Comment on 'Towards gravitationally assisted negative refraction of light by vacuum'
Reference arXiv:physics/0408021 claims that matter--free regions of spacetime may support electromagnetic waves for which the phase velocity 3-vector is oppositely directed to the power flow 3-vector. This was the definition of negative refraction adopted in \cite{ML}. We here show that this cannot occur in regions of spacetime for which the electromagnetic energy density is positive.
physics.optics↗