# From Poisson Geometry to Quantum Fields on Noncommutative Spaces

I will be attending the autumn school “From Poisson Geometry to Quantum Fields on Noncommutative Spaces” Oct 05–10, in Würzburg, Germany.

There will be a series of lectures:

• Francesco D’Andrea (University of Naples)
Topics in Noncommutative Differential Geometry
• Martin Bordemann (Univ. Haute Alsace, Mulhouse)
Algebraic Aspects of Deformation Quantization
• Henrique Bursztyn (IMPA, Rio de Janeiro)
Poisson Geometry and Beyond
• Simone Gutt (ULB, Brussels)
Symmetries in Deformation Quantization
• Gandalf Lechner (University of Cardiff)
Strict Deformation Quantization and Noncommutative Quantum Field Theories
• Eva Miranda (University of Barcelona)
Poisson Geometry and Normal Forms: A Guided Tour through Examples

It should be very interesting and I hope to learn a lot about subjects that are aligned with my general research area, but alas I have not yet looked into properly.

Also I will be presenting a poster on ‘Graded bundle in the category of Lie groupoids’ which is based on recent work with K. Grabowska and J. Grabowski (arXiv preprint)

The website for the school states that places may still be available.

# On a variant of rhodonea curves

Rhodonea curves or rose curves are plots of a polar equation of the form
$$r = \cos(k \theta)$$.

If we specialise to equations with

$$k= \frac{n}{d}$$

for n and d integers (>0), then we have plots of the form below. In the table n runs across and d down

Now, just for fun I considered a slight variant of this given by

$$r = \cos( k \theta) – k$$

The plots are as follows

For another variant I considered

$$r = \cos( k \theta) – k^{-1}$$

I am not sure there is anything mathematically deep here, I just like the images and classify this as some basic mathematical art.

# On the physics of chocolate

 Researchers at Technische Universität München, Germany, have reported that molecular dynamics can be used to gain new insights into the chocolate conching [1].

Chocolate conching is the stage of manufacturing where aromatic sensation, texture and mouthfeel are developed.

This work seems to be the first to attempt to properly understand the role of lecithins in chocolate production.

Physics, helping to build a tasty more palatable world.

Reference
[1] M Kindlein, M Greiner, E Elts and H Briesen, Interactions between phospholipid head groups and a sucrose crystal surface at the cocoa butter interface, 2015 J. Phys. D: Appl. Phys. 48 384002.

Chocolate physics: how modelling could improve mouthfeel, IOP website.

# An interview with Prof. Christopher Lintott

 Prof. Christopher Lintott of Oxford University, winner of this years Kelvin Medal and Prize from the Institute of Physics and regular on the BBC’s Sky at Night agreed to answer a few questions.

Science and Popularisation

1. What first got you involved in science, and in particular astronomy?

I was a small kid who loved looking through telescopes – first of all a neighbour’s small reflector, then a larger telescope at school. I loved the idea that we could understand what’s happening in space despite being stuck on the surface of a small insignificant planet – and that there was lots left for us still to find out.

2. What was your first telescope?

The same one I have now, a 6” reflector. It’s nothing fancy – it doesn’t even have a motor – but it allows me to explore the sky. I’m a great fan of astrophotography, but I spend too much time looking at my computer as it is. When I’m observing I want the photons to be hitting my eyeballs!

3. How did you get involved in the BBC’s Sky at Night?

I’d been doing some science writing and got invited to be a guest on the show. From there, I was lucky enough to be part of the team and I gradually did more and more. I think Sky at Night’s a wonderful show, with the chance to explore so many fascinating aspects of our relationship with the Universe.

4. What is ‘Citizen Science’?

It’s a modern term for an old idea, which is that anyone can participate in the scientific process. These days, we use the term to cover the kind of projects we build on Zooniverse.org – projects which allows professional astronomers and volunteers together to comb through the vast stores of data which modern surveys produce.

5. Which medium do you think is the most effective at popularising science?

It depends what you’re trying to do, but one of the things that I think we need to remember is that we can’t rely on people choosing to seek out scientific content. A large proportion of the public have been put off science through experiences at school, or through a lack of confidence, and we need to find ways to reach them. In the old days, that meant big budget TV shows, but now that audience is fragmenting we need to find new ways for people to stumble across science. As an example, the Adler Planetarium in Chicago runs a Telescopes in the City program in which they take scopes (and astronomers) to random locations, surprising people with the sky. I think that kind of experience can be life-changing.

6. What, in your opinion, should be the ultimate goal of science popularisation?

I’m not sure it’s an ultimate goal, but there are lots of people who I believe would enjoy following science as it happens, and maybe even participating. I want that crowd to feel like they’re part of the journey, rather than just consumers of pre-packaged scientific results. We just reported on the New Horizons encounter with Pluto, which threw up all sorts of wonderful surprises. Someone said to me that they hadn’t realised that scientists smile when they say they don’t know something – I’d like more people to participate in the joy of not knowing.

Research

These days I’m interested in galaxies – in particular, we’re trying to find out why some galaxies form stars and why some appear to have shut down. Most of this work is done with the wonderful data provided by Galaxy Zoo volunteers.

2. Which one of your papers are you most proud of, and why?

The discovery paper for the Voorwerp – I had to learn a lot to write it, and we had the most tremendous battles with the referee, but it came out well in the end. Plus it’s about a wonderful object, and consisted of what I thought astronomers did when I was a kid. Find an interesting object, and point telescopes at it until you know what it is…

# On contact and Jacobi geometry

 I have placed a preprint on the arXiv Remarks on contact and Jacobi geometry, which is joint work with K. Grabowska and J. Grabowski.

In the preprint we explain how the proper framework of contact and Jacobi geometry is that of $$\mathbb{R}^{\times}$$-principal bundles equipped with homogeneous Poisson structures. Note that in our approach homogeneity is with respect to a principal bundle structure and not just a vector field. This framework allows a drastic simplification of many standard results in Jacobi geometry while simultaneously generalising them to the case of non-trivial line bundles. Moreover, based on what we learned from our previous work, it became clear that this framework gives a very natural and general definition of contact and Jacobi groupoids.

The key concepts of the preprint are Kirillov manifolds and Kirillov algebroids, i.e. homogeneous Poisson manifolds and, respectively, homogeneous linear Poisson manifolds. Among other results we

• describe the structure of Lie groupoids with a compatible principal G-bundle structure
• present the `integrating objects’ for Kirillov algebroids
• define contact groupoids, and show that any contact groupoid has a canonical realisation as a contact subgroupoid of the latter

Our motivation
The main motivation for this work was to put some order and further geometric understanding into the subject of contact and Jacobi geometry. We take the ‘poissonisation’ as the true starting definition of a ‘Jacobi structure’ and accept all the consequences of that choice. Importantly, once phrased in the correct way, that is in terms of $$\mathbb{R}^{\times}$$-principal bundles and their actions, the true nature of Jacobi geometry as a specialisation and not a generalisation of Poisson geometry becomes clear.

Non-trivial line bundles makes it easier?
Almost oxymoronically, passing to structures on non-trivial line bundles and then the language of $$\mathbb{R}^{\times}$$-principal bundles really does simplify the overall understanding.

This is particularly evident for contact and Jacobi groupoids where insisting on working with a trivialisation leads to unnecessary complications.

In conclusion
We hope that this work will really convince people that contact and Jacobi geometry need not be as complicated as it is often presented in the literature. Quite often the constructions become very ‘computational’ and ‘algebraic’, and in doing so the underlying geometry is obscured. In this work we really try to stick to geometry and avoid algebraic computations.

# One nation science

The UK Science Minister Jo Johnson wants to reshape the funding of science and make sure that the funds are distributed wider.

Currently the ‘Golden Triangle’, Oxford, Cambridge and London’s UCL, Imperial and King’s receive 35% of the total £2.7bn annual funding.

I think that spreading the money could be a good idea, though it has to be done carefully and objectively.

# The 2nd Conference of the Polish Society on Relativity

 I will be attending the 2nd conference of the Polish Society of Relativity which will celebrate 100 years of general relativity.

The conference is in Warsaw and will be held over the period 23-28 November 2015.

The invited speakers include George Ellis, Roy Kerr, Roger Penrose and Kip Thorne. I am a little excited about this.

Registration is now open and you can follow the link below to find out more.

# Homotopy versions of Jacobi structures

 I have placed a preprint on the arXiv ‘Jacobi structures up to homotopy’ (arXiv:1507.00454 [math.DG]) which is joint work with Alfonso G. Tortorella (a PhD student from Universita degli Studi di Firenze, Italy). Our motivation for the work comes from the increasing presence of higher Poisson and Schouten structures in mathematical physics.

In the preprint we ask and answer the question of ‘how to equip sections of (even) line bundles over a supermanifold with the structure of an L-algebra’.

It turns out that the most conceptionally simple way to do this is to adopt the philosophy of [1] and study homogeneous higher Poisson geometry. In essence, we take the ‘higher Poissonisation’ of a ‘homotopy Kirillov structure’ as the starting definition. In this way we go around trying to carefully define homotopy Kirillov structure in the so-called ‘intrinsic set-up’; which would be quite complicated for non-trivial line bundles. We define a higher Kirillov manifold as a principal ℜx-bundle equipped with a homogeneous higher Poisson structure.

We also study the notion of a higher Kirillov algebroid, which is essentially a higher Kirillov manifold with an addition compatible regular action of ℜ. This additional action encodes a vector bundle structure [2].

Interestingly, from the structure of a higher Kirillov algebroid we derive a line bundle equipped with a ‘higher representation’ of an associated L-algebroid. This is very similar to the classical case of Jacobi algebroids and is a geometric realisation of Sh Lie-Rinehart representations as define by Vitagliano [4]. As a special example we see that the line bundle underlying a higher Kirillov manifold comes equipped with a higher representation of the first jet vector bundle of the said line bundle (which is naturally an L-algebroid).

Finally we present the higher BV-algebra associated with a higher Kirillov manifold following the ideas of Vaisman [3].

References
[1] J. Grabowski, Graded contact manifolds and contact Courant algebroids, J. Geom. Phys. 68 (2013) 27-58.

[2] J. Grabowski & M. Rotkiewicz, Higher vector bundles and multi-graded symplectic manifolds, J. Geom. Phys. 59 (2009), 1285-1305

[3] I. Vaisman, Annales Polonici Mathematici (2000) Volume: 73, Issue: 3, page 275-290.

[4] L. Vitagliano, Representations of Homotopy Lie-Rinehart Algebras, Math. Proc. Camb. Phil. Soc. 158 (2015) 155-191.

# Interview with Journal of Physics A

 The paper ‘Higher Order Mechanics on Graded Bundles’ has been chosen for ‘Publisher’s Pick’ by the Journal of Physics A.

As part of this, my collaborators and I have given a short interview about the motivation and reasoning behind the paper. You can read the interview at the link below.

# Riemannian Lie algebroids and harmonic maps

 I have placed a preprint on the arXiv ‘Killing sections and sigma models with Lie algebroid targets’ (arXiv:1506.07738 [math.DG]). In the paper I recall the notion of a Riemannian Lie algebroid, collect the basic theory and proceed to define Killing sections.

Lie algebroids are a generalisation of the tangent bundle of a manifold. The mantra here is that whatever you can do on a tangent bundle you can do on a Lie algebroid. This includes developing a theory of Riemannian geometry on them.

The notion of a Riemannian metric on a Lie algebroid is just that of a metric on the underlying vector bundle. There is no compatibility condition or anything like that. So, as all vector bundles can be equipped with metrics, all Lie algebroids can be given a metric. The interesting fact is that the Lie algebroid structure allows you to build the theory of Riemannian geometry in exactly the same way as you would on a standard Riemannian manifold. A Lie algebroid with a metric are known as Riemannian Lie algebroids.

In particular we have a good notion of torsion (which is generally missing) and have the notion of a Levi-Civita connection. Moreover, we have the fundamental theorem that says that such a connection is uniquely defined by metric compatibility and vanishing torsion, just as we have in the classical case. All the formula generalise directly with little fuss.

This all begs the question of developing general relativity on a Lie algebroid. Indeed one can formulate the Einstein field equations in this context, see [1]. The geometry here is clear and very neat, the applications to the theory of gravity are less clear.

Killing sections
Something I noticed that was generally missing in the literature was the notion of a Killing section of a Riemannian Lie algebroid. Such a section is a natural generalisation of a Killing field on a Riemannian manifold; they represent infinitesimal isometries. In the paper I show how the basic idea generalises to Riemannian Lie algebroids giving the notion of a Killing section. Moreover, I show how the common ways of expressing the notion of a Killing field directly generalise to Lie algebroids.

Sigma models and harmonic maps
With the above technology in place, I then look at the theory of sigma models that have a Riemannian Lie algebroid as their target. I took the work of Martinez [2] on classical field theory on Lie algebroids, and applied it to this class of theories. The basic idea is that the fields of such a theory are Lie algebroid morphisms from the tangent bundle of our source manifold to a Lie algebroid target. Equipping both the source and target with a metric allows us to build a model in exactly the same way as a standard sigma model on the space of maps between two Riemannian manifolds. The critical points of the Lie algebroid sigma model are seen to be a generalisation of harmonic maps.

I show, as expected, that the infinitesimal internal symmetries of the Lie algebroid sigma model are described by the Lie algebra of Killing section.

After thoughts
Non-linear sigma models represent a large class of models that have found applications in high energy physics, string theory and condensed matter physics. From a mathematical perspective, sigma models provide a strong link between differential geometry and field theory. In this work, I do not attempt to find such applications of the Lie algebroid sigma model, I focus on the differential geometry. However, studying such models seems very natural and hopefully useful.

References
[1] M. Anastasiei & M. Girtu, Einstein equations in Lie algebroids, Sci. Stud. Res. Ser. Math. Inform. 24 (2014), no. 1, 5-16.

[2] E. Martinez, Classical field theory on Lie algebroids: variational aspects, J. Phys. A: Math. Gen. 38 (2005) 7145.