Beyond the light-cone propagation of relativistic wavefunctions: numerical results

TL;DR

This paper investigates the non-causal propagation of relativistic wavefunctions beyond the light cone, showing that such effects are typically small transient phenomena with limited practical impact.

Contribution

It provides a quantitative analysis of non-causality in relativistic wavefunctions for various wavepackets, highlighting that these effects are minor and transient.

Findings

Non-causality appears as a small transient effect.

The effect can often be neglected in practical scenarios.

Numerical results support the limited impact of non-causality.

Abstract

It is known that relativistic wavefunctions formally propagate beyond the light cone when the propagator is limited to the positive energy sector. By construction, this is the case for solutions of the Salpeter (or relativistic Schr\"odinger) equation or for Klein-Gordon and Dirac wavefunctions defined in the Foldy-Wouthuysen representation. In this work we investigate quantitatively the degree of non-causality for free propagation for different types of wavepackets all having initially a compact spatial support. In the studied examples we find that non-causality appears as a small transient effect that can in most cases be neglected. We display several numerical results and discuss the fundamental and practical consequences of our findings concerning this peculiar dynamical feature.