Between classical and quantum

TL;DR

This paper explores the complex relationship between classical and quantum physics, analyzing how classical phenomena emerge from quantum mechanics through various limits and decoherence, emphasizing the approximate and relative nature of classical reality.

Contribution

It provides a comprehensive, rigorous analysis of the emergence of classical physics from quantum theory, integrating historical, conceptual, and mathematical perspectives with over 500 references.

Findings

Classical world emerges as an approximate appearance relative to certain states and observables.

Decoherence leads to the selection of robust, 'einselected' states.

Classicality results from the elimination of certain quantum states and observables.

Abstract

The relationship between classical and quantum theory is of central importance to the philosophy of physics, and any interpretation of quantum mechanics has to clarify it. Our discussion of this relationship is partly historical and conceptual, but mostly technical and mathematically rigorous, including over 500 references. On the assumption that quantum mechanics is universal and complete, we discuss three ways in which classical physics has so far been believed to emerge from quantum physics, namely in the limit h -> 0 of small Planck's constant (in a finite system), in the limit of a large system, and through decoherence and consistent histores. The first limit is closely related to modern quantization theory and microlocal analysis, whereas the second involves methods of C*-algebras and the concepts of superselection sectors and macroscopic observables. In these limits, the classical world does not emerge as a sharply defined objective reality, but rather as an approximate appearance relative to certain "classical" states and observables. Decoherence subsequently clarifies the role of such states, in that they are "einselected", i.e. robust against coupling to the environment. Furthermore, the nature of classical observables is elucidated by the fact that they typically define (approximately) consistent sets of histories. We make the point that classicality results from the elimination of certain states and observables from quantum theory. Thus the classical world is not created by observation (as Heisenberg once claimed), but rather by the lack of it.