Topological quantum field theory is an interest of mine which has grown in intensity over the last year. I have, however, made the unfortunate mistake of attempting to describe some of these concepts to family and friends with results ranging from eyes-glazed-over incomprehension to “That’s really cool but my brain hurts now.” As a result of this, I have decided to try to improve my exposition by way of writing a cursory introduction to the subject requiring little more than a familiarity with sets, functions, and vector spaces.
Disclaimer: the notes presented herein should not be taken to be rigorous or comprehensive in any way (or, even, necessarily coherent). I refer the interested reader to V. Turaev’s Quantum Invariants of Knots and 3-Manifolds or any of John Baez’s excellent papers on the topic. Although this post is primarily mathematical in its intent, I will follow the latter’s approach to the heuristics of topological quantum field theory.
For the physically inclined, topological quantum field theory – or, more properly, the theory of topological quantum field theories, which I shall abbreviate as TQFTs – makes rigorous certain analogies between Einstein’s theory of general relativity (i.e. gravity) and quantum mechanics. Historically, topological quantum field theory arose out of attempts to make rigorous the notion of a Feynman path integral, a problem which has proven enormously difficult owing to the nonexistence of certain complex valued measures. (I may write a post on measure theory at some point, though I reserve the right not to.)
Formally a topological quantum field theory is a functor from the category of -cobordisms to the category of Hilbert spaces obeying certain axioms. However, this definition requires a great deal of machinery largely inaccessible to the layman and even to most undergraduate students of either mathematics or physics so we must first develop that here.
In order to accomplish this, corners must be cut and many essential theorems completely ignored but that is, sadly, the price one must pay for even the vaguest semblance of brevity.
We begin, then, with the notion of a topological space. Let be a set and denote by the set of subsets of . We say that is a topology on if it is closed under arbitrary unions and finite intersections and contains both the empty set and itself, and we refer to its elements as open sets. That is to say that if is a collection of open sets, then and if , then . A topological space is then an ordered pair consisting of a set and a topology on – by abuse of notation, we will often write when we mean .
Let and be topological spaces. We say that a function is continuous if, given an open set , the preimage of under is open.
A homeomorphism between topological spaces is a bicontinuous bijection from to . This means that is continuous, as is its inverse, and we additionally have that and if , then .