Reflectionless Boundary Propagation Formulas for Partial Wave Solutions to the Wave Equation

Abstract

We consider solutions to the wave equation in 3+1 spacetime dimen- sions whose data is compactly supported at some initial time. For points outside a ball containing the initial support, we develop an outgoing wave condition, and associated one-way propagation formula, for the partial waves in the spherical-harmonic decomposition of the solution. The prop- agation formula expresses the l-th partial wave at time t and radius a in terms of order-l radial derivatives of the partial wave at time t − ∆t and radius a − ∆t. The boundary propagation formula can be applied to any differential equation that is well-approximated by the wave equation outside a fixed ball.