In quantum field theory, observables are associated with regions of spacetime, not with individual particles. This article introduces one of the simplest models in which that distinction really is important: a nonrelativistic model of multiple interacting particles with zero spin, all of the same species, sometimes called identical particles. A generalization to multiple species, with multiple particles of each species, is also introduced. Despite their shortcomings (they are nonrelativistic, they ignore the dynamics of the electromagnetic field, and their elementary particles all have zero spin), these models can still exhibit complex phenomena that resemble real molecules, fluids, and solids.
Article 15939 introduces a different way of constructing these models, using field operators.
