Gravitational wave propagation beyond geometric optics
  It is standard practice to study the lensing of gravitational waves (GW)
using the geometric optics regime. However, in many astrophysical
configurations this regime breaks down as the wavelength becomes comparable to
the Schwarzschild radius of the lens. We revisit the lensing of GW including
corrections beyond geometric optics. We propose a perturbative method for
calculating these corrections simply solving first order decoupled differential
equations. We study the behavior of a single ray and find that the polarization
plane defined in geometric optics is smeared due to diffraction effects, which
leads to the rise of apparent vector and scalar polarization modes. We analyze
how these modes depend on the observer choice, and we study the impact of
diffraction on the pseudo-stress energy momentum tensor of the gravitational
field.
