  An explanation is proposed for the fact that pp-waves superpose linearly when
they propagate parallely, while they interact nonlinearly, scatter and form
singularities or Cauchy horizons if they are antiparallel. Parallel pp-waves do
interact, but a generalized gravitoelectric force is exactly cancelled by a
gravitomagnetic force. In an analogy, the interaction of light beams in
linearized general relativity is also revisited and clarified, a new result is
obtained for photon to photon attraction, and a conjecture is proved. Given
equal energy density in the beams, the light-to-light attraction is twice the
matter-to-light attraction and four times the matter-to-matter attraction.
