On the Pre- and Promonoidal Structure of Spacetime

TL;DR

This paper develops a categorical framework for spacetime using promonoidal categories, allowing for a generalized notion of joint systems that includes both actual and virtual systems, extending previous partial monoidal approaches.

Contribution

It introduces a concrete formulation of spacetime with promonoidal structures for all system pairs, incorporating virtual systems and two logical ways to combine systems, broadening the categorical modeling of spacetime.

Findings

Representable presheaves correspond to actual systems from spacelike combinations.

General presheaves model virtual systems with logical properties.

Two methods of combining systems relate to relativistic conjunction and disjunction.

Abstract

The notion of a joint system, as captured by the monoidal (a.k.a. tensor) product, is fundamental to the compositional, process-theoretic approach to physical theories. Promonoidal categories generalise monoidal categories by replacing the functors normally used to form joint systems with profunctors. Intuitively, this allows the formation of joint systems which may not always give a system again, but instead a generalised system given by a presheaf. This extra freedom gives a new, richer notion of joint systems that can be applied to categorical formulations of spacetime. Whereas previous formulations have relied on partial monoidal structure that is only defined on pairs of independent (i.e. spacelike separated) systems, here we give a concrete formulation of spacetime where the notion of a joint system is defined for any pair of systems as a presheaf. The representable presheaves correspond precisely to those actual systems that arise from combining spacelike systems, whereas more general presheaves correspond to virtual systems which inherit some of the logical/compositional properties of their ``actual'' counterparts. We show that there are two ways of doing this, corresponding roughly to relativistic versions of conjunction and disjunction. The former endows the category of spacetime slices in a Lorentzian manifold with a promonoidal structure, whereas the latter augments this structure with an (even more) generalised way to combine systems that fails the interchange law.