  A graviton laser works, in principle, by the stimulated emission of coherent
gravitons from a lasing medium. For significant amplification, we must have a
very long path length and/or very high densities. Black holes and the existence
of weakly interacting sub-eV dark matter particles (WISPs) solve both of these
obstacles. Orbiting trajectories for massless particles around black holes are
well understood \cite{mtw} and allow for arbitrarily long graviton path
lengths. Superradiance from Kerr black holes of WISPs can provide the
sufficiently high density \cite{ABH}. This suggests that black holes can act as
efficient graviton lasers. Thus directed graviton laser beams have been emitted
since the beginning of the universe and give rise to new sources of
gravitational wave signals. To be in the path of particularly harmfully
amplified graviton death rays will not be pleasant.
