Time in quantum mechanics: A fresh look at the continuity equation

TL;DR

This paper explores how replacing classical time with a quantum clock affects the continuity equation in quantum mechanics, revealing that nuclei serve as the relevant clock in electron-nuclear dynamics under the Born-Oppenheimer approximation.

Contribution

It derives a clock-dependent continuity equation using a quantum clock, providing a new framework to analyze quantum dynamics with internal clocks.

Findings

Nuclei act as the relevant quantum clock for electrons under the Born-Oppenheimer approximation.

The derived continuity equation incorporates a clock-dependent operator instead of a traditional time derivative.

The approach offers insights into internal time concepts in quantum mechanics.

Abstract

The local conservation of a physical quantity whose distribution changes with time is mathematically described by the continuity equation. The corresponding time parameter, however, is defined with respect to an idealized classical clock. We consider what happens when this classical time is replaced by a non-relativistic quantum-mechanical description of the clock. From the clock-dependent Schr\"odinger equation (as analogue of the time-dependent Schr\"odinger equation) we derive a continuity equation, where, instead of a time-derivative, an operator occurs that depends on the flux (probability current) density of the clock. This clock-dependent continuity equation can be used to analyze the dynamics of a quantum system and to study degrees of freedom that may be used as internal clocks for an approximate description of the dynamics of the remaining degrees of freedom. As an illustration, we study a simple model for coupled electron-nuclear dynamics and interpret the nuclei as quantum clock for the electronic motion. We find that whenever the Born-Oppenheimer approximation is valid, the continuity equation shows that the nuclei are the only relevant clock for the electrons.