An exact self-consistent gravitational shock wave in semiclassical   gravity
  We find a self-consistent pp-gravitational shock wave solution to the semiclassical Einstein equations resulting from the $1/N$ approach to the effective action. We model the renormalized matter stress-energy-momentum tensor by $N$ massless scalar fields in the Minkowski vacuum plus a classical particle. We show that quantum effects generate a milder singularity at the position of the particle than the classical solution, but the singularity does not disappear. At large distances from the particle, the quantum correction decreases slowly, as $1/\rho^2$ ($\rho$ being the distance to the particle in the shock wave plane). We argue that this large distance correction is a necessary consequence of quantum gravity. 
Carlos O. Lousto (Univ. Utah) and Francisco D. Mazzitelli (Univ.   Buenos Aires)