  A generalization of the limiting procedure of Penrose, which allows non-zero
cosmological constants and takes into account metrics that contain homogeneous
functions of degree zero, is presented. It is shown that any spacetime which
admits a spacelike conformal Killing vector has a limit which is conformal to
plane waves. If the spacetime is an Einstein space, its limit exists only if
the cosmological constant is negative or zero. When the conformal Killing
vector is hypersurface orthogonal, the limits of Einstein spacetimes are
certain AdS plane waves. In this case the nonlinear version of the
Randall-Sundrum zero mode is obtained as the limit of the brane world
scenarios.
