Macroscopical matter derived from Quantum Field Theory

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