Abstract

This chapter develops a case for attributing quantum field theory a particle ontology. When the space of physical states of the theory resides in a Fock space, the observables constituting a particle notion circumscribed by that Fock space are fundamental in the sense that they form a complete set, so that every state of the theory can be understood in terms of values assigned those observables. The Unruh effect, in which an observer accelerating uniformly through Minkowski spacetime in its vacuum (i.e. no-particle) state observes a thermal distribution of particles, is often presented as a challenge to particle interpretations. Arguing that the standard form of the challenge is miscast, the chapter recasts it in a stronger form: unitarily inequivalent Fock space representations circumscribe incommensurable particle notions, so if physical states reside in multiple, unitarily inequivalent Fock space representations, no single particle notion can be fundamental.