  Gravitational plane waves (when Ricci flat) belong to the VSI family. The
achronym VSI stands for vanishing scalar invariants, meaning that all scalar
invariants built out of Riemann tensor and its derivatives vanish, although the
Riemann tensor itself does not. In the particular case of plane waves many
interesting phenomena have been uncovered for strings propagating in this
background. Here we comment on gravitons propagating in such a spacetime, which
itself presumably consists of an Avogadro number of such gravitons.
