# Local quantum field theory

The **Haag–Kastler axiomatic framework** for quantum field theory, introduced by Haag and Kastler (1964), is an application to **local quantum physics** of C*-algebra theory. Because of this it is also known as **algebraic quantum field theory** (**AQFT**). The axioms are stated in terms of an algebra given for every open set in Minkowski space, and mappings between those.

## Overview

Let **Mink** be the category of open subsets of Minkowski space M with inclusion maps as morphisms. We are given a covariant functor
from **Mink** to **uC*alg**, the category of unital C* algebras, such that every morphism in **Mink** maps to a monomorphism in **uC*alg** (**isotony**).

The Poincaré group acts continuously on **Mink**. There exists a pullback of this action, which is continuous in the norm topology of
(Poincaré covariance).

Minkowski space has a causal structure. If an open set *V* lies in the causal complement of an open set *U*, then the image of the maps

and

commute (spacelike commutativity). If
is the causal completion of an open set *U*, then
is an isomorphism (primitive causality).

A state with respect to a C*-algebra is a positive linear functional over it with unit norm. If we have a state over , we can take the "partial trace" to get states associated with for each open set via the net monomorphism. The states over the open sets form a presheaf structure.

According to the GNS construction, for each state, we can associate a Hilbert space representation of Pure states correspond to irreducible representations and mixed states correspond to reducible representations. Each irreducible (up to equivalence) is called a superselection sector. We assume there is a pure state called the vacuum such that the Hilbert space associated with it is a unitary representation of the Poincaré group compatible with the Poincaré covariance of the net such that if we look at the Poincaré algebra, the spectrum with respect to energy-momentum (corresponding to spacetime translations) lies on and in the positive light cone. This is the vacuum sector.

More recently, the approach has been further implemented to include an algebraic version of quantum field theory in curved spacetime. Indeed, the viewpoint of local quantum physics is in particular suitable to generalize the renormalization procedure to the theory of quantum fields developed on curved backgrounds. Several rigorous results concerning QFT in presence of a black hole have been obtained.

## List of researchers in local quantum field theory

- Detlev Buchholz
- Rudolf Haag
- Daniel Kastler
- Karl-Henning Rehren
- Bert Schroer
- Robert Wald
- Klaus Fredenhagen

## References

- Haag, Rudolf; Kastler, Daniel (1964), "An algebraic approach to quantum field theory",
*Journal of Mathematical Physics*,**5**: 848–861, Bibcode:1964JMP.....5..848H, doi:10.1063/1.1704187, ISSN 0022-2488, MR 0165864 - Haag, Rudolf (1996) [1992],
*Local quantum physics*, Texts and Monographs in Physics (2nd ed.), Berlin, New York: Springer-Verlag, ISBN 978-3-540-61451-7, MR 1405610

## External links

- Local Quantum Physics Crossroads 2.0 – A network of scientists working on local quantum physics
- Papers – A database of preprints on algebraic QFT
- Algebraic Quantum Field Theory – AQFT resources at the University of Hamburg