Moving black holes: Energy extraction, absorption cross section, and the ring of fire

Abstract

We consider the interaction between a plane wave and a (countermoving) black hole. We show that energy is transferred from the black hole to the wave, giving rise to a negative absorption cross section. Moving black holes absorb radiation and deposit energy in external radiation. Due to this effect, a black hole hole of mass M moving at relativistic speeds in a cold medium will appear to be surrounded by a bright "ring" of diameter 33 GM/c2 and thickness ∼GM/c2.