  We propose that relativistic wave--particle duality can be embodied in a
relation $\langle u^i\rangle=\bar{\psi}\gamma^i\psi/\bar{\psi}\psi$, which
determines the mean four-velocity of the fermion particle associated with a
Dirac wave function. We use the Einstein--Cartan theory of gravity with
torsion, which incorporates the spin-orbit interaction in curved spacetime.
This relation is satisfied by a spinor plane wave and it is consistent with the
energy-momentum tensor for particles. We suggest that spacetime guides the
evolution of a wave function, that in turn guides the mean motion of the
associated particle. Consequently, spacetime guides the motion of particles.
The exact motion is limited by the uncertainty principle.
