FQFT and cohomology
Types of quantum field thories
|analytic integration||cohomological integration|
|measurable space||Poincaré duality|
|measure||orientation in generalized cohomology|
|volume form||(virtual) fundamental class|
|Riemann/Lebesgue integration of differential forms||push-forward in generalized cohomology/in differential cohomology|
The notion of path integral originates in and is mainly used in the context of quantum mechanics and quantum field theory, where it is a certain operation supposed to model the notion of quantization.
The idea is that the quantum propagator – in FQFT the value of the functor on a certain cobordism – is given by an integral kernel where is something like the integral of the exponentiated action functional over all field configurations with prescribed boundary datat and . Formally one writes
and calls this the path integral. Here the expression is supposed to allude to an measure integral on the space of all . The main problem with the path integral idea is that it is typically unclear what this measure should be, or, worse, it is typically clear that no suitable such measure does exist.
The name path integral originates from the special case where the system is the sigma model describing a particle on a target space manifold . In this case a field configuration is a path in , hence the integral over all field configurations is an integral over all paths.
The idea of the path integral famously goes back to Richard Feynman, who motivated the idea in quantum mechanics. In that context the notion can typically be made precise and shown to be equivalent to various other quantization prescriptions.
The central impact of the idea of the path integral however is in its application to quantum field theory, where it is often taken in the physics literatire as the definition of what the quantum field theory encoded by an action functional should be, disregarding the fact that in these contexts it is typically quite unclear what the path integral actually means, precisely.
Notably the Feynman perturbation series summing over Feynman graphs is motivated as one way to make sense of the path integral in quantum field theory and in practice usually serves as a definition of the perturbative path integral.
We start with stating the elementary description of the Feynman-Kac formula? as traditional in physics textbooks in
A simple form of the path integral is realized in quantum mechanics, where it was originally dreamed up by Richard Feynman and then made precise using the Feynman-Kac formula?. (Most calculations in practice are still done using perturbation theory, see the section Perturbatively in BV-formalism below).
The Schrödinger equation says that the rate at which the phase of an energy eigenvector rotates is proportional to its energy:
Therefore, the probability that the system evolves to the final state after evolving for time from the initial state is
Chop this up into time steps and use the fact that
Assume we have the free Hamiltonian Looking at an individual term we can insert a factor of 1 and solve to get
and letting we get
For arbitrary Hamiltonians we get
where is the action functional.
Is there an easy way to see how the Hamiltonian transforms into the Lagrangian in the exponent?
The Wiener measure on the space of stochastic paths in ,we may write suggestively write as
for it combines what in the physics literature is the kinetic action and a canonical measure on paths.
(This is a general phenomenon in formalizations of the process of quantization: the kinetic action (the free field theory-part of the action functional) is absorbed as part of the integration measure against with the remaining interaction terms are integrated. )
the integral kernel for the time evolution propagator is
This “holonomy integrated against the Wiener measure” is the path integral in the form in which it notably appears in the worldline formalism for computing scattering amplitudes in quantum field theory. See (Strassler 92, (2.9), (2.10)). Notice in particular that by the discussion there this is the correct Wick rotated form: the kinetic action is not a complex phase but a real exponential while the gauge interaction term (the holonomy) is a complex phase (locally ).
From the point of view of higher prequantum field theory this means that the path integral sends a correspondence in the slice (infinity,1)-topos of smooth infinity-groupoids over the delooping groupoid
(essentially a prequantized Lagrangian correspondence) to another correspondence, now in the slice over the stack (now an actual 2-sheaf) of modules over the complex numbers, hence of complex vector bundles:
For more discussion along these lines see at motivic quantization.
BV-BRST formalism is a means to formalize the path integral in perturbation theory as the passage to cochain cohomology in a quantum BV-complex. See at The BV-complex and homological integration for more details.
|action functional||kinetic action||interaction||path integral measure|
|BV differential||elliptic complex +||antibracket with interaction +||BV-Laplacian|
Ours is the age whose central fundamental theoretical physics question is:
What is quantum field theory?
A closely related question is:
What is the path integral ?
After its conception by Richard Feynman in the middle of the 20th century It was notably Edward Witten’s achievement in the late 20th century to make clear the vast potential for fundamental physics and pure math underlying the concept of the quantum field theoretic path integral.
And yet, among all the aspects of QFT, the notion of the path integral is the one that has resisted attempts at formalization the most.
While functorial quantum field theory is the formalization of the properties that the locality and the sewing law of the path integral is demanded to have – whatever the path integral is, it is a process that in the end yields a functor on a (infinity,n)-category of cobordisms – by itself, this sheds no light on what that procedure called “path integration” or “path integral quantization” is.
The single major insight into the right higher categorical formalization of the path integral is probably the idea indicated in
which says that
it is wrong to think of the action functional that the path integral integrates over as just a function: it is a higher categorical object;
accordingly, the path integral is not something that just controls the numbers or linear maps assigned by a -dimensional quantum field theory in dimension : also the assignment to higher codimensions is to be regarded as part of the path integral;
More recently, one sees attempts to formalize this observation of Freed’s, notably in the context of the cobordism hypothesis:
and something similar or is indicated in section 3 and section 6 of
based on material (on categories of “families”) in On the Classification of Topological Field Theories .
The original textbook reference is
Lecture notes include
Textbook accounts include
G. Johnson, M. Lapidus, The Feynman integral and Feynman’s operational calculus, Oxford University Press, Oxford, 2000.
Barry Simon, Functional integration and quantum physics AMS Chelsea Publ., Providence, 2005
Vassili Kolokoltsov, Path integration: connecting pure jump and Wiener processes (pdf)
Bruce Driver, Anton Thalmaier, Heat equation derivative formulas for vector bundles, Journal of Functional Analysis 183, 42-108 (2001) (pdf)
The following articles discuss (aspects of) the path integral for the charged particle coupled to a background gauge field, in which case the path integral is essentially the integration of the holonomy/parallel transport functional against the Wiener measure.
Marc Arnaudon and Anton Thalmaier, Yang–Mills fields and random holonomy along Brownian bridges, Ann. Probab. Volume 31, Number 2 (2003), 769-790. (Euclid)
Dana Fine, Stephen Sawin, A Rigorous Path Integral for Supersymmetric Quantum Mechanics and the Heat Kernel (arXiv:0705.0638)
following Norris 92, theorem (34).
Other references on mathematical aspects of path integrals include
Sergio Albeverio, Raphael Hoegh-Krohn, Sonia Mazzucchi, Mathematical theory of Feynman path integrals. An introduction (ZMATH
Lecture notes on quantum field theory, emphasizing mathematics of the Euclidean path integrals and the relation to statistical physics are at
MathOverflow questions: mathematics-of-path-integral-state-of-the-art,path-integrals-outside-qft, doing-geometry-using-feynman-path-integral, path-integrals-localisation, finite-dimensional-feynman-integrals, the-mathematical-theory-of-feynman-integrals