Superluminal Caustic is Just a Common Misconception: A Comment on   astro-ph/0001199 by Zheng Zheng and Andrew Gould
  When angular objects in lensing are considered as linear objects, interesting phenomena start happening. Tachyonic caustics are one example. We review that the intrinsic variables of the lens equation are angular variables. We argue that the "fast glance effect" of a caustic curve that is far away from lenses does not share the physical bearing of the well-known (apparent) superluminal motion. There is no dbout that it would be a useful exercise to study the null geodesics in the metric of, say, a rapidly rotating black hole binary. Lienard-Wiechert potentials ($A_\mu$) satisfy Maxwell's equations in Minkowski space. Authors' claim that swapping eQ and GM makes the time component ($A_0$) of the Lienard-Wiechert potentials into "the gravitational analog" that governs the behavior of the null geodesics near a relativistic binary system seems to be unfounded. 
Sun Hong Rhie