  We develop the WKB expansion to relate Quantum Field Theory variables with
those describing macroscopical matter. We find that, up to the first quantum
correction, free scalar fields correspond to perfect fluids with pressure. We
also find the law of motion for the associated particles which takes into
account deviations from the classical trajectories, showing that they do not
follow unaccelerated straight line trajectories.
  Keywords: Quantum Field Theory, Fluid dynamics.
  PACS number (s): 11.10.-z, 03.40.Gc
