Topological quantum field theory
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A topological quantum field theory (or topological field theory or TQFT) is a quantum field theory which computes topological invariants. In condensed matter physics, topological quantum field theories are the low energy effective theories of topologically ordered states, such as fractional quantum Hall states.
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[edit] Overview
In a topological field theory, the correlation functions do not depend on the metric on spacetime. This means that the theory is not sensitive to changes in the shape of spacetime; if the spacetime warps or contracts, the correlation functions do not change. Consequently, they are topological invariants. (Strictly speaking, the argument above only shows that they are diffeomorphism invariants. Showing that they are homotopy invariants takes more effort.)
Topological field theories are not very interesting on the flat Minkowski spacetime used in particle physics. Minkowski space can be contracted to a point, so a TQFT on Minkowski space computes only trivial topological invariants. Consequently, TQFTs are usually studied on curved spacetimes, such as, for example, Riemann surfaces. Most of the known topological field theories are defined on spacetimes of dimension less than five. It seems that a few higher dimensional theories exist, but they are not very well understood.
Although TQFTs were invented by physicists (notably Witten), they are primarily of mathematical interest, being related to, among other things, knot theory and the theory of four-manifolds in algebraic topology, and to the theory of moduli spaces in algebraic geometry. (Donaldson, Jones, Witten, and Kontsevich have all won Fields Medals for work related to topological field theory.) However, in condensed matter physics, TQFTs describe the internal order -- the topological order -- in fractional quantum Hall states, string-net condensed states, and other strongly correlated quantum liquid states.
Physically speaking, topological field theories are not especially interesting. They are toy models, far simpler in structure than the quantum field theories which describe real-world physics. They can be thought of as warm-up exercises for the much harder task of quantizing gravity. Quantum gravity is believed to be background-independent (in some suitable sense), and TQFTs provide examples of background independent quantum field theories. Unfortunately, they are only background independent in a fairly trivial sense: they are independent of the spacetime metric (thought of as a background gravitational field), but they do not admit any local degrees of freedom. There is no radiation in a TQFT: no propagating waves, no gluons, no gravitons.
(Caveat: It is often said that TQFTs have only finitely many degrees of freedom. This is not a fundamental property. It happens to be true in most of the examples that physicists and mathematicians study, but it is not necessary. A topological sigma model with target infinite-dimensional projective space, if such a thing could be defined, would have countably infinitely many degrees of freedom.)
[edit] Specific Models
The known topological field theories fall into two general classes: Schwarz-type TQFTs and Witten-type TQFTs. Witten TQFTs are also sometimes referred to as cohomological field theories.
[edit] Schwarz-type TQFTs
In Schwarz-type TQFTs, the correlation functions computed by the path integral are topological invariants because the path integral measure and the quantum field observables are explicitly independent of the metric. For instance, in the BF model, the spacetime is a two-dimensional manifold M, the observables are constructed from a two-form F, an auxiliary scalar B, and their derivatives. The action (which determines the path integral) is
The spacetime metric does not appear anywhere in this theory, so the theory is explicitly topologically invariant. Another, more famous example is Chern-Simons theory, which can be used to compute knot invariants.
[edit] Witten-type TQFTs
In Witten-type topological field theories, the topological invariance is more subtle. [ed. NEEDS FINISHING]
[edit] Mathematical formulations
[edit] Atiyah-Segal axioms
Atiyah suggested a set of axioms for topological quantum field theory which was inspired by Segal's proposed axioms for conformal field theory. These axioms have been relatively useful for mathematical treatments of Schwarz-type QFTs, although it isn't clear that they capture the whole structure of Witten-type QFTs. The basic idea is that a TQFT is a functor from a certain category of cobordisms to the category of vector spaces.
There are in fact two different sets of axioms which could reasonably be called the Atiyah axioms. These axioms differ basically in whether or not they study a TQFT defined on a single fixed n-dimensional Euclidean spacetime M or a TQFT defined on all n-dimensional spacetimes at once.
- [ed. What follows is still in rough draft form and should be regarded suspiciously.]
[edit] The case of a fixed spacetime
Let BordM be the category whose morphisms are n-dimensional submanifolds of M and whose objects are connected components of the boundaries of such submanifolds. Regard two morphisms as equivalent if they are homotopic via submanifolds of M, and so form the quotient category hBordM: The objects in hBordM are the objects of BordM, and the morphisms of hBordM are homotopy equivalence classes of morphisms in BordM. A TQFT on M is a functor from hBordM to the category of vector spaces.
Note that cobordisms can, if their boundaries match up, be sewn together to form a new bordism. We would like the functor to preserve this structure, so that a morph [ed. unfinished]
[edit] All n-dimensional spacetimes at once
To consider all spacetimes at once, it is necessary to replace hBordM by a larger category. So let Bordn be the category of bordisms, i.e. the category whose morphisms are n-dimensional manifolds with boundary, and whose objects are the connected components of the boundaries of n-dimensional manifolds. (Note that any (n-1)-dimensional manifold may appear as an object in Bordn.) As above, regard to morphisms in Bordn as equivalent if they are homotopic, and form the quotient category hBordn. Bordn is a monoidal category under the operation which takes two bordisms to the bordism made from their disjoint union. A TQFT on n-dimensional manifolds is then a functor from hBordn to the category of vector spaces, which takes disjoint unions of bordisms to the tensor product f [ed. unfinished]
[edit] Generalizations
For some applications, it is convenient to demand extra topological structure on the morphisms, such as a choice of orientation.
