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 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 E8E8 heterotic string are related to eleven-dimensional supergravity .
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 , for example, leads to the low-energy limit of E8E8 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 , 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: , which describes an equivalence between radius and inverse radius, with the exchange of momentum modes and the intrinsically stringy winding modes 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 . 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 -dimensions are compactified on a -torus, we may generalise T-duality transformations under the group .
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: . 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 , where he showed that the type IIB string in 10-dimensions with coupling was equivalent to the same theory with coupling constant . This is quite beautiful because, from the perspective of non-perturbative theory, S-duality of the type-IIB can be observed as a consequence of M-theory diffeomorphism invariance . It can also be observed how S-duality relates the heterotic 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 (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 , 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 limit of the type IIA superstring, where 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 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 , we find the type IIB string compactified on a circle . 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 . 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 . But it also seems widely agreed that the hidden symmetry groups often denoted under and their compact subgroups for an -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 . 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) -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.
As I had to summarise the other day when it comes to de Sitter solutions in string theory: careful systematic definition of M-theory is important because, without a complete non-perturbative theory, I am skeptical we can expect to rigorously deduce an answer about the fundamental structure of string vacua. Indeed, many attempts to construct dS solutions rely on brane physics, and again there is still so much to be learned about non-perturbative effects on the string level. Additionally, it seems both reasonable and logical that unambiguous proof of predictions from the non-perturbative theory will prove important when supporting any such quantum gravity arguments that make empirical statements, or that try to answer notable conjectures like the one that suggests dS space cannot be obtained in string theory (being in contradiction with current observations that point toward a quasi-dS cosmological expansion). No doubt others will disagree, and in no way are these comments meant to take away from the wonderful insights and accomplishments of research belonging to the Swampland programme. It is just that from my current vantage, I’ve come to view any claims or empirical predictions without a complete mathematical formulation of M-theory, from which we may rigorously deduce answers to questions about fundamental structures or from which we may construct unambiguous proofs, as premature. There are too many pieces of the tapestry missing. So this is where I place my primary focus, because without answers as to these missing pieces it is hard to settle on anything else.
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.https://platform.twitter.com/widgets.js
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 on which 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.
 Edward Witten. String theory dynamics in various dimensions.Nuclear PhysicsB, 443(1):85 – 126, 1995.
 Balachandran Sathiapalan. Duality in statistical mechanics and string theory.Phys. Rev. Lett., 58:1597–1599, Apr 1987.
 Ashoke Sen. Strong – weak coupling duality in four-dimensional string theory.Int. J. Mod. Phys. A, 9:3707–3750, 1994.
 John H. Schwarz. The power of m theory.Physics Letters B, 367(1-4):97–103,Jan 1996.
 N.A. Obers and B. Pioline. U-duality and m-theory.Physics Reports, 318(4-5):113–225, Sep 1999.
 John H. Schwarz. Introduction to m theory and ads/cft duality.Lecture Notesin Physics, page 1–21, 1999.
 M. P. Garcia del Moral. Dualities as symmetries of the supermembrane theory,2012.
 David S. Berman and Daniel C. Thompson. Duality symmetric string and m-theory, 2013.
 B. Julia. GROUP DISINTEGRATIONS.Conf. Proc. C, 8006162:331–350, 1980.
 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.