Quentin Ruyant

Classical chaos is frequently claimed to pose a problem for quantum-classical correspondence. Recent work on quantum decoherence purportedly solves this problem. This essay attempts to rationally reconstruct these claims. When comparing quantum and classical distributions, it is argued that classical chaos poses no problem for quantum mechanics, even in the absence of quantum decoherence. By restricting our attention to relevant physical details—actions, timescales, and the limited measurement resolution of classical observables—we find that quantum distributions do not appreciably diverge from their classical, chaotic counterparts over relevant timescales, even without decoherence. This point has been obscured in the literature by an inattention to realistic physical parameters and measurement capacities. The remaining problem posed by chaos for quantum-classical correspondence is that chaos is a large natural channel for generating macroscopic quantum superpositions. Thus, the problem of quantum chaos is deeply tied up with the quantum measurement problem. The way in which this problem shows up, and the way in which decoherence “solves” it, is idiosyncratic to different interpretations of quantum mechanics. I illustrate this point using the Everett and Bohmian interpretations.