Hawking radiation with dispersion versus breakdown of the WKB approximation

Inspired by the condensed matter analogues of black holes (a.k.a. dumb holes), we study Hawking radiation in the presence of a modified dispersion relation which becomes superluminal at large wave numbers. In the usual stationary coordinates (t, x), one can describe the asymptotic evolution of the wave packets in WKB, but this WKB approximation breaks down in the vicinity of the horizon, thereby allowing for a mixing between initial and final creation and annihilation operators. Thus, one might be tempted to identify this point where WKB breaks down with the moment of particle creation. However, using different coordinates (tau, U), we find that one can evolve the waves so that WKB in these coordinates is valid throughout this transition region, which contradicts the above identification of the breakdown of WKB as the cause of the radiation. Instead, our analysis suggests that the tearing apart of the waves into two different asymptotic regions (inside and outside the horizon) is the major ingredient of Hawking radiation.