*[This is akin to a living review, which will hopefully improve from time to time. Last edited 2020-4-8.]*

This post will collect some models of decoherence and branching. We don’t have a rigorous definition of branches yet but I crudely define models of branching to be models of decoherence^{a } which additionally feature some combination of amplification, irreversibility, redundant records, and/or outcomes with an intuitive macroscopic interpretation.

(Note in particular that I am *not* just listing models for which you can mathematically take a classical limit ( or ) and recover the classical equations of motion; Yaffe has a pleasingly general approach to that task [1], but I’ve previously sketched why that’s an incomplete explanation for classicality.)

I have the following desiderata for models, which tend to be in tension with computational tractability:

- physically realistic
- symmetric (e.g., translationally)
- no ad-hoc system-environment distinction
- Ehrenfest evolution along classical phase-space trajectories (at least on Lyapunov timescales)

Regarding that last one: we would like to recover “classical behavior” in the sense of classical Hamiltonian flow, which (presumably) means continuous degrees of freedom.… [continue reading]