Comment on `Do electromagnetic waves always propagate along null geodesics?'

TL;DR

This paper investigates electromagnetic wave propagation in curved spacetimes, clarifies the geometrical optics limit, and refutes a recent claim suggesting ill-defined limits in G"odel spacetime, reaffirming established understanding.

Contribution

It provides a detailed analysis of Maxwellian wave propagation in curved spacetime and resolves conflicting claims about the geometrical optics limit in G"odel spacetime.

Findings

Confirmed the well-defined nature of the geometrical optics limit in G"odel spacetime

Refuted the claim that wave propagation is ill-defined in curved spacetime

Reaffirmed the consistency of Maxwellian theory in curved backgrounds

Abstract

We study the propagation of Maxwellian electromagnetic waves in curved spacetimes in terms of the appropriate geometrical optics limit, notions of signal speed, and minimal coupling prescription from Maxwellian theory in flat spacetime. In the course of this, we counter a recent major claim by Asenjo and Hojman (2017) to the effect that the geometrical optics limit is partly ill-defined in G\"odel spacetime; we thereby dissolve the present tension concerning established results on wave propagation and the optical limit.