Bohrification of operator algebras and quantum logic Chris Heunen · Nicolaas P. Landsman · Bas Spitters

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 × 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” A 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 measures on the lattice of projections on a
Hilbert space H and probability valuations on the internal Gelfand spectrum of A for
A = B(H).