Abstract
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 first review in detail some 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: 1.) the Bargmann superselection rule for overall mass in ordinary quantum mechanics, and 2.) the superselection rule for charge in quantum electrodynamics.
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