2000/10/26 by Domenico Giulini
Physics and Astronomy · #quant-ph #gr-qc #hep-th
published as Lect.Notes Phys. 559 (2000) 67-92 · 25 pages, LaTeX-2e, no figures, Springer style file <cl2emult.sty> needed
arxiv created 2000/10/26 · arxiv updated 2009/11/30
It is well known that the dynamical mechanism of decoherence may cause apparent superselection rules, like that of molecular chirality. These `environment-induced' or `soft' superselection rules may be contrasted with `hard' superselection rules, like that of electric charge, whose existence is usually rigorously demonstrated by means of certain symmetry principles. We address the question of whether this distinction between `hard' and `soft' is well founded and argue that, despite first appearance, it might not be. For this we give a detailed and somewhat pedagogical exposition of the basic structural properties of the spaces of states and observables in order to establish a fairly precise notion of superselection rules. We then discuss two examples: the Bargmann superselection rule for overall mass in ordinary quantum mechanics, and the superselection rule for charge in quantum electrodynamics.