  We consider quantum mechanics written in hydrodynamic formulation for the
case of relativistic spinor fields to study their velocity: within such a
hydrodynamic formulation it is possible to see that the velocity as is usually
defined can not actually represent the tangent vector to the trajectories of
particles. We propose an alternative definition for this tangent vector and
hence for the trajectories of particles, which we believe to be new and in fact
the only one that is possible. Finally we discuss how these results are a
necessary step to take in order to face further problems, like the definition
of trajectories for the multi-particle systems.
