  We show that the vacuum expectation value of the stress-energy tensor of a
scalar particle on the background of a spherical gravitational shock wave does
not give a finite expression in second order perturbation theory, contrary to
the case seen for the impulsive wave. No infrared divergences appear at this
order. This result shows that there is a qualitative difference between the
shock and impulsive wave solutions which is not exhibited in first order.
