Collapses and Avoiding Wave Function Spreading

TL;DR

This paper explores the fundamental limitations of time reversal and wave function spreading in multi-degree-of-freedom systems, discussing classical-quantum transition, environment interactions, and implications for quantum measurement.

Contribution

It provides a general argument on the impossibility of exact time reversal and analyzes quantum spreading in multi-particle systems, connecting to quantum-classical transition mechanisms.

Findings

Exact time reversal is fundamentally impossible in systems with many degrees of freedom.

Quantum evolution causes wave functions to spread and become uniform over configuration space.

Environmental interactions can potentially suppress quantum spreading, relating to quantum measurement processes.

Abstract

We address the impossibility of achieving exact time reversal in a system with many degrees of freedom. This is a particular example of the difficult task of "aiming" an initial classical state so as to become a specific final state. We also comment on the classical-to-quantum transition in any non-separable closed system of $n \geq 2$ degrees of freedom. Even if the system is initially in a well defined WKB, semi-classical state, quantum evolution and, in particular, multiple reflections at classical turning points make it completely quantum mechanical with each particle smeared almost uniformly over all the configuration space. The argument, which is presented in the context of $n$ hard discs, is quite general. Finally, we briefly address more complex quantum systems with many degrees of freedom and ask when can they provide an appropriate environment to the above simpler systems so that quantum spreading is avoided by continuously leaving "imprints" in the environment. We also discuss the possible connections with the pointer systems that are needed in the quantum-to-classical "collapse" transitions.