Gravitational glint: Detectable gravitational wave tails from stars and
  compact objects
  Observations of a merging neutron star binary in both gravitational waves, by
the Laser Interferometer Gravitational-wave Observatory (LIGO), and across the
spectrum of electromagnetic radiation, by myriad telescopes, have been used to
show that gravitational waves travel in vacuum at a speed that is
indistinguishable from that of light to within one part in a quadrillion.
However, it has long been expected mathematically that, when electromagnetic or
gravitational waves travel through vacuum in a curved spacetime, the waves
develop "tails" that travel more slowly. The associated signal has been thought
to be undetectably weak. Here we demonstrate that gravitational waves are
efficiently scattered by the curvature sourced by ordinary compact objects --
stars, white dwarfs, neutron stars, and planets -- and certain candidates for
dark matter, populating the interior of the null cone. The resulting
"gravitational glint" should imminently be detectable, and be recognizable (for
all but planets) as briefly delayed echoes of the primary signal emanating from
extremely near the direction of the primary source. This opens the prospect for
using GRAvitational Detection And Ranging (GRADAR) to map the Universe and
conduct a comprehensive census of massive compact objects, and ultimately to
explore their interiors.
