Classical resummation and breakdown of strong-field QED

TL;DR

This paper demonstrates that classical electrodynamics and strong-field QED experience a breakdown at lower field strengths than previously thought, which can be mitigated through resummation techniques, restoring unitarity.

Contribution

It reveals the breakdown of perturbation theory in classical electrodynamics at lower fields and proposes resummation as a solution, impacting strong-field QED analysis.

Findings

Breakdown occurs at lower field strengths than previously believed.

Resummation restores unitarity and cures the breakdown.

Diagrammatic analysis shows unitarity removes problematic diagrams.

Abstract

QED perturbation theory has been conjectured to break down in sufficiently strong backgrounds, obstructing the analysis of strong-field physics. We show that the breakdown occurs even in classical electrodynamics, at lower field strengths than previously considered, and that it may be cured by resummation. As a consequence, an analogous resummation is required in QED. A detailed investigation shows, for a range of observables, that unitarity removes diagrams previously believed to be responsible for the breakdown of QED perturbation theory.