# Contents

## Idea

The Borchers property is a property of local nets in the Haag-Kastler approach to quantum field theory that follows, by a theorem of Borchers, from three physically motivated axioms on local nets. If every local algebra of the net is a type III factor, then the Borchers property is also a direct consequence.

The Borchers property is therefore often used as an “intermediate technical assumption”.

Sometimes this property is also abbreviated as “property B”.

## Definition

Let $\mathcal{M}(\mathcal{O})$ be a net of von Neumann algebras indexed by bounded open subsets of Minkowski spacetime, for more details see Haag-Kastler vacuum representation,

###### Definition

The net $\mathcal{M}(\mathcal{O})$ satisfies the Borchers property if for every double cones $K_1, K_2$ with $\bar K_1 \subseteq K_2$ and $E \in \mathcal{M}(\mathcal{K}_1)$ a nonzero projection, $E$ is (Murray-von Neumann) equivalent to the identity with respect to $\mathcal{M}(\mathcal{K}_2)$. That is, there is a partial isometry $V \in \mathcal{M}(\mathcal{K}_2)$ such that $VV^* = E, V^*V = \mathbb{1}$.

## Properties

###### Proposition

A net satisfying causality, the spectrum condition and weak additivity (see Haag-Kastler vacuum representation for definitions) satisfies the Borchers property.

Remark : In particular the local algebras of nets that satisfy the Borchers property cannot be finite where finite is meant in the sense of the Murray-von Neumann classification of factors.

## References

• H.-J. Borchers: A remark on a theorem of B. Misra (available online from project euclid here)

Revised on June 30, 2010 10:42:23 by Tim van Beek (192.76.162.8)