Connecting the quantum and classical worlds

TL;DR

This paper argues that the quantum-classical transition can be understood through the limited scope of unitary evolution in practical quantum mechanics, emphasizing the role of classical environments and nonunitary processes.

Contribution

It offers a perspective that embeds the quantum-classical transition within current theories without requiring controversial interpretations, highlighting the role of classical environments and nonunitary effects.

Findings

Classical behavior emerges from finite-temperature macroscopic systems.

Nonunitary projections and symmetry breaking are common in quantum calculations.

Quantum correlations are limited in macroscopic environments, leading to classicality.

Abstract

By considering (non-relativistic) quantum mechanics as it is done in practice in particular in condensed-matter physics, it is argued that a deterministic, unitary time evolution within a chosen Hilbert space always has a limited scope, leaving a lot of room for embedding the quantum-classical transition into our current theories without recurring to difficult-to-accept interpretations of quantum mechanics. Nonunitary projections to initial and final states, the breaking of time-reversal symmetry, a change of Hilbert space, and the introduction of classical concepts such as external potentials or localized atomic nuclei are widespread in quantum mechanical calculations. Furthermore, quantum systems require classical environments that enable the symmetry breaking that is necessary for creating the atomic configurations of molecules and crystals. This paper argues that such classical environments are provided by finite-temperature macroscopic systems in which the range of quantum correlations and entanglement is limited. This leads to classical behavior on larger scales, and to collapse-like events in all dynamical processes that become coupled to the thermalized degrees of freedom.