Radiative Transfer Along Rays in Curved Spacetimes
  Radiative transfer in curved spacetimes has become increasingly important to understanding high-energy astrophysical phenomena and testing general relativity in the strong field limit. The equations of radiative transfer are physically equivalent to the Boltzmann equation, where the latter has the virtue of being covariant. We show that by a judicious choice of the basis of the phase space, it is generally possible to make the momentum derivatives in the Boltzmann equation vanish along an arbitrary (including nongeodesic) path, thus reducing the problem of radiative transfer along a ray to a path integral in coordinate space. 
Avery E. Broderick (Institute for Theory and Computation,   Harvard-Smithsonian Center for Astrophysics)