Geometrical vs wave optics under gravitational waves

TL;DR

This paper compares geometric and wave optics approaches to analyze how plane gravitational waves affect light, revealing phase modulation effects and generalizing time-delay formulas for massive particles.

Contribution

It provides new derivations of gravitational wave effects on light using both geometric and wave optics, unifying phase modulation understanding from two different frameworks.

Findings

Gravitational waves cause phase modulation of electromagnetic waves.

The time-delay formula extends to massive particles.

Electromagnetic fields experience the same phase modulation as the potentials.

Abstract

We present some new derivations of the effect of a plane gravitational wave on a light ray. A simple interpretation of the results is that a gravitational wave causes a phase modulation of electromagnetic waves. We arrive at this picture from two contrasting directions, namely null geodesics and Maxwell's equations, or, geometric and wave optics. Under geometric optics, we express the geodesic equations in Hamiltonian form and solve perturbatively for the effect of gravitational waves. We find that the well-known time-delay formula for light generalizes trivially to massive particles. We also recover, by way of a Hamilton-Jacobi equation, the phase modulation obtained under wave optics. Turning then to wave optics - rather than solving Maxwell's equations directly for the fields, as in most previous approaches - we derive a perturbed wave equation (perturbed by the gravitational wave) for the electromagnetic four-potential. From this wave equation it follows that the four-potential and the electric and magnetic fields all experience the same phase modulation. Applying such a phase modulation to a superposition of plane waves corresponding to a Gaussian wave packet leads to time delays.