1. Uniform motion can induce negative phase velocity in electromagnetic plane waves which propagate in materials such as isotropic dielectric-magnetic materials and isotropic chiral materials that only support positive phase velocity while at rest.
2. The gravitational fields associated with certain spacetime metrics can support the propagation of electromagnetic plane waves with negative phase velocity. Such metrics include those of de Sitter, Schwarzschild-de Sitter, Kerr and Kerr-Newman spacetimes. In particular, negative phase velocity can arise in the ergosphere of a rotating black hole; furthermore this phenomenon is distinct from superradiance, although both phenomenons involve negative energy densities.