  We investigate a relativistic self-interacting gas in the field of an
external {\it pp} gravitational wave. Based on symmetry considerations we ask
for those forces which are able to compensate the imprint of the gravitational
wave on the macroscopic 4-acceleration of the gaseous fluid. We establish an
exactly solvable toy model according to which the stationary states which
characterize such a situation have negative entropy production and are
accompanied by instabilities of the microscopic particle motion. These features
are similar to those which one encounters in phenomena of self-organization in
many-particle systems.
