  We study the holographic description of string theory in a plane wave
spacetime by taking the Penrose limit of the usual AdS/CFT correspondence. We
consider three-point functions with two BMN operators and one non-BMN operator;
the latter should go over to a perturbation of the dual CFT. On the string side
we take the Penrose limit of the metric perturbation produced by the non-BMN
operator, and the BMN state propagates in this perturbed background. The work
of Lee, Minwalla, Rangamani, and Seiberg shows that for chiral operators the
AdS three-point functions agree with those calculated in the free gauge theory.
However, when this is reduced to an effective plane wave amplitude by
truncating the amplitude to propagate from the AdS boundary to the Penrose
geodesic, we find a puzzling mismatch. We discuss possible resolutions, and
future directions.
