Introduction to Non-Commutative Geometry

I gave a course of lectures at the ESI in Vienna on Non-Commutative Geometry in October 2022. The draft lecture notes are uploaded here. A similar series of lectures will be given at the Galileo Galilei Institute June 12-16th 2023.

Chapters: 1. Introduction 2. Fuzzy spaces with symmetry 3. Commutative analogues 4. Dirac operator 5. Real structures 6. Real spectral triples 7. Standard model charges 8. Standard Model masses 9. The fuzzy sphere 10. The fuzzy torus (+4 more chapters to come)

The old October version is here for those that did the coursework.

Please let me know any corrections or improvements via email.

Hope and glory

In a given field of research, there’s a big distinction to be made between the scientific achievements in that field and the future prospects for the field. The “glory” is a matter of record, the stuff of review articles, whilst the “hope” is a matter of opinion. So what’s the role for opinion in scientific research?

In very pure mathematics there’s a presumption against expressing your opinions (in print anyway) in case it misleads readers, who would rather construct their own (and perhaps very different) picture of how the subject might develop. This certainly means that opinion and fact are never confused, but the problem is that there is consequently no debate about the aims of a subject or about the value of what has been achieved.

One might think that opinion has no place in science but that isn’t really right. Treating every possible avenue of future research as equally worthy of attention is not what we do; it would be a good way to waste resources. Every scientist has to make a judgement about what is worth doing and what is not. So we have opinions on the way forward, and also often share them when we communicate science. It seems to succeed in a subject it is necessary to be optimistic about its prospects, and to convince others that what you are doing is worthwhile you have to communicate that optimism.

My view is that expressing that optimism is quite legitimate but one should be careful not to let it be confused with the story about what has been achieved.

So where are we in quantum gravity research? As far as the “glory” is concerned: nowhere. Approaches based on quantum field theory have lots of “approximation” schemes but no central definition (so one cannot say they are actually approximating anything). Superstring theory does not even have a complete perturbation series, and a general non-perturbative formulation does not exist. Loop quantum gravity does not exist either; while there is a description of  a state space that is “too big”, there is no definition of the constraints that should reduce this space to the physical states. Discrete approaches such as spin foam models avoid troubles with the continuum, but the models that have been investigated so far are not supposed to be a complete physical theory; there have both technical and conceptual shortcomings.

As for the “hope”, there are plenty of reasons to continue to work in these fields. For example, superstring perturbation theory is the subject of current research and there are grounds to hope that a more complete perturbation theory will emerge. And maybe this will help in looking for the non-perturbative theory. String theorists could do themselves a favour by dropping the pretence that a complete string theory exists. After all, how can you recruit people to work on this problem if the wider scientific community thinks it has all been done already?

However I’m not going to get excited about superstrings because experimental tests of supersymmetry have a habit of coming out negative. Whilst this does not prove the absence of supersymmetry, in my opinion it is not looking very likely.

I’m continuing to work on the overlap between higher category theory and quantum physics. Some of this is work on understanding the spin foam models of quantum gravity. But I think there is more physics to be extracted from higher category theory. Indeed the mathematics is already deeply involved with both topological quantum field theory and conformal field theory, both of which are mathematically well-defined.

Moreover, there is a lot of unexplored territory. For example, there are “defects” in topological models that are point-like, string-like, brane-like etc., and there is a role there for deformations of geometry that are non-commutative, non-associative or other such things that don’t even have names yet. There are non-commutative geometries in particle physics that look a whole lot more enticing than supersymmetry. How do these examples fit into categorical quantum field theories? I hope to find out.