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 it 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 currentlt 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. As a new researcher, 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 start my formal PhD years, the 2018 Durham Symposium seemed momentous. 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 events 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 exciting.

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 a 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 premise of introduction, which resonated with me. 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, in 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 alreadt 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, as a mathematical framework, 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 fundamental important. 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 comply-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 constraints to the homotopy class, an equivalent expression for the path integral may be found. And really, one can probably already start to see this 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 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 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, 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 this 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.


[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.

The duality symmetric string: A return to Tseytlin

The case of the duality symmetric string is a curious one (in a recent post we began discussing this string in the context of building toward a study of duality symmetric M-theory). In this essay, which may serve as the first of a few on the topic, I want to offer an introduction to some of the characteristic features of the duality symmetric string – what I will also refer to as the doubled string – as well as discuss some of its historical connections. One thing that we will focus on at the outset is the deep connection between this extended formulation of string theory, string field theory (SFT), and the more recent development of double field theory (DFT). Such a connection is prominent not least in how we treat the string fields in constructions in which T-duality is a manifest symmetry. For the purposes of this essay, these constructions may be defined in terms of what are called double sigma models.

To help lay this out, let’s quickly review some history. In the early 1990s, a series of papers appeared by Tseytlin [1,2], Siegel [3, 4], and Duff [5]. In these papers, the important topic of string dualities was explored, particularly the fundamental role target-space duality (T-duality) plays in string theory. T-duality is of course an old subject in string theory, and we have already spoken several times in the past about its key features. Recall, for instance, that the existence of this fundamental symmetry is a direct consequence of the existence of the string as a generalisation of point particle theory. Given how for the closed string in the presence of {d} compact dimensions T-duality interchanges the momentum modes {k} of a string with its winding modes {w} around a compact cycle, one of the deep implications is that in many cases two different geometries for the extra dimensions are found to be physically equivalent.

From the space-time perspective, T-duality is a solution generating symmetry of the low energy equations of motion. However, from a world-sheet point of view, T-duality is a non-perturbative symmetry. The fact that it is an exact symmetry for closed strings suggests, firstly, that one should be able to extend the standard formulation of string theory based famously on the Polyakov action (for review, see the first chapter of Polchinski). The idea is that we may do this at the level of the world-sheet sigma-model Lagrangian density, by which I mean the motivation is to construct a manifestly T-duality invariant formulation of closed string theory on the level of the action, remembering from past discussions that we may capture T-duality transformations under the group O(D,D,\mathbb{Z}) . When we extend the theory in this way, we find that we are obliged to introduce the compact coordinates {X} and the dual ones {\tilde{X}} in the sigma model, which means we double the string coordinates in the target-space. This gives the name double string theory.

Let’s explain what this all means in clearer terms, as many of these ideas can be sketched cleanly in the context of SFT. In 1992/93, around the same time as the first duality symmetric string papers, field theory emerged as a complete gauge-invariant formulation of string dynamics [6, 7]. This led to the development of a precise spacetime action whose gauge symmetry arguably takes the most elegant possible form [8]. What was observed, furthermore, is how the momentum and winding modes may be treated symmetrically and on equal footing. For instance, let us explicitly denote the compact coordinates {X^{a}} and the non-compact coordinates {X^{\mu}} , with {X^{I} = (X^{a}, X^{\mu})} . Conventionally, we define the indices such that {I = 1,...,D} , {\mu = 1,...,d} , and {a = 1,...,n} . If the string field gives component fields that depend on momentum {p^{a}} and winding {w^{a}} , then in position space we may assign the coordinates {X^{a}} conjugate to the momentum and, as alluded above, new periodic dual coordinates {\tilde{X}_{a}} conjugate to the winding modes.

The key point is as follows: if one attempts to write the complete field theory of closed strings in coordinate space, then as stated the full theory depends naturally on dual coordinates {X^{a}} and {\tilde{X}_{a}} . This is also to say that naturally the full phase space of the theory accompanies both the momentum and the winding modes. Or, to phrase it in a slightly different manner, for toroidal compactification there is a zero mode {X^{a}} and {\tilde{X}_{a}} , and, as the expansion of a string field provides component fields that depend on both momentum and winding, we come to the statement that the arguments of all fields in such a theory are doubled. For the doubled fields {\phi(X^{a}, \tilde{X}_{a}, X^{\mu})} we may write the following seemingly simple action

\displaystyle S = \int dX^{a}  d\tilde{X}_{a}  dX^{\mu} \mathcal{L}(X^{a}, \tilde{X}_{a}, X^{\mu}) \ (1)  .

The Lagrangian in (1) may seem straightforward, but in fact it proves incredibly complicated. One issue has to do with how the physical content of the theory becomes buried underneath unphysical and computationally inaccessible data, with the full closed string field theory comprising an infinite number of fields. This is where DFT may be motivated from first-principles; because, in response, DFT answers this problem by issuing the following simplification strategy: what if we instead choose some finite subset of string fields? An obvious choice for such a subsector of the full theory is the massless sector. In the study of DFT, we may then ask, if for the standard bosonic string the low-energy effective action is famously

\displaystyle S = \int dX \sqrt{-g} e^{-2\phi} [R + 4(\partial \phi)^2 - \frac{1}{12}H^2]  + \text{higher derivative terms}, \ (2)

what does this action become in the case of doubled coordinates on tori? Is T-duality manifest? What about for non-trivial geometries? Historically, DFT emerged with the aim to answer such questions. In fact, following Nigel Hitchin’s introduction of generalised geometry [9, 10], itself inspired by the existence of T-duality, serious efforts materialised to incorporate this mathematical insight into the study of the target-space geometry in which strings live [11, 12, 13, 14], beginning especially with the study of phase space and invariance of respective Hamiltonians. This culminated in 2009, when Hull and Zwiebach formulated such a T-duality invariant theory explicitly [11], formalising DFT almost two decades after the original duality symmetric string papers. What one finds is a theory constructed on the product manifold {\mathbb{R}^{d-1,1} \times T^{n}} with coordinate space fields {\phi(X^{\mu}, X^a, \tilde{X}_{a})} . The torus is doubled, containing the spacetime torus and the torus parameterised by the winding modes, such that {(X^a, \tilde{X}_{a})} are periodic on {T^{2n}} . The spectrum for the massless fields is then described in terms of the supergravity limit of string theory.

By taking this approach, DFT has presented fresh insight on T-duality in string theory, leading to the development of deeper connections between frontier theoretical physics and mathematics through the appearance and use of Courant brackets, and by gaining new insight on the deepening role generalised geometry seems to play in string theory.

Much like field theory, the doubled world-sheet theory has also been reinvigorated in the last decade or more. This follows from breakthrough work by Hull [16, 17], who established the doubled formalism to define strings in a class of non-geometric backgrounds known as T-folds. These are non-geometric manifolds where locally geometric regions are patched together such that the transition functions are T-duality transformations.


Currently, there are primarily two doubled string actions that we may consider when constructing double sigma models: Tseytlin’s first-principle construction of the duality symmetric string [1, 2, 15] and Hull’s doubled formalism [16, 17]. Both actions satisfy the requirement of T-duality appearing as a manifest symmetry, with the former possessing general non-covariance and the latter possessing general covariance.

Hull’s doubled formalism is interesting for several reasons. In this formulation we have manifest 2-dimensional Lorentz invariance from the outset, and a notable advantage is that there is a priori doubling of the string coordinates in the target space. In other words, both the Tseytlin approach and the Hull approach are formulated such that both the string coordinates and their duals are treated on equal footing. But in Hull’s formulation, {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 takes the form {R^{1, d-1} \otimes T^{2n}} , in which we have a non-compact spacetime and a doubled torus. From the torus identifications we have manifest {GL(2n; \mathbb{Z})} symmetry. Then, after imposing what is defined as the self-duality constraint of the theory, which contains the O(D,D) metric, invariance of the theory reduces directly to O(D,D; \mathbb{Z}) . In other words, while the doubled formalism starts with a covariant action that involves doubled coordinates, the invariance of this theory under O(D,D) is generated by imposing this self-duality constraint, which, similar to DFT, effectively halves the degrees of freedom and ensures that the remaining fields are physical.

Think of it this way: in Hull’s doubled formalism the essential motivation is to double the torus by then adding {2n} coordinates such that the fibre is {T^{2n}} ; however, typically the fields depend only on the base coordinates. Finally, the strategy is generally to proceed with a patch-wise splitting {T^{2n} \rightarrow T^{n} \oplus \tilde{T}^n} so that we have demarcated a strictly physical subspace {T^n} and its dual {\tilde{T}^{n}} . For a geometric background local patches are glued together with transition functions which include group {GL(n, \mathbb{Z})} valued large diffeomorphisms of the fibre. For the non-geometric case, this is approached by gluing local patches with transition functions that take values in {GL(n, \mathbb{Z})} as well as in the complete T-duality group, such that {O(D,D,\mathbb{Z})} is a subgroup of {GL(2n, \mathbb{Z})} large diffeomorphisms of the doubled torus.

On the other hand, Tseytlin’s first-principle formulation of the duality symmetric string and world-sheet theory for interacting chiral scalars, which presents a direct stringy extension (or stringification) of the Floreanini-Jackiw Lagrangians [31] for chiral fields, does not possess {O(D,D)} by principle of construction. Instead, we find that it emerges rather organically as an intrinsic characteristic of the doubled string, with the caveat being that we lose manifest Lorentz covariance on the string world-sheet. What one finds is that we must instead impose local Lorentz invariance on-shell. The equivalence of the Tseytlin and Hull actions on a classical and quantum level has been shown in [32, 33, 34]. Like DFT, both of these approaches are constructed around the generalised metric \mathcal{H}_{IJ} which we’ll touch on later.

It is no surprise that earlier formulations of the duality symmetric string were a primary reference in the development of DFT. In [1, 2], Tseytlin argues that the existence of the intrinsically stringy winding modes, which appear in the spectrum of the closed string compactified on a torus (created by vertex operators involving both {X} and {\tilde{X}} ), can result in 2d field theories with interactions indeed involving {X} and {\tilde{X}} . Similar models have been explored in statistical mechanics, with the key point in closed string theory being how for fully-fledged local quantum field theories we are required to treat {X} and {\tilde{X}} as independent 2d fields (dual to each other on-shell). An advantage of such an extended formulation of string theory is that we may obtain more vacua than the standard formulation. Furthermore, as one may have guessed, the notion of the duality symmetric string is based on the fact that duality symmetry becomes an off-shell symmetry of the world-sheet action. Thus, T-duality for example may be made manifest in the scattering amplitudes and on the level of the effective action.

To study the construction of the duality symmetric string, we note that directly from 2-dimensional scalar field theory constructed to be symmetric in {\phi} and {\tilde{\phi}} , Tseytlin derives the Lagrangian density

\displaystyle \mathcal{L}_{sym} = \mathcal{L}_{+}(\phi_{+}) + \mathcal{L}_{-}(\phi_{-}) \ (3)


\displaystyle \mathcal{L}_{\pm}(\phi_{\pm}) = \pm \frac{1}{2}\dot{\phi}_{\pm}\phi^{\prime}_{\pm} - \frac{1}{2} \phi^{\prime 2}_{\pm}. \ (4)  .

Here {\mathcal{L}_{+}} and {\mathcal{L}_{-}} are the Floreanini-Jackiw Lagrangian densities for chiral and anti-chiral fields, with {\dot{\phi} = \partial /\partial_{\tau}} and {\phi^{\prime} = \partial / \partial_{\sigma}} . The total Lagrangian {\mathcal{L}_{sym}} is itself constructed so that it is manifestly invariant under the exchange of {\phi = \frac{1}{\sqrt{2}} (\phi_{+} + \phi_{-})} with its Hodge dual {\tilde{\phi} = \frac{1}{\sqrt{2}} (\phi_{+} - \phi_{-})} . Directly from the equations of motion one can derive chirality conditions for this theory (for a complete review see also [32, 33, 34]).

For our present purposes it is important to note that the goal for Tseytlin is to realise from 2-dimensional scalar field theory the corresponding formulation of string theory, which indeed proves general enough to incorporate the world-sheet dynamics of the winding sector. Writing the Lagrangian (3) for {D} scalar fields {X^{I}} and with a general background, in the Tseytlin approach we famously obtain the action

\displaystyle S [e^{a}_{n}, X^{I}] = - \frac{1}{2} \int_{\sum} d^{2}\xi  e [ \mathcal{C}^{ab}_{IJ}(\xi)  \nabla_{a} X^{I} \nabla_{b} X^{J}]. \ (5)  .

Here {I, J = 1,...,D} . We define the coordinates on {\sum} such that {\xi^{0} \equiv \tau} and {\xi^{1} \equiv \sigma} . The two-dimensional scalar fields {X^{I}} depend on {\xi} and they are vectors in {N} -dimensional target space {\mathcal{M}} . The number {N} of embedding coordinates is kept general, because the purpose of this action is to be as generic as possible while minimising assumptions for its construction. We also note that {C_{IJ}} need not necessarily be symmetric and, from the outset, we can treat it completely generically. We also have the zweibein {e^{a}_{n}} , where {e = \det e^{a}_{n}} . This term appears in the definition of the covariant derivative of the scalar field {X^{I} : \nabla_{a} X^{I} \equiv e^{a}_{n}\partial_{a} X^{I}} , where {a} is a flat index and {n} is a curved index.

In its first principle construction, which occupies the earliest sections of [2], one can recover from this generic action (5) the standard manifestly Lorentz invariant sigma model action for strings propagating in a curved background. Furthermore, if we exclude the dilaton for simplicity we may define {\mathcal{C}^{ab}_{IJ} = T(\eta^{ab}G_{IJ} - \epsilon^{ab}B_{IJ})} , where we reintroduce explicit notation for the string tension {T} , {G} is the metric tensor on the target space, and {B} is the Kalb-Ramond field.

Keeping to a generic analysis with a general {C} , after a number of steps one finds that (5) may be rewritten in the following way,

\displaystyle S = -\frac{1}{2} \int d^{2}\xi  e[ \mathbb{C}_{IJ}(\xi) \nabla_{0} X^{I} \nabla_{1} X^{J} + M_{IJ} \nabla_{1} X^{I} \nabla_{1} X_{J}]. \ (6)  .

Here it is conventional to define {\mathbb{C}_{IJ} = C_{IJ}^{01} + C_{JI}^{10}} and {M_{IJ} = M_{JI} = C^{11}_{IJ}} . The action is manifestly diffeomorphism {\xi^{n} \rightarrow \xi^{\prime n}(\xi)} and Weyl {e^{a}_{n} \rightarrow \lambda(\xi)e^{a}_{n}} invariant, but it is not manifestly invariant under local Lorentz transformations. Moreover, notice that (6) must be invariant for the finite transformation of the zweibein, because the physical theory should be independent of {e^{a}_{n}} . This means that if under such a transformation we have {e^{a}_{n} \rightarrow e^{\prime a}_{n} = \Lambda^{a}_{b}(\xi)e^{b}_{n}} , where one may recognise {\Lambda^{a}_{b}} is a Lorentz {SO(1,1)} matrix dependent on {\xi} , we also have an induced infinitesimal transformation of the form {\delta e^{a}_{n} = \omega^{a}_{b}(\xi)e^{b}_{n}} with {\omega_{ab} = - \omega_{ba}} . Now, substituting {\omega^{a}_{b}(\xi) = n(\xi)\epsilon^{a}_{b}} , we obtain

\displaystyle \delta e^{a}_{n} = n (\xi)\epsilon^{a}_{b}(\xi)e^{b}_{n}. \ (7)  ,

however, as stated, the action is not manifestly invariant under such transformations. The requirement of on-shell local Lorentz invariance is fundamental to the entire discussion at this point. As Tseytlin comments in a footnote [2], alternatively we may prefer Siegel’s manifestly Lorentz covariant formulation, but with that we obtain extra fields and gauge symmetries; whereas in extending the Floreanini-Jackiw formulation it is fairly simple to introduce interactions and, ultimately, we find that the condition in the Siegel approach that requires decoupling of the Lagrange multiplier corresponds to what we will review as the Lorentz invariance condition in the Floreanini-Jackiw approach.

For the action (6), a way to attack the requirement of on-shell Lorentz invariance is by seeing in [2] that it demands we satisfy the condition

\displaystyle \epsilon^{ab} t_{ab} = 0, \text{where} \ t_{a}^{b} \equiv \frac{2}{\epsilon} \frac{\delta S}{\delta e^{a}_{n}}e^{b}_{n}. \ (8)  .

The general idea is that the tree-level string vacua should be assumed to correspond to {S[X, \tilde{X}, e]} , which define the Weyl and Lorentz invariant quantum field theory. In performing the background field expansion, we may take the expansion to be near the classical solution of the {(X, \tilde{X})} equations of motion with the trace of the expectation value of the energy-momentum tensor as well as the {\epsilon^{ab}} trace vanishing on-shell. In Tseytlin’s formulation, {\hat{t}} denotes precisely this epsilon trace such that {\hat{t} = \epsilon^{a}_{b} t_{a}^{b}} . The vanishing of {\hat{t}} shows local Lorentz invariance. So let us now vary (6) under local Lorentz transformation, which is proportional to the equations of motion

\displaystyle t^{b}_{a} = - \delta_{a}^{b} [\mathbb{C}_{IJ}(\xi) \nabla_{0} X^{I} \nabla_{1} X^{J} \ + \ M_{IJ} \nabla_{1} X^{I} \nabla_{1} X^{J}] \ + \ \delta_{0}^{b}[C_{IJ}\nabla_{a} X^{I} \nabla_{1} X^{J}] \ + \ \delta_{1}^{b} [C_{IJ}\nabla_{0} X^{I} \nabla_{a} X^{J}] \ + \ 2\delta_{1}^{b}M_{IJ}\nabla_{a}X^{I}\nabla_{1}X^{J}. \ (9)

This equation for {t^{b}_{a}} is equivalent to equation 4.3 in [2]. In order for the variation of the action to vanish under such a transformation, we derive the condition

\displaystyle \epsilon^{ab}t_{ab} = 0. \ (10)

In other words, the condition that must be satisfied to recover local Lorentz invariance depends on the solution of the equations of motion for the zweibein. In fact, one will recognise that what is observed is completely analogous to the standard string theory formulation based on the Polyakov action, where one will recall that the equations of motion for the world-sheet metric determines the vanishing of the energy-momentum tensor [35].

This constraint must be imposed on a classical and quantum level.

The key point is that now we can choose the flat gauge {e_{n}^{a} = \delta_{n}^{a}} , thanks to the invariances under diffeomorphisms, Weyl transformations, and finally local Lorentz invariance imposed on-shell. This is crucial for the formulation of the dual symmetric string in that, using the flat gauge for the zweibein, we are effectively performing the analogous procedure as when fixing the conformal gauge in standard string theory. Keeping {C} and {M} constant, we can compute the equations of motion for the field {X^{I}} to give

\displaystyle  \nabla_{1} [e (C_{IJ} \nabla_{0} X^{J} + M_{IJ}\nabla_{1} X^{J}] = 0. \\\ (11)

In the flat gauge this result becomes

\displaystyle \partial_{1} [C_{IJ} \partial_{0} \xi^{J} + M_{IJ} \partial_{1} \xi^{J}] = 0. \ (12)  .

From (12) a now famous identity appears, where, in the flat gauge and along the equations of motion for {\xi^{I}} , the following constraint on {C} and {M} is obtained [2]:

\displaystyle  C = MC^{-1}M. \ (13)

One may recognise the tensor structure of (13) in terms of the action of an {O(D,D,\mathbb{Z})} element. The important thing to highlight is that throughout the lengthy calculation to get to this point, {C} and {M} are held constant. (When {C} and {M} are not treated as constant, a number of interesting questions arise which extend beyond the scope of the present discussion). What is also important is that, after rotating {\xi^{I}} , the matrix {C} can always be put into diagonal form such that

\displaystyle  C = \ \textbf{diag} \ (1,...,1,-1,...,-1). \ (14)

It remains to be said that {C = C^{-1}} , which means that the constraint (13) defines the indefinite orthogonal group {O(p,q)} of {N \times N} matrices {M} with {N = p + q} in {\mathbb{R}^{p,q}} . The inner product may now be written as

\displaystyle  C = MCM, \ (15)

in which the matrix {C} eventually takes on the explicit definition of an {O(D,D,\mathbb{R})} invariant metric in the 2D target space {M} . Although, admittedly, this cursory review has omitted many important and interesting details, the pertinent point in terms of this essay is as follows. The action (6) turns out to describe rather precisely a mixture of {D} chiral {\xi^{\mu}_{-}} and {D} anti-chiral {\xi^{\mu}_{+}} scalars. In demanding local Lorentz invariance and the vanishing of the Lorentz anomaly, this requires that {p = q = D} with {2D = N} . In working through the complete logic of the calculation, we observe quite explicitly that inasmuch the requirement of local Lorentz invariance is imposed through the condition (10), this leads one naturally to an interpretation of the matrix {C} as a 2D target space metric with coordinates

\displaystyle  \xi^{I} = (\xi^{\mu}_{-}, \xi^{\mu}_{+}), \ ds^{2} = dX^{I} C_{IJ} d X^{J}, \ I = 1,...,2D, \ \text{and} \ \mu = 1,...,D. \ (16)

If we make a change of coordinates in the target space, particularly by defining a set of new chiral coordinates, the matrix {C} takes on the off-diagonal form of the {O(D,D)} constant metric {L} typically considered in DFT (for review, see [36]) and elsewhere. The chiral coordinates we define are

\displaystyle  X^{I} = \frac{1}{\sqrt{2}} (X_{+}^{\mu} + X_{-}^{\mu}), \tilde{X}_{I} = \frac{1}{\sqrt{2}} (X_{+}^{\nu} - X_{-}^{\nu}). \ (17)

In this frame, the matrix {C} is then shown to be

\displaystyle C_{IJ} = - \Omega_{IJ} = -\begin{pmatrix}  0 & \mathbb{I} \\ \mathbb{I} & 0 \end{pmatrix}. \ (18)

It follows that the condition (13) transforms into the constraint

\displaystyle  M^{-1} = \Omega^{-1}M\Omega^{-1} \ (19)

on the symmetric matrix {M} , which can be parametrised by a symmetric matrix {G} and an antisymmetric matrix {B} . Therefore, remarkably, the symmetric matrix {M} takes the precise form of the generalised metric in which {M} is found to be positive definite.

To conclude, in the chiral coordinates we arrive at a famous form of the Tseytlin action,

\displaystyle  S = \frac{1}{2} \int d^{2}\xi \ e[ \Omega_{IJ} \nabla_{0} X^{I} \nabla_{1} X^{J} - M_{IJ}\nabla_{1} X^{I} \nabla_{1} X^{J}]. \ (20)

This action is manifestly {O(D,D)} invariant. When {O(D,D)} transformations are applied to (20), we obtain exactly what we would anticipate for the standard string in the sense of T-duality invariance under {X \rightarrow\tilde{X}} and for the generalized metric {M \rightarrow M^{-1}} .

For completeness, from the action (6) in arriving at (20), it should be clear that what we are working with is a sigma model for the dual symmetric string. The generalised version of the celebrated action (20) is indeed often written as

\displaystyle  S_{General} = \frac{1}{2} \int d^{2}\xi \ [- (C_{IJ} + \eta_{IJ}) \partial_{0} X^{I} \partial_{1} X^{J} + \mathcal{H}_{IJ} \partial_{1} X^{I} \partial_{1}X^{J})]. \ (21)

This final action can be argued to be a very natural generalisation for the standard string on a curved background. It not only contains the generalised metric {\mathcal{H}_{IJ}} , but also another symmetric metric {\eta_{IJ}} with {(D,D)} signature and an antisymmetric 2-tensor {C_{IJ}} . The coordinates are defined {X^{I} = \{ X^{I}, \tilde{X}_{I} \}} with the background fields in general depending on {X^{I}} .


In the last decade especially, Tseytlin’s formulation has been refocused in various studies concerning the nature of the doubled string and its geometry. One notable example to which we will return in a moment, pre-dates the first primary collection of DFT papers and, in many ways, can be interpreted to give a prediction to DFT. I am refering to the 2008 paper David S. Berman, Neil B. Copland, and Daniel C. Thompson [18], where they investigated the background field equations for the duality symmetric string using an action equivalent to that of Tseytlin’s but constructed in the context of Hull’s doubled formalism. In recent years, a series of publications on doubled sigma models have appeared in connection [19, 20, 21, 22], where in [20] the double sigma model is for example directly related to DFT.

Another example refers directly to both Tseytlin and DFT from a different perspective. In the years after 2009 when Hull and Zwiebach published their important paper formalising DFT, it was recognised that while a deep connection exists between DFT and generalised geometry, with the former locally equivalent to the latter, it does not completely come into contact with its formal mathematical structures. In fact, an open research question remains motivated by the unmistakeable resemblance DFT has with generalised geometry and the formal gap that remains between them. Recent work in mathematics and physics has displayed some promise, suggesting that the use of para-Hermitian and para-Kähler manifolds may be the solution [23, 24, 25]. Related to these efforts is a recent reformulation of string theory under the heading metastring theory [24, 26, 27, 28, 29], which begins, similar to the studies on double sigma models, with a generalised version of the first-principle Tseytlin action for the duality symmetric string. The metastring is therefore a chiral T-duality invariant theory that, in many ways, wants to generalise from DFT and make direct connection with things like Born geometry [26], relying on the consistency of Tseytlin’s formulation.

If a direct consequence of making T-duality manifest is that the winding modes are treated on equal footing with momentum, then for DFT all of these properties are incorporated into one field theory. The result, as mentioned, is a doubled coordinate space. In metastring theory, on the other hand, the target space of the world-sheet formulation is a phase space, much like in Tseytlin’s original construction. The coordinates of this phase space are indeed doubled, but unlike in DFT they are also conjugate such that in this case the dual coordinates are related directly to energy-momentum coordinates. In other words, {\tilde{X}} is now identified with {p} . This means that, instead of a physical spacetime formulation, the goal of metastring theory is to construct a sigma model as a phase space formulation of the string and its dynamics.

The implications of metastring theory, as they have so far been conjectured, are intriguing. For example, there have been claims toward obtaining a family of models with a 3+1-dimensional de Sitter spacetime, argued to be realised in the standard tree-level low-energy limit of string theory in the case of a non-trivial anisotropic axion-dilaton background [29]. A key statement here is that, while string theory has purely stringy degrees of freedom (from first principles consider simply the difference between the left and right-moving string modes), these are not captured by standard effective field theory approaches and their spacetime descriptions. Such approaches are usually employed when investigating de Sitter space. In the phase-space formulation of the metastring, these purely stringy degrees of freedom (generally chiral and non-commutating) are argued to be captured explictly. When it comes to the hope of obtaining an effective de Sitter background, one of the major claims in this non-commutative phase-space formalulation is how, in the doubled and generalised geometric description, the effective spacetime action translates directly into the see-saw formula for the cosmological constant. Furthermore, in this cosmic-string-like solution related to the concept of an emergent de Sitter space, it is argued that the metastring leads naturally to an expression of dark energy, represented by a positive cosmological constant to lowest order. Finally, it is argued that the intrinsic stringy non-commutativity provides a vital ingredient for an effective field theory that reproduces to lowest order the sequestering mechanism [29, 30] and thus a radiatively stable vacuum energy.


Building from the Tseytlin action (21), this world-sheet theory of chiral bosons not only takes the heterotic string to its maximal logical completion (a point to be discussed another time), the total doubled space that it sees naturally accomodates stringy non-geometries. With the development of DFT and Hull’s doubled formalism in mind, one interesting question that we can ask concerns whether the best features of all of these approaches can be put together under a more general formulation. There is already a lot in Tseytlin’s original first-principle construction, and so one idea is to generalise from this theory. This was one motivation for my MRes thesis. Another question concerns the presence of generalised geometry and finally how, given a completely generalised treatment of the duality symmetric string, how may we extend the ideas toward the study of duality symmetric M-theory, where exceptional field theory seeks to promote the U-duality group to a manifest symmetry of the spacetime action [37, 38].

These comments take us back to the work of Berman et al. [18], who started to point toward the same question of generalisation in their approach that combines Tseytlin’s action with Hull’s doubled formalism. It is a very interesting entry into the ideas described, and it is this paper where my own MRes thesis more or less entered the picture.

Moreover, the approach in my MRes was basically to follow the prescription first adopted by Berman et al; however, the action they used to study the doubled beta-functionals for the interacting chiral boson model was constructed in the case where the background fields depend trivially on the doubled coordinates but non-trivially on the non-compact spacetime coordinates. This means that in their approach the target-space was constructed in terms of a torus fibration {T^{n}} over a base {N} . One may think of this as a description of string theory in which the target space is locally a {T^n} bundle, while {N} is some generic base manifold that may be thought of simply as a base space.

While such constructions are important and deserve attention moving forward – we will certainly discuss cases in the future of more complicated bundles, for example – for my MRes the idea was to first strip everything back and generalise the result with minimal assumptions. The first step, for example, was to not demand anything about the dependence of the background fields. What we arrived at was an action of the form

\displaystyle  S_{Maximally \ doubled} = \frac{1}{2} \int d^{2}\sigma [-\mathcal{H}_{AB}(X^{A}) \partial_{1} X^{A} \partial_{1} X^{B} + L_{AB}(X^{A}) \partial_{1}X^{A} \partial_{0} X^{B}], \ (22)

where {\mathcal{H}} is the generalised metric and we also have a generic 2-tensor {L} (that we continued to treat generically). In doing away with a base-fibre split (we also dropped a topological term, which isn’t so important here), what we have is the sort of action considered originally by Tseytlin. In fact, (22) is the most general doubled action we can write without manifest Lorentz invariance, because it allows us to calculate the background fields in a way in which the fields maintain arbitrary dependence on the full doubled geometry. That is to say, in taking the democratic approach in which everything becomes doubled, we’re ultimately seeking an effective spacetime theory that corresponds to completely generic non-geometric geometries. At the same time, the structure of the action is precisely the sort proposed to lead directly to DFT [20], and it also remains equivalent to the Polyakov action in the standard formulation of string theory.

Due to the fact that there are papers pending on these calculations and associated topics, I will leave more details for future entries and for when they more formally appear on arxiv.


[1] Arkady A. Tseytlin. Duality Symmetric Formulation of String World Sheet Dy-namics.Phys. Lett. B, 242:163–174, 1990.

[2] Arkady A. Tseytlin. Duality symmetric closed string theory and interactingchiral scalars.Nucl. Phys. B, 350:395–440, 1991.

[3] Warren Siegel. Superspace duality in low-energy superstrings.Phys. Rev. D,48:2826–2837, 1993.

[4] Warren Siegel. Two vierbein formalism for string inspired axionic gravity.Phys.Rev. D, 47:5453–5459, 1993.

[5] M.J. Duff. Duality Rotations in String Theory.Nucl. Phys. B, 335:610, 1990.

[6] Taichiro Kugo and Barton Zwiebach. Target space duality as a symmetry ofstring field theory.Prog. Theor. Phys., 87:801–860, 1992.

[7] Barton Zwiebach. Closed string field theory: An introduction, 1993.

[8] Theodore Erler. Four lectures on closed string field theory.Physics Reports,851:1–36, Apr 2020.

[9] Nigel Hitchin. Lectures on generalized geometry, 2010.

[10] Nigel Hitchin. B-fields, gerbes and generalized geometry, 2005.

[11] Chris Hull and Barton Zwiebach. Double field theory.Journal of High EnergyPhysics, 2009(09):099–099, Sep 2009.

[12] Chris Hull and Barton Zwiebach. The gauge algebra of double field theory andcourant brackets.Journal of High Energy Physics, 2009(09):090–090, Sep 2009.

[13] Olaf Hohm, Chris Hull, and Barton Zwiebach. Generalized metric formulationof double field theory.JHEP, 08:008, 2010.

[14] Olaf Hohm, Chris Hull, and Barton Zwiebach. Background independent actionfor double field theory.Journal of High Energy Physics, 2010(7), Jul 2010.

[15] Arkady A. Tseytlin. Duality symmetric string theory and the cosmological con-stant problem.Phys. Rev. Lett., 66:545–548, 1991.12.

[16] Christopher M Hull. A Geometry for non-geometric string backgrounds.JHEP,10:065, 2005.

[17] Christopher M Hull. Doubled Geometry and T-Folds.JHEP, 07:080, 2007.

[18] David S. Berman, Neil B. Copland, and Daniel C. Thompson. Background fieldequations for the duality symmetric string.Nuclear Physics B, 791(1-2):175–191,Mar 2008.

[19] David S. Berman and Daniel C. Thompson. Duality symmetric strings, dilatonsand o(d,d) effective actions.Physics Letters B, 662(3):279–284, Apr 2008.

[20] Neil B. Copland. A Double Sigma Model for Double Field Theory.JHEP, 04:044,2012.

[21] Spyros D. Avramis, Jean-Pierre Derendinger, and Nikolaos Prezas. Conformalchiral boson models on twisted doubled tori and non-geometric string vacua.Nuclear Physics B, 827(1-2):281–310, Mar 2010.

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

[23] Laurent Freidel, Felix J. Rudolph, and David Svoboda. Generalised kinematicsfor double field theory.Journal of High Energy Physics, 2017(11), Nov 2017.

[24] Laurent Freidel, Robert G. Leigh, and Djordje Minic. Quantum gravity, dynam-ical phase-space and string theory.International Journal of Modern Physics D,23(12):1442006, Oct 2014.

[25] David Svoboda. Algebroid structures on para-hermitian manifolds.Journal ofMathematical Physics, 59(12):122302, Dec 2018.

[26] Laurent Freidel, Robert G. Leigh, and Djordje Minic. Metastring theory andmodular space-time, 2015.

[27] Laurent Freidel, Robert G. Leigh, and Djordje Minic. Modular Spacetime andMetastring Theory.J. Phys. Conf. Ser., 804(1):012032, 2017.

[28] Laurent Freidel, Robert G. Leigh, and Djordje Minic. Noncommutativity ofclosed string zero modes.Physical Review D, 96(6), Sep 2017.

[29] Per Berglund, Tristan H ̈ubsch, and Djordje Mini ́c. On stringy de sitter space-times.Journal of High Energy Physics, 2019(12), Dec 2019.

[30] Per Berglund, Tristan H ̈ubsch, and Djordje Mini ́c. Dark energy and string theory.Physics Letters B, 798:134950, Nov 2019.13

[31] Roberto Floreanini and Roman Jackiw. Selfdual Fields as Charge Density Soli-tons.Phys. Rev. Lett., 59:1873, 1987.

[32] Franco Pezzella. Two double string theory actions: Non-covariance vs. covari-ance, 2015.

[33] Gianguido Dall’Agata and Nikolaos Prezas.Worldsheet theories for non-geometric string backgrounds.Journal of High Energy Physics,2008(08):088–088, Aug 2008.

[34] L. De Angelis, G. Gionti S.J, R. Marotta, and F. Pezzella. Comparing doublestring theory actions.Journal of High Energy Physics, 2014(4), Apr 2014.

[35] Joseph Polchinski.String theory. Vol. 1: An introduction to the bosonic string.Cambridge Monographs on Mathematical Physics. Cambridge University Press,12 2007.

[36] Barton Zwiebach. Double Field Theory, T-Duality, and Courant Brackets.Lect.Notes Phys., 851:265–291, 2012.

[37] David S. Berman and Malcolm J. Perry. Generalized Geometry and M theory.JHEP, 06:074, 2011.

[38] Olaf Hohm and Henning Samtleben. Exceptional Form of D=11 Supergravity.Phys. Rev. Lett., 111:231601, 2013.

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.

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.

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.


[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.

Roger Penrose, Reinhard Genzel, and Andrea Ghez share the Nobel Prize in Physics

I am absolutely delighted for Roger Penrose. He has contributed so much to mathematical and theoretical physics, it is a surprise to me that he hadn’t won the Nobel Prize sooner.

When I was first getting into mathematical physics twistor theory, originally proposed by Penrose in 1967, was a topic that I had been obsessed about, particularly as it then came to twistor string theory and the amplituhedron. It remains a notebook project for the future, but the great enthusiasm and interest I developed for the twistor programme I think aided my focus in mathematical physics before arriving at university. The geometry of twistors and the twistor description of massless fields, as well as his description of Grassmannian space and Grassmann algebra were things that left me inspired. In hindsight it is nice to think about how, as twistor space is chiral and treats the left and right handed parts of physical fields differently, my current thesis is building on what I think is a wider project involving the chiral string which is not so far removed.

In the world of modern mathematical and theoretical physics it seems one is never too far from a Penrose contribution. It was not too long ago when I was thinking about the Ward-Penrose transform, which has a wonderful relation to the geometry of strings. Then of course there are the beautiful Penrose Tilings. But I don’t think the discussion is complete without also recognising the importance of his views on different scientific and philosophical matters. Whether one agrees or disagrees, Penrose’s views on quantum mechanics and objective collapse, with his theory of dual fields, serve as more than sampling food for thought. Additionally, I’ve personally taken a lot from his views on mathematical Platonism in which he relates the Platonic mathematical to the physical and the physical to the mental, and so on. The topic of a separate essay, there is quite a lot here to unpack and develop in the context of the wider history of mathematical thought.

For the layperson or even the engaged student of physics, The Road to Reality is one popular book that I would not hesitant to recommend. It is a proper masterclass, which goes into many important ideas in modern physics and does not spare on the mathematical details. The stories of Am-tep and Amphos in the prologue followed by the discussion on the roots of science is perhaps one of my favourite introductions to a book.

Having said all that, it is not possible to write a post celebrating the work of Penrose without mention of the singularity theorems for which the Nobel Prize was awarded. He showed that, as a consequence of General Relativity, black holes are a deeply general phenomena predicted to emerge in cases of gravitational collapse irrespective of symmetry. A key concept here is that of a singular manifold and trapped surface as related to geodesic incompleteness.

The proof of the Penrose singularity theorem is quite nice. There isn’t enough space here to detail it in full, but a summary of what the proof looks like might be a nice way to conclude the present entry (should one like to work through the complete proof here is a good text as well as notes and review).

First some definitions. Let us consider some spacetime (M,g), which we may think of as being singular if it is not geodesically complete. Now, in a global analysis, let us define (M,g) to be a globally hyperbolic spacetime in which we denote S the Cauchy hypersurface and n some future pointing unit normal vector field. For a compact 2-dimensional submanifold \Sigma \in S with unit normal vector field v, the proposition is that v is trapped if the expansions \theta_{+} and \theta_{-} of the null geodesics are negative everywhere on \Sigma. The initial conditions for the null geodesics are assumed to be n + v and n - v, and they satisfy the null energy condition.

The proof then goes something like this. Let t : M \rightarrow \mathbb{R} be a global time function such that S = t^{-1}(0). The integral curves of grad \ t are timelike, and they only intersect the hypersurface S once and \partial I^{+}(\Sigma) once, where I^{+}(\Sigma) is an open set and defines the chronological future along each point of the compact surface. What we obtain is a continuous injective map with an open image \pi : \partial I^{+}(\Sigma) \rightarrow S. What is really cool is that if we have a point p that chronologically precedes the point q, with the future-directed chronological (timelike) curve from p to q, following a number of definitions and corollaries, it can be shown that should q = \pi(p) then in some neighbourhood of q are images of points in \partial I^{+}(\Sigma). In the full proof one can then show that there is a contradiction in the intersection of the integral curves of grad \ t, and since \Sigma is trapped there exists some \theta <  0 such that the null geodesics \theta^{+} and \theta_{-} orthogonal to \Sigma satisfy \theta^{+}, \theta^{-} \leq 0. One can complete the proof from this point, finding that indeed should (M,g) be a connected globally hyperbolic spacetime, where the Cauchy hypersurface S is non-compact and satisfies the null energy condition typically written T^{\mu \nu} n_{\mu} n_{\nu } \geq 0, then if S contains the trapped surface \Sigma the spacetime is singular. Like I said, it is quite nice!

Before this post grows too long. I would also like to leave a special note congratulating Reinhard Genzel and Andrea Ghez in sharing the Nobel Prize in Physics. I cannot profess to have read their papers, but a review of their history and extensive work shows that for decades they have made significant contributions to long-term scientific research focusing on Sagitarius A*! The astronomers in the blogosphere will be able to speak more to the methods both Genzel and Ghez have developed, as well as the technicalities of the brilliant techniques they have prioneered along the way to providing the most convincing evidence to date of the presence of a supermassive black hole at the centre of our very own galaxy! For myself, I look forward to reading a few of their respective papers. Meanwhile, here is a 2017 article discussing some of their research.