetric approach to M-theory. As we started to review in the last entry, one of the first goals is to formulate and study a general field theory in which T-duality is a manifest symmetry. It was discussed how this was the first-principle goal of double field theory, and it was similarly featured as a motivation in our introductory review of double sigma models. There is a lot to be discussed about the duality symmetric approach moving forward, including the effective theory for this doubled string prior to ultimately looking at lifting to M-theory, where, instead of double field theory we will be working with what is known as exceptional field theory. What also remains an important question has to do with obtaining a global formulation of such duality symmetric actions. What is clear is that higher geometry and algebra are important to achieving such a formulation, and there is much ground to cover on this topic.

Meanwhile, in the present entry I would like to share what I have been studying and learning about as it relates to the other side of my PhD research: the higher structure approach to M-theory. If the duality symmetric approach is a sort of bottom-up way to attack the M-theory proposal, particularly insofar that we are building from the field theory point of view, the higher structure approach can be looked at here as a sort of top-down way to access the question of string theory’s non-perturbative completion. Although this language is a bit schematic, as there is a lot of overlap between the two approaches and their machinery, it does lend some intuition to the different perspectives being undertaken.

***

In William Thurston’s 1994 essay, ‘On proof and progress in mathematics‘ [1], it was argued that progress in mathematics is driven not only by proof of new theorems. Progress is also made by aiding in human beings ways to think about and understand mathematics. Emily Riehl made this a point of emphasis at the beginning of her notes on categorical homotopy theory [2], including on the usefulness of qualitative insights, and I think a similar emphasis may be made here in the context of our focus in mathematical physics and particularly M-theory. A further point of philosophical emphasis in this essay is Eugene Wigner’s article on the unreasonable effectiveness of mathematics in physics and, finally, the more recent presentation by Robbert Dijkgraaf on the unreasonable effectiveness of string theory in mathematics. In my view, M-theory represents one of a few research topics at the frontier of mathematical physics. What parametrises the boundaries of this frontier is the interface between foundational maths and fundamental physics. Indeed, I take this as Dijkgraaf’s point in his presentation at String Math 2020, namely both the need for this engagement and how, historically, progress is often made when the two sides (mathematics and physics) interact. For myself, I almost joined the maths school prior to deciding my future was in mathematical physics, and I find great interest in working at this interface, where, furthermore, when thinking of M-theory Thurston’s notion of progress appears particularly apt.

The motivation may be stated thusly [3]: there presently exists many interconnected hints in support of the proposed existence of M-theory. But a systematic formulation of the full theory – i.e., string theory’s non-perturbative completion – remains an important open problem. A key issue here ultimately concerns the lack of clarity about the underlying principles of M-theory (there are many references on this point, but as one example see [4]). I look at the current situation as a puzzle or as a patchwork quilt. There are pieces of the total picture that we can identify and start to fill in. There are others that remain unknown, leaving empty spaces in our picture of M-theory. And then, finally, how all of the pieces relate or connect is another question that we need to answer but cannot currently access.

To advance the problem, there is ample reason to suggest and to argue that what is needed is new mathematical machinery. As a new researcher, this need was something that I started thinking about a year or more ago. Let me put it this way: our world is best described by quantum field theory. If M-theory is the correct description of fundamental physics, we should end up with a quantum field theoretic description. But it seems unlikely that M-theory will be captured or defined by some Lagrangian, or some S-matrix, or other traditional approaches [3]. Indeed, the tools we need are more than just fibre bundles, standard topology, or differential geometry. Although much of modern physics is built using tools and approaches that deal with local, approximate, perturbative descriptions of reality, in investigating the M-theory problem we need to find ways of dealing with the global and non-perturbative structure of physical fields, and thus we are dealing with the difficulty of employing non-perturbative methods. Entering into my PhD, this is the challenge that I see. I also see this challenge, from the perspective of fundamental physics, as being similar to the situations that have historically arisen many times. A large part of the history of fundamental physics is described by the search for new mathematical language required to aid the modelling of physical phenomena. Hisham Sati and Urs Schreiber [5] presented the argument well, describing the situation explicitly, when discussing the motivation for pursuing a rigorous mathematical foundation for quantum field theory and perturbative string theory. As an example, they cited the identification of semi-Riemannian differential geometry as the underlying structure of gravity. Or, think of the use of representation theory in particle physics. In truth, there are many examples and, to Dijkgraaf’s point, we should embrace this history.

I think this is why, as I prepare to start my formal PhD years, the 2018 Durham Symposium seems momentous, particularly as I begin to generate my thoughts on M-theory and what sort of research I might find meaningful. Although it was slightly before my time, as I was only a first-year undergraduate when the Durham symposium had taken place, I was already developing an interest in non-perturbative theory and I remember learning of the symposium with enthusiasm. It gave me confidence and, I suppose, assurance that my thoughts are moving in the right direction. I’ve also taken confidence from many other important conferences, such as the 2015 conference organised around the theme of new spaces in mathematics and physics. But, for me, the Durham symposium has become a tremendous reference, because the culmination of this search for new mathematical language is apparent, organised under the study of higher structures, and I find this programme of research immensely stimulating.

Similar to the situation in QFT where, over the past decade or more, progress has been made to understand its fundamental nature – for instance, efforts to define QFT on arbitrary corbordism – higher structures provides a concise language of gauge physics and duality that has seemed, in recent years, to open pathways to rigorously attack the M-theory question. Indeed, efforts toward an axiomatic formulation of QFT (for instance, see recent developments in the area of algebraic and topological QFT [6]) and those toward string theory’s full non-perturbative completion to M-theory have a lot in common. Furthermore, an important motivation for the study of higher structures (and higher differential geometry, higher gauge theory and symmetry algebras, and so on) comes directly from decisive hints about the inner workings of M-theory. Hence, the title of the Durham Symposium and its guiding document, ‘Higher structures in M-theory‘.

To give some immediate examples and sketch a few more introductory thoughts, the higher algebraic structures we know to govern closed string field theory is something I started to investigate as related to my recent MRes thesis. But the most basic example of a higher structure in string theory arguably goes back to the first quantisation of the bosonic string. Indeed, as I described in a past note (I think from my first-year undergrad), if I were to teach strings one day my opening lecture would be on generalising point particle theory and emphasising the motivation on why we want to do this. From this approach, I think one can show in a wonderfully pedagogical way that, when generalising from 0-dimensional point particle theory to the 1-dimensional string (and so on), higher dimensionality is a natural consequence and is essentially forced upon us. (As an aside, I remember reading a comment by Schreiber about this very same point of introduction. I recommend reading Schreiber’s many notes over the years. For instance, here is a forethoughtful contribution from 2004 that begins to motivate some of the concepts we will discuss below. A helpful online resource is also ncatlab that covers many of the topics we will be discussing on this blog, along with appropriate references). And, it turns out, this is one way we might also motivate in fundamental physics the study of higher structures; because, in this picture, the Kalb-Ramond 2-form can be seen as an example of a higher structure as it is generalised from the gauge potential 1-form [3]. Of course, since the mid-1990s, a growing body of evidence urged the string theory community to study extended objects of dimension $> 1$, and around the same time attempts were already developing to use category theory (more on categories in a moment) to study string diagrams [7], as one can certainly see that string diagrams possess a powerful logic when it comes to composition.

***

So what do we mean by higher structures? From my current vantage, I would describe a higher structure as a categorified mathematical structure, which I also take to mean higher homotopy theory. But we can perhaps begin to build toward the idea by reviewing briefly two main ingredients: category theory and homotopy theory. As a matter of correspondence between mathematics and physics, category theory is the mathematical language of duality and homotopy theory is the mathematical language of the gauge principle.

We may think of category theory as being positioned at the foundations of modern mathematics [8], but, in many ways, it is quite elementary. Similar to the use of a venn diagram when teaching basic set theory, we can build the idea of a category in a fairly intuitive way.

A category ${\mathcal{C}}$ consists of the following data [9]:

* A collection of mathematical objects. If ${X}$ is an object of ${\mathcal{C}}$, then we write ${X \in \mathcal{C}}$.

* Every pair of objects ${X, Y \in \mathcal{C}}$, we may define a set of morphisms ${X \rightarrow Y}$ denoted as ${\text{Hom}\mathcal{C}(X,Y)}$.

* For every ${X \in \mathcal{C}}$, there needs to exist an identity morphism ${Id_{X} \in \mathcal{C}(X,X)}$.

* For every triple ${X,Y,Z \in \mathcal{C}}$, we may define a composition map ${\circ : \mathcal{C}(X,Y) \times \mathcal{C}(Y, C) \rightarrow C(X, Z)}$.

* Composition is associative and unital.

If category theory is the mathematics of mathematics, I would currently emphasise in a physics context [10] the approach to category theory as the language that describes composition. Think of the trivial example of moving in some space (let’s not get too stuck on definitions at this point). We can compose the journey from points A to B to C to D in the following way,

$\displaystyle A \rightarrow B \rightarrow C \rightarrow D \ (1).$

We can also compose the same journey in terms of pairs of vertices or what we are presently calling points such that

$\displaystyle A \rightarrow C, B \rightarrow D \ (2)$,

and then we may write the entire journey as ${A \rightarrow D}$ giving the same description in (1).

The idea of a category can be constructed using similar logic. Given a collection of objects ${A,B,C,D}$, paths ${A \rightarrow B \rightarrow C \rightarrow D}$ denoted by the arrows may be defined as the relation amongst the objects in terms of structure preserving maps ${f,g,h}$ called morphisms.

So at its most basic, a category is a collection of objects and arrows between those objects. It is, in some sense, a relational set, which must follow the conditions stated above.

Example. The category of sets, denoted by Set. The category of R-modules, denoted by RMod. A morphism ${f : X \rightarrow Y}$ is said to be an isomorphism if there exists ${g : Y \rightarrow X}$ such that ${g \circ f = Id_{X}}$ and ${f \circ g = Id_{Y}}$. In the category Set, isomorophisms are bijections.

The concept of functors is of deep importance in this language. In short, a functor is a morphism between categories. If ${\mathcal{C}}$ and ${\mathcal{D}}$ are categories, we may define a functor ${F : \mathcal{C} \rightarrow \mathcal{D}}$ such that it assigns an object ${FX \in \mathcal{D}}$ for any ${X \in \mathcal{C}}$, and a morphism ${Ff : FX \rightarrow FY}$ for any ${f : X \rightarrow Y}$, where associativity and unitality are preserved. So, for instance, if ${f : X \rightarrow Y}$, ${g : Y \rightarrow X}$, associativity is preserved such that

$\displaystyle Fg \circ Ff = F(g \circ f) \ (3).$

We may also define the notion of a natural transformation as a morphism between functors. If ${F,G : \mathcal{C} \rightarrow \mathcal{D}}$ define two functors, then a natural transformation ${F \implies D}$ assigns any ${X \in \mathcal{C}}$ a morphism ${FX \rightarrow GX}$.

There is a lot to be said about functors, categorical products, and also the important role duality plays in category theory. In the next entries, we will formally define these ideas as well as many others. For now, I am simply trying to provide some sense of an early introduction into some of the machinery used when we speak of higher structures, such as by giving an intuitive example of a category, with a mind toward formal definition in a following post. The same can be said for all ideas presented here, as, in the present entry, we are simply encircling concepts and sketching a bit of land, similar as a geoscientist would do when first preparing to sketch a topological map.

What one will find, on further inspection, is that category theory is deeply interesting for a number of reasons. At its deepest, there is something to be said about it as a foundational framework. One of the most inspiring realisations about category theory comes from something that seems incredibly basic: the idea in set theory of taking the product of two sets. Indeed, one may have seen this notion of a product as fundamental. But what we observe is that this most basic concept of taking a product of two sets is not fundamental in the way we may have been used to thinking, because one of the amazing things about the story of category theory is how the idea of products is more deeply defined in terms of a categorical product. The reward for this realisation, aside from shear inspiration, is technically immense.

Indeed, a category can contain essentially any mathematical object, like sets, topological spaces, modules, and so on. In many constructions, one will seek to study very generally the products of these objects – so, for example, the product of topological spaces – and the concept of a product in category theoretic language can capture all such instances and constructions. In later discussions we will see how this language allows us to look at mathematics at a large scale, which is to say that, in the abstract, we can take any collection of mathematical objects and study the relations between them. So if the goal is a completely general view, using category theory we are able to strip back a lot of inessential detail so as to drill fundamentally into things.

Additionally, there is a deep relationship between category theory and homotopy theory, which, in this post, I would like to highlight on the way to offering a gentle introduction to the concept of a higher structure. Down the road we will discuss quite a bit about higher-dimensional algebra, such as n-categories and operads, which are algebraic structures with geometric content, as we drive toward a survey of the connection between higher categorical structures and homotopy theory. In physics, there is also connection here with things like topological quantum field theory. Needless to say, there is much to cover, but when thinking of homotopy theory at its most basic, it is appropriate to go all the way back to algebraic topology.

The philosophical motivation is this: there are many cases in which we are interested in solving a geometrical problem of global nature, and, in algebraic topology, the method is generally to rework the problem into a homotopy theoretic one, and thus to reduce the original geometric problem to an algebraic problem. Let me emphasise the key point: it is a fundamental achievement of algebraic topology to enable us to reduce global topological problems into homotopy theory problems. One may motivate the study of homotopy theory thusly: if we want to think about general topological spaces – for example, arbitrary spaces that are not Hausdorff or even locally contractible – what this amounts to is that we relax our interest in the notion of equivalence under homeomorphism (i.e., topological equivalence) and instead work up to homotopy equivalence.

Definition 1 Given maps ${f_0,f_1: X \rightarrow Y}$, we may write ${f_0 \simeq f_1}$, which means ${f_0}$ is homotopic to ${f_1}$, if there exists a continuous map ${F : X \times I \rightarrow Y}$, called a homotopy, such that ${F(x,0) = f_0(x)}$ and ${ F(x,1) = f_1(x)}$. We may also write ${F: f_0 \implies f_1}$ to denote the homotopy.

As suggested a moment ago, a homotopy relation ${\simeq}$ is an equivalence relation. This is true if ${F_{01} : f_0 \implies f_1}$ and ${F_{12} : f_1 \implies f_2}$ for the family of maps ${f_i : X \rightarrow Y}$, then

$F_{02} (t,x) = \begin{cases} F_{01}(2t,x) : 0 \leq t \leq 1/2 \\ F_{01}(2t-1,x) : 1/2 \leq t \leq 1 \\ \end{cases} \ (4)$

gives a homotopy ${F_{02} : f_0 \implies f_2}$.

As an aside, what is both lovely and interesting is how, from a physics perspective, we may think of homotopy theory and ask how it might relate to the path integral; because, on first look, it would seem intuitive to ask this question. There is a long and detailed way to show it to be true, but, for simplicity, the argument goes something as follows. Think, for starters, of what we’re saying in the definition of homotopy. Given some ${X}$, which for now we’ll define as a set but later understand as a homotopy type, let us define two elements ${x,y \in X}$ such that we may issue the following simple proposition ${x = y}$. The essential point, here, is that there may be more than one way that ${x}$ is equal to ${y}$, or, in other words, there may be more than one reason or more than one path. Hence, we can construct a homotopy ${\gamma}$ such that $x \xrightarrow[]{\gamma} y$ is a homotopy from ${x}$ to ${y}$ and then an identity map ${Id_{X}(x,y)}$ for the set of homotopies from ${x}$ to ${y}$ in ${X}$. One can then proceed to follow the same reasoning and construct a higher homotopy by defining a homotopy of homotopy and so on.

The analogy I am drawing is that, in the path integral formalism, given some simply-connected topological space, recall that we can continuously deform the path ${x(t)}$ to ${x(y)}$. In this deformfation, ${\phi[x(t)]}$ approaches ${\phi[y(t)]}$ continuously such that, taking the limit, we have

$\displaystyle \phi[y(t)]=\lim\phi[x(t)]=e^{iS[y(t)]}, \ \text{as }x(t)\rightarrow y(t) \text{continuously}. \ (5)$

The principle of the superposition of quantum states, or, the sum of many paths, in a simply-connected space can be constructed as a single path integral; because, when all of the dust settles, the paths in this space can be shown to contribute to the total amplitude with the same phase (this is something we can lay out rigorously in another post). The result is that we end up with the Feynman path integral.

In homotopy theory, on the other hand, the analogous is true in that paths in the same homotopy class contribute to the total amplitude with the same phase. So, if one defines the appropriate propagator and constrains appropriately to the homotopy class, an equivalent expression for the path integral may be found. And really, one can probably already start to suspect this in the basic example of homotopy theory of topological spaces. Typically, given a topological space ${X}$ and two continuous functions from this space to another topological space ${Y}$ such that

$\displaystyle f,g : X \rightarrow Y \ (6)$,

it is straightforward to define, with two points in the mapping space, ${f,g \in \text{Maps}(X,Y)}$ a homotopy ${\eta}$

$\displaystyle f \xrightarrow[]{\eta} g \ (7).$

This is just a collection of continuous paths between the points.

But I digress. The focus here is to build up to the idea of higher structures.

The reason that a brief introduction to homotopy theory aids this purpose is because, if we think of a higher structure as a categorified mathematical structure, what we are referring to is a phenomenon in which natural algebraic identities hold up to homotopy. In other words, we’re speaking of mathematical structure in homotopy theory and thus of higher algebra, higher geometry, and so forth. Higher algebra consists of algebraic structure within higher category theory [11, 12]. As we discussed earlier, categories have a set of morphisms between objects, and, so, in the example of the category of sets, elements of a set may or may not be equal. Higher categories, much like higher algebra, are a generalisation of these sort of constructions we see in ordinary category theory. In the higher case we now have homotopy types of morphisms, which are called mapping spaces. And so, unless we are working with discrete objects, we must deal with homotopy as an equivalence relation should two so-called elements of a homotopy type, typically represented by vertices, be connected in a suitable way.

When we speak of higher structures as mathematical structures in homotopy theory, this is more specifically a mathematical structure in ${(\infty, 1)-\text{category theory}}$. This is a special category such that, from within the collection of all ${(n, r)-\text{categories}}$, which is defined to be an ${\infty-\text{category}}$ satisfying a number of conditions, we find an ${(\infty, 1)-\text{category theory}}$ to be a weak ${\infty-\text{category}}$ in which all n-morphisms for ${n \geq 2}$ are equivalences. I also think of a higher structure almost as a generalisation of a Bourbaki mathematical structure. But perhaps this comment should be reserved for another time.

In summary, if as motivation it is the case that we often want to study homotopy theory of homotopy theories, for instance what is called a Quillen model category, what we find is a hierarchy of interesting structures, which is described in terms of the homotopy theoretic approach to higher categories. And it is from this perspective that homotopy theories are just ${(\infty, 1)-\text{category theory}}$, where ${\infty}$ denotes structure with higher morphisms (of all levels) and the 1 refers to how all the 1-morphisms and higher morphisms are weakly invertible. Hence, too, in higher category theory we may begin to speak of ${(\infty, n)-\text{categories}}$, which may be described as:

1. An n-category up to homotopy (satisfying the coherence laws, more on this in a later post);

2. An ${(r, n)-\text{categories}}$ for ${r = \infty}$;

3. A weak ${\infty-\text{category}}$ or ${\omega-\text{category}}$ where all k-morphisms are equivalences satisfying the condition ${k > n}$.

There are different ways to define ${(\infty, n)-\text{categories}}$, and their use can be found in such places as modern topological field theory. If category theory is a powerful language to study the relation between objects, n-categories enables us to then go on and study the relations between relations, and so on. As an example, consider the category of all small categories. For two categories ${\mathcal{C}}$, ${\mathcal{D}}$, whose morphisms are functors, the set or collection of all morphisms hom-set ${\text{Fun}(C, D)}$ are then functors from ${\mathcal{C}}$ to ${\mathcal{D}}$. This forms a functor category in which all morphisms are natural transformations, given that the natural transformations are morphisms between morphisms (functors). Hence, in this way, we scratch the surface of the idea of higher categories, because, taking from what was mentioned above, these are categories equipped with higher ${n}$-morphisms between ${(n-1)}$-morphisms for all ${n \in \mathbb{N}}$.

Moreover, if in ordinary category theory there are objects and morphisms between those objects, from the higher category view these are seen as 1-morphisms. Then, we may define a 2-category, which is just a generalisation that includes 2-morphisms between the 1-morphisms. And we can therefore continue this game giving definition to ${n}$-category theory. We will eventually get into more detail about the idea of ${n}$-categories, including things like weak ${n}$-categories where associativity and identity conditions are no longer given by equalities (i.e., they are no longer strict), instead satisfied up to an isomorphism of the next level. But for now, in thinking of the basic example of a composition of paths and this notion of generalising to 2-morphisms between the 1-morphism, the emphasis here is on the idea that the two conditions of associativity and identity must hold up to reparameterisation (the topic of reprematerisation being a whole other issue) – hence, up to homotopy – and what this amounts to is a 2-isomorphism for a 2-category. If none of this is clear, hopefully more focused future notes will help spell it all out with greater lucidity.

***

In using the language of higher structures in M-theory, there have been many promising developments. For instance, it can be seen how core structures of string/M-theory emerge as higher structures in super homotopy theory [4, 13], leading to a view of M-theory beginning from the superpoint in super Minkowski spacetime going up to 11-dimensions. An interesting part of this work was the use of Elmendorf’s theorem on equivariant homotopy theory. It has led to exciting new developments in our picture of brane physics, with an updated brane bouquet.

Of course, the higher structures programme is far-reaching. From double and exceptional field theory and the global formulation of such actions to the study of homotopy algebras in string field theory, M-branes, sigma models on gerbes, and even modern views on anomalies in which field theories are treated as functors – this merely scratches the surface. Some nice lecture notes on higher structures in M-theory, focusing for example on M5-brane systems and higher gauge theory were recently offered by Christian Saemann [14]. Hopefully we will be able to cover many of these ideas (and others) moving forward. Additionally, I am currently enjoying reading many older works, such as Duiliu-Emanuel Diaconescu’s paper on enhanced D-brane categories in string field theory [15], and I’ve been working through Eric Sharpe’s 1999 paper [16], which was the first to explicitly draw the correspondence between derived categories and Dp-branes in his study of Grothendieck groups of coherent sheaves. These and others will be fun papers to write about in time.

To conclude, we’ve begun to introduce, even if only schematically, some important ideas at their most basic when it comes to studying higher structures in M-theory. In the next entries, we can deepen our discussion with more detailed notes and definitions, perhaps beginning with a formal discussion on category theory and then homotopy theory, and then a more rigorous treatment of the idea of a higher structure.

References

[1] William P. Thurston. On Proof and Progress in Mathematics, pages 37–55. Springer New York, New York, NY, 2006.

[2] Emily Riehl. Categorical Homotopy Theory. New Mathematical Monographs. Cambridge University Press, 2014.

[3] Branislav Jurco, Christian Saemann, Urs Schreiber, and Martin Wolf. Higher structures in m-theory, 2019.

[4] Domenico Fiorenza, Hisham Sati, and Urs Schreiber. The rational higher structure of m-theory. Fortschritte der Physik, 67(8-9):1910017, May 2019.

[5] Hisham Sati and Urs Schreiber. Survey of mathematical foundations of qft and perturbative string theory, 2012.

[6] J. Baez and J. Dolan. Higher dimensional algebra and topological quantum field theory. Journal of Mathematical Physics, 36:6073–6105, 1995.

[7] Daniel Marsden. Category theory using string diagrams, 2014.

[8] Birgit Richter. From Categories to Homotopy Theory. Cambridge Studies in Advanced Mathematics. Cambridge University Press, 2020.

[9] Carlos T. Simpson. Homotopy theory of higher categories, 2010.

[10] Bob Coecke and Eric Oliver Paquette. Categories for the practising physicist, 2009.

[11] T. Leinster. Topology and higher-dimensional category theory: the rough idea. arXiv: Category Theory, 2001.

[12] J. Baez. An introduction to n-categories. In Category Theory and Computer Science, 1997.

[13] John Huerta, Hisham Sati, and Urs Schreiber. Real ade-equivariant (co)homotopy and super m-branes. Communications in Mathematical Physics, 371(2):425–524, May 2019.

[14] Christian Saemann. Lectures on higher structures in m-theory, 2016.

[15] Duiliu-Emanuel Diaconescu. Enhanced d-brane categories from string field theory. arXiv: High Energy Physics – Theory, 2001.

[16] E. Sharpe. D-branes, derived categories, and grothendieck groups. Nuclear Physics, 561:433–450, 1999.

*Image: ‘Homotopy theory harnessing higher structures’, Newton Institute.

*Edited for spelling, grammar, and syntax.

# M-theory, the duality symmetric string, and fundamental mathematical structure

In quantum gravity, there presently exists a tight web of hints as well as numerous plausibility arguments in support of the proposed existence of M-theory; however, a systematic formulation of the non-perturbative theory remains an open problem. Without a fundamental formulation of M-theory, all we have is a hypothetical theory of which splinters of clues intimate 11-dimensional supergravity and the five string theories are each a limiting case of some deeper structure.

Mike Duff once described the situation like a patchwork quilt. We have corners – for instance, matrix theory – and we have some bits of stitching here and there, great successes in themselves, but the total object of the quilt is not understood.

In my opinion, this is one of the most deeply interesting and challenging problems one can currently undertake. In pursuing M-theory a great ocean lays undiscovered, in the words of Duff, the depths of which we may not yet be able to fully imagine but of which we anticipate to lead to new mathematics.

We still have no fundamental formulation of “M-theory” – the hypothetical theory of which 11-dimensional supergravity and the five string theories are all special limiting cases. Work on formulating the fundamental principles underlying M-theory has noticeably waned. […]. If history is a good guide, then we should expect that anything as profound and far-reaching as a fully satisfactory formulation of M-theory is surely going to lead to new and novel mathematics. Regrettably, it is a problem the community seems to have put aside – temporarily. But, ultimately, Physical Mathematics must return to this grand issue.’ – Greg Moore, from his talk at Strings 2014

For our present purposes, to better explain the opening paragraphs and the two programmes of research in non-perturbative theory in which I am currently most interested, we should go back a couple of decades in time. The story begins as late as 1995, when it was believed that the five superstring theories – type I, type IIA, type IIB, and the two flavours of heterotic string theory (SO(32) and E8 ${\times}$ E8) were distinct. At this time, the situation in quantum gravity appeared messy. There were five theories and no obvious mechanism to select the correct one. When it was eventually observed that these theories are deeply related by non-trivial dualities, it was proposed by Edward Witten that rather than being distinct they actually represent different limits of an overarching theory. This overarching theory, M-theory, was indeed found to possess an extraordinary unifying power, giving conception to the notion of a web of dualities based firstly on Witten’s observation that the type IIA string and the E8${\times}$E8 heterotic string are related to eleven-dimensional supergravity [1].

More specifically, it was seen that the 10-dimensional type IIA theory in the strong coupling regime behaves as an 11-dimensional theory whose low-energy limit is captured by 11-dimensional supergravity. This mysterious 11-dimensional theory was then seen to give further clue at its parental status when it was observed how supergravity compactified on unit interval ${\mathbb{I} = [0,1]}$, for example, leads to the low-energy limit of E8${\times}$E8 heterotic theory.

So far, these two examples provide only a few pieces of the web. A common way to approach a picture of M-theory today is to start with target-space duality (T-duality) and strong-weak duality (S-duality), which are two examples of string symmetries. T-duality, first observed by Balachandran Sathiapalan [2], is a fundamental consequence of the existence of the string, and we may describe it as a fundamental symmetry. Indeed, it famously constitutes an exact symmetry of the bosonic string, encoded by the transformations: ${R \leftrightarrow \frac{\alpha^{\prime}}{R}, k \leftrightarrow w}$, which describes an equivalence between radius and inverse radius, with the exchange of momentum modes ${k}$ and the intrinsically stringy winding modes ${w}$ in closed string theory, or in the case of the open string an exchange of Dirichlet and Neumann boundary conditions. In that closed strings can wrap around non-contractible cycles in space-time, the winding states present in string theory have no analogue in point particle theory, and it is the existence of both momentum and winding states that allows T-duality.

For example, the type IIA and type IIB string theories are found to be equivalent on a quantum level when compactified on a 1-dimensional torus ${\mathbb{T}^{1}}$. T-duality also relates the two heterotic theories. In that it is closely related to mirror symmetry in algebraic geometry, which in string theory is related to the important study of Calabi-Yau manifolds, T-duality in many cases enables us to observe how different geometries for the compact dimensions are physically equivalent, and when ${d}$ -dimensions are compactified on a ${n}$ -torus, we may generalise T-duality transformations under the group ${O(n,n,\mathbb{Z})}$.

S-duality, on the other hand, may be thought of in terms of a familiar description from classical physics, notably invariance of Maxwell’s equations under the exchange of electric and magnetic fields: ${E \rightarrow B, B \rightarrow - \frac{1}{c^2}E}$. As suggested by its name, S-duality transformation displays physical equivalence between strong and weak couplings of a theory. The existence of S-duality in string theory was first proposed by Ashoke Sen [3], where he showed that the type IIB string in 10-dimensions with ${g}$ coupling was equivalent to the same theory with coupling constant ${\frac{1}{g}}$. This is quite beautiful because, from the perspective of non-perturbative theory, S-duality ${SL(2, \mathbb{Z})}$ of the type-IIB can be observed as a consequence of M-theory diffeomorphism invariance [4]. It can also be observed how S-duality relates the heterotic ${SO(32)}$ string with the type I theory.

Together T-duality and S-duality unify all ten-dimensional superstring theories. And it is the web of dualities that unifies all five string theories, which provides one of the clues that M-theory is a unique theory of quantum gravity.

As summarised in a previous post, although we do not know the degrees of freedom of M-theory, we can begin to trace a picture. Starting with a point in parameter space, noting that there are different ways we can transform in this space, we may begin for the sake of example with type IIA string theory. We may then consider another point, for instance 11-dimensional supergravity – the classical limit of M-theory. What we find is how we can move between these two theories depending on the string coupling limit. If we go to the weak coupling limit ${g_{s} \rightarrow 0}$ (or when the dilaton has a large negative expectation value), then we go to a perturbative type IIA string theory. On the other hand, when we go to the strong coupling limit ${g_{s} \rightarrow \infty}$, we have strongly coupled type IIA string theory and, in this case, we should transform to a description of supergravity. This coincides with taking the large $N$ limit of the type IIA superstring, where $N$ is the number of D0-branes. The idea is that we can similarly carry on through each corner of the theory.

From the perspective that M-theory can be obtained from strings at strong coupling, one interesting fact is that this unique theory of quantum gravity in 11-dimensions does not in itself contain strings; instead, the fundamental objects are membranes and the theory describes the dynamics of M2-branes and M5-branes (i.e., 2-dimensional and 5-dimensional branes). When we compactify M-theory on a circle ${S^1}$ it is equivalent to type IIA string theory. What we see more technically is that a fundamental string is associated to an M2-brane wrapped around the circle. The other objects of type IIA string theory like D2- and D4-branes appear similarly from the fundamental objects of the non-perturbative theory [5,6,7]. If instead we take M-theory and compactify it on a torus ${T^2}$, we find the type IIB string compactified on a circle ${S^1}$. The idea, again, is that we may continue to play this game, from the view of the underlying theory, with the limiting cases for this unique theory of quantum gravity in 11-dimensions giving the zoo of perturbative string theories.

When T-duality and S-duality transformations are combined they then define the unified duality (U-duality). At present, I’m not entirely sure how to think about the U-dualities of M-theory as it is something I am actively working through. What I can say is that there are a few ways to look at and approach them. For instance, we can approach U-duality as the hidden continuous symmetry group of supergravity [8]. It is well-known that when compactifying 11-dimensional supergravity on tori of various dimensions, we observe a wealth of symmetries. This was first observed by Julia in 1980 [9]. But it also seems widely agreed that the hidden symmetry groups often denoted under ${G}$ and their compact subgroups ${H}$ for an ${n}$ -torus are suspected to play a discrete role within the U-dualities of M-theory in its complete form. In other words, there is suspicion going back to 1989 that some appropriately discrete version of these symmetries survive, and that they define the fundamental U-dualities of M-theory [10]. This discussion deserves a separate post in order to fully lay out the hidden symmetry groups and provide greater detail in explanation; what might be said, for now, is that the content of the dualities, as well as the way in which the duality groups describe or perhaps even fundamentally define the theory, are questions still requiring unambiguous answers.

2. Approaching M-theory: Top-down and bottom-up

There are a few more or less textbook approaches to M-theory and the important study of non-perturbative duality relations, which one can easily review. For instance, one may use low-energy effective actions, which, as we have touched on, are supergravity theories (that describe massless field interactions in the string spectrum). Within a restricted regime this approach can offer great insight into the physics at strong coupling. One can also study non-perturbative duality relations by exploiting known properties of things like Bogomolny-Prasad-Sommerfield (BPS) ${p}$ -branes, utilising a technique known as the saturation of a BPS bound. In general, the idea in both cases is to extrapolate from the weakly coupled theory to the strongly coupled theory (again, why we can trust such extrapolations is touched on briefly here).

However, going back to the discussion at the outset, one issue I currently share with other researchers in the field concerns a lack of mathematical rigour in the study of M-theory and its objects. While the existence of branes was posited during the Second Superstring Revolution’, and while there are many hints toward this non-perturbative proposition, a lot about brane physics has not been proven or rigorously derived. Moreover, there is a lot about the dynamics of branes that we still do not understand, and, impliededly, the non-perturbative effects in string theory require greater knowledge and clarity. The thing about M-theory and its properties in 11-dimensions, as presently being studied, is that it governs or is suspected to impact many aspects of the lower dimensional string theories. What the completion of M-theory should mean is greater systematic understanding of non-perturbative D/M-brane physics without ambiguity, including brane dynamics, as well as many curious properties and processes in quantum gravity, like what happens in the mathematical process when 10-dimensional space-time of string theory transforms into the 11-dimensional space-time of M-theory. It should also offer insight into the structure of things like perturbative string vacua, not to mention provide a final say on fundamental string cosmology as a whole. This refers to another concern.

For me, I would say as I so far understand, there are a number of interesting approaches to the non-perturbative theory that seem to be contributing overall to the right direction. The two approaches that I find most interesting and that I am currently focusing on for my PhD are relatively new and, while quite different from each other, I think they both have tremendous potential.

The first is a systematic top-down research programme that aims to capture a complete mathematical formulation of M-theory. This is the approach of Sati, Schreiber, and others, which I will write about quite a bit on this blog. It entails some of the best and most stimulating work in M-theory that I’ve seen to date, offering some wonderfully deep and potentially fundamental insight into the non-perturbative theory, if such a theory can in fact be rigorously proven. Here we have some fantastic developments in the form of Hypothesis H, such as the observation that the M-theory C-field is charge-quantized in Cohomotopy theory, or, as I have it in my mind, in cohomology theory M-brane charge quantisation is in cohomotopy. Recent updates to the brane bouquet are also magnificent and evoke wonderful emergent images. I can’t wait to write about these sorts of things in the coming months.

The other approach that I am interested in can be pictured as almost diagrammatically opposite to Schreiber et al. In some sense, it takes a bottom-up approach to M-theory by way of the duality symmetric string. This is what I began to study and work on for my recent MRes thesis. There is so much to be said about the doubled string and its many amazing qualities, which I may break up into several posts. For now, it is worth sharing that one of a host of reasons to study the duality symmetric string is to then look at analogous extensions of ideas and techniques in the study of duality symmetric M-theory.

The theory of the duality symmetric string is importantly a chiral theory, in which T-duality is made manifest on the level of the action, and so it is one that takes a world-sheet perspective such that we want to employ a sigma model description of the maximally doubled string. This world-sheet theory of chiral bosons that sees the total doubled space – especially when treated in a very generic way – naturally accommodates stringy non-geometries. This means that from a study of the maximally doubled string, in addition to seeking very general formulations of chiral boson models for generic doubled geometries, we can also look to construct models that realise completely the full web of string dualities.

I think there is quite a bit of potential insight to be gained when building from the duality symmetric string toward duality invariant M-theory. This relates, in no small part, to non-perturbative investigations leading to new global solutions combining spacetime geometry and quantum field theory defined as generalised geometry, if we take the view of understanding such geometry in terms of a study of conventional geometry with a metric and B-field on some D-dimensional manifold ${M}$ on which ${O(D, D)}$ finds natural action. In M-theory, generalised geometry may be extended to exceptional generalised geometry, and one implication is the extension of spacetime itself, with a further consequence being the possibility that geometry and gravity are emergent concepts. Indeed, there is the lingering idea, one that was first formulated in the late 1990s, that a complete theory of quantum gravity should give access to whatever extent to pre-geometrical features of space-time – a non-commutative geometry at very short distances. Working backwards, this is almost like a disolution of space-time in the emergent picture. And, in the quilt analogy, we should see patches defined as large groups of hidden symmetries, which contain extensions of stringy dualities – what we have described as U-duality – and even potentially a new self-dual string theory. By an analogous extension of ideas, from what we learn about the duality symmetric string, perhaps we can drill a bit more into the true meaning of hidden symmetry groups in the full M-theory. What does it mean when such symmetries are made manifest? I think these sorts of approaches, questions, and conceptual possibilities are exciting.

References

[1] Edward Witten. String theory dynamics in various dimensions.Nuclear PhysicsB, 443(1):85 – 126, 1995.

[2] Balachandran Sathiapalan. Duality in statistical mechanics and string theory.Phys. Rev. Lett., 58:1597–1599, Apr 1987.

[3] Ashoke Sen. Strong – weak coupling duality in four-dimensional string theory.Int. J. Mod. Phys. A, 9:3707–3750, 1994.

[4] John H. Schwarz. The power of m theory.Physics Letters B, 367(1-4):97–103,Jan 1996.

[5] N.A. Obers and B. Pioline. U-duality and m-theory.Physics Reports, 318(4-5):113–225, Sep 1999.

[6] John H. Schwarz. Introduction to m theory and ads/cft duality.Lecture Notesin Physics, page 1–21, 1999.

[7] M. P. Garcia del Moral. Dualities as symmetries of the supermembrane theory,2012.

[8] David S. Berman and Daniel C. Thompson. Duality symmetric string and m-theory, 2013.

[9] B. Julia. GROUP DISINTEGRATIONS.Conf. Proc. C, 8006162:331–350, 1980.

[10] Bernard de Wit and Hermann C Nicolai. d = 11 supergravity with local SU(8)invariance.Nucl. Phys. B, 274(CERN-TH-4347-86):363–400. 62 p, Jan 1986.

# Literature: Duality Symmetric String and the Doubled Formalism

When it comes to a T-duality invariant formulation of string theory, there are two primary actions that are useful to study as a point of entry. The first is Tseytlin’s non-covariant action. It is found in his formulation of the duality symmetric string, which presents a stringy extension of the Floreanini-Jackiw Lagrangians for chiral fields. In fact, for the sigma model action in this formulation, one can directly reproduce the Floreanini-Jackiw Lagrangians for antichiral and chiral scalar fields. The caveat is that, although we have explicit $O(D,D)$ invariance, which is important because ultimately we want T-duality to be a manifest symmetry, we lose manifest Lorentz covariance on the string worldsheet. What one finds is that we must impose local Lorentz invariance on-shell, and from this there are some interesting things to observe about the constraints imposed at the operator level.

The main papers to study are Tseytlin’s 1990/91 works listed below. Unfortunately there is no pre-print available, so these now classic string papers remain buried behind a paywall:
1) Tseytlin, ‘Duality Symmetric Formulation of String World Sheet Dynamics
2) Tseytlin, ‘Duality Symmetric Closed String Theory and Interacting Chiral Scalars

For Hull’s doubled formalism, on the other hand, we have manifest 2-dimensional invariance. In both cases the worldsheet action is formulated such that both the string coordinates and their duals are on equal footing, hence one thinks of the coordinates being doubled. However, one advantage in Hull’s formulation is that there is a priori doubling of the string coordinates in the target space. Here, $O(D,D)$ invariance is effectively built in as a principle of construction. This is because for the covariant double sigma model action, the target space may be written as $R^{1, d-1} \otimes T^{2D}$, in which we have a non-compact spacetime and a doubled torus. From the torus identifications we have manifest $GL(2D; Z)$ symmetry. Then after imposing what we define as the self-duality constraint of the theory, which contains an $O(D,D)$ metric, invariance of the theory reduces directly to $O(D,D; \mathbb{Z})$.

1. Hull, ‘Doubled Geometry and T-Folds
2. Hull and Reid-Edwards, ‘Non-geometric backgrounds, doubled geometry and generalised T-duality

What is neat about the two formulations is that, turning off interactions, they are found to be equivalent on a classical and quantum level. It is quite fun to work through them both and prove their equivalence, as it comes down to the constraints we must impose in both formulations.

I think the doubled formalism (following Hull) for sigma models is most interesting on a general level. I’m still not comfortable with different subtleties in the construction, for example the doubled torus fibration background or choice of polarisation from T-duality. The latter is especially curious. But, in the course of the last two weeks, things are finally beginning to clarify and I look forward to writing more about it in time.

Related to the above, I thought I’d share three other supplementary papers that I’ve found to be generally helpful:

1) Berman, Blair, Malek, and Perry, ‘O(D,D) Geometry of String Theory
2) Berman and Thompson, ‘Duality Symmetric String and M-theory
3) Thompson, ‘T-duality Invariant Approaches to String Theory

There are of course many other papers, including stuff I’ve been studying on general double sigma models and relatedly the Pasti, Sorokin and Tonin method. But those listed above should be a good start for anyone with an itch of curiosity.