An electromagnetic analog of gravitational wave memory

TL;DR

The paper develops an electromagnetic analogue of gravitational wave memory, showing that an EM pulse left a residual kick in detector charges rather than a persistent distortion. In the slow-motion regime, the kick is Δv = (q/m r) P[ Σ q_k v_k(∞) − Σ q_k v_k(−∞) ], analogous to GW memory's displacement, with a far-field E ~ (1/r) d^2p/dt^2. In the general case, Maxwell theory at null infinity yields an EM memory with two contributions: an ordinary kick tied to changes in the field and a null kick tied to charge radiated to infinity, the latter calculable via a spherical-harmonic expansion of the radiated charge per solid angle F. This work clarifies the EM memory structure, reveals a linear-theory realization of a nonlinear memory analogue, and provides a pathway to better understanding gravitational memory through simpler electromagnetic models.

Abstract

We present an electromagnetic analog of gravitational wave memory. That is, we consider what change has occurred to a detector of electromagnetic radiation after the wave has passed. Rather than a distortion in the detector, as occurs in the gravitational wave case, we find a residual velocity (a "kick") to the charges in the detector. In analogy with the two types of gravitational wave memory ("ordinary" and "nonlinear") we find two types of electromagnetic kick.