Bohrification of operator algebras and quantum logic

TL;DR

This paper develops a topos-theoretic approach to quantum logic by replacing the traditional non-distributive lattice of projections with a distributive Heyting algebra derived from commutative subalgebras, leading to an intuitionistic logic framework.

Contribution

It introduces a new logical framework for quantum mechanics using Bohrification and topos theory, generalizing previous matrix-based results to Rickart C*-algebras.

Findings

The Heyting algebra from commutative subalgebras forms a basis for the internal Gelfand spectrum.

The construction relates to partial Boolean algebras and Bruns-Lakser completions.

Probability measures on projections correspond to valuations on the internal spectrum.

Abstract

Following Birkhoff and von Neumann, quantum logic has traditionally been based on the lattice of closed linear subspaces of some Hilbert space, or, more generally, on the lattice of projections in a von Neumann algebra A. Unfortunately, the logical interpretation of these lattices is impaired by their nondistributivity and by various other problems. We show that a possible resolution of these difficulties, suggested by the ideas of Bohr, emerges if instead of single projections one considers elementary propositions to be families of projections indexed by a partially ordered set C(A) of appropriate commutative subalgebras of A. In fact, to achieve both maximal generality and ease of use within topos theory, we assume that A is a so-called Rickart C*-algebra and that C(A) consists of all unital commutative Rickart C*-subalgebras of A. Such families of projections form a Heyting algebra in a natural way, so that the associated propositional logic is intuitionistic: distributivity is recovered at the expense of the law of the excluded middle. Subsequently, generalizing an earlier computation for n-by-n matrices, we prove that the Heyting algebra thus associated to A arises as a basis for the internal Gelfand spectrum (in the sense of Banaschewski-Mulvey) of the "Bohrification" of A, which is a commutative Rickart C*-algebra in the topos of functors from C(A) to the category of sets. We explain the relationship of this construction to partial Boolean algebras and Bruns-Lakser completions. Finally, we establish a connection between probability measure on the lattice of projections on a Hilbert space H and probability valuations on the internal Gelfand spectrum of A for A = B(H).