Scattering amplitudes and electromagnetic horizons

TL;DR

This paper explores the quantum and classical behavior of charged particles interacting with electromagnetic fields that mimic gravitational horizons, revealing phenomena like pair production and unitarity issues, and introduces a method to compute relevant scattering amplitudes.

Contribution

It introduces a novel approach to analyze scattering on electromagnetic horizons and constructs amplitudes for pair creation and radiation in this context.

Findings

Particles are causally excluded beyond the electromagnetic horizon.

Pair production occurs at the horizon via the Schwinger effect.

Unitarity appears violated when crossing the horizon, and the S-matrix is ill-defined.

Abstract

We consider the scattering of charged particles on particular electromagnetic fields which have properties analogous to gravitational horizons. Classically, particles become causally excluded from regions of spacetime beyond a null surface which we identify as the `electromagnetic horizon'. In the quantum theory there is pair production at the horizon via the Schwinger effect, but only one particle from the pair escapes the field. Furthermore, unitarity appears to be violated when crossing the horizon, and there is no well-defined S-matrix. Despite this, we show how to use the perturbiner method to construct `amplitudes' which contain all the dynamical information required to construct observables related to pair creation, and to radiation from particles scattering on the background.