The most compelling aspect of Quantum Topology for me is its connection to analytic number theory. Today I’d like to draw your attention to recent work of Minhyong Kim on Arithmetic Chern-Simons Theory (see his paper for more details). I was fortunate to hear him give a talk about this last Wednesday and a colloquium talk on related subjects the day before. People have been talking about such “quantum topological number theory” for a long time- e.g. this 2010 MO question – but we haven’t seen much of an uptake so far. This isn’t an easy direction to pursue because one needs to know both quantum topology and analytic number theory, but I was left with the strong feeling that “There’s gold in them thar hills”, both for topologists and for number theorists.
Chern-Simons theory is concerned with canonically associating functions to representations. In a typical topological context, we would be looking at the moduli space of representations of a knot group into a group such as . The associated functions are topological invariants, and such invariants are of primary interest in quantum topology. Examples of invariants arising as or from such functions are the Jones polynomial and the Alexander polynomial (indirectly; what actually shows up is the square root of analytic torsion).
I find it deeply unsatisfying that Chern-Simons theory for topologists (indeed all of quantum topology) all happens over the complex numbers (some stuff can happen over or , but that’s as far as it goes). There is no conceptual justification for introducing complex numbers that I can see- for technical reasons we just seem to need the complex structure on the moduli space in order to be able to prove anything. There have been various attempts to study quantum topology over other fields or over rings such as the integers, but as far as I know the results are weak. For example, a fundamental result in quantum knot topology, that Vassiliev invariants are uniquely specified by weight systems, is only known over a few fields, as lamented e.g. by Bar-Natan.
In analytic number theory the goal is once again to canonically associate functions to representations. The parallel nature of the task is striking- the role of the knot complement is taken by an arithmetic scheme , and the role of the group is played by a so-called motivic sheaf which is uniquely built up from a representation of the arithmetic fundamental group of . On the other side, the properties the canonical functions must satisfy nicely parallel those of Chern-Simons theory.
The functions thus constructed, assuming that they exist, are topological invariants of . No general construction for these functions is known, but L-functions, whose constructions have been via ad-hoc methods, are examples. In fact, two major conjectures in analytic number theory, the Iwasawa Main Conjecture and the Hasse-Weil Conjecture, can both be framed as conjectures that such a canonical assigment of functions to representations exists.
I ought to mention parenthetically that quantum topological analytic number theory has already happened, when Le and Murakami used the Kontsevich invariant to discover relations between multiple-zeta values that had not been known previously, which analytic number theorists have since assimilated (see this recent survey by Furusho).
Arithmetic Chern-Simons promises to be exciting for both communities. For topologists, we may dream of a more flexible version of Chern-Simons theory which works over more general rings than just the complex numbers (although sadly we have a dearth of good conjectures in this direction at the moment). For number theorists, perhaps quantum topology can provide ideas to help attack some conjectures of interest. One may fantasize that, by way of these goals, we will gain an understanding of how knots and 3-manifolds fit into the main body of the big picture of mathematics, and how they might act special model cases not only in topology, but perhaps also in number theory.