Causality condition and reflection on event horizon
  A new way to implement the causality condition on the event horizon of black holes is discovered. The metric of a black hole is shown to be a function of the complex-valued gravitational radius r_g => r_g + i0. The relation between this modification of the metric and the causality condition is established using the analyticity of the S-matrix, which describes scattering of probing particles on a back hole. The found property of the metric has strong manifestations in scattering and related phenomena. One of them is the unexpected effect of reflection of incoming particles on the event horizon, which strongly reduces the absorption cross section. 
M.Yu.Kuchiev, V.V.Flambaum