  In classical relativistic mechanics, a "preferred" proper direction in
spacetime for each particle is determined by the direction of its 4-momentum.
Analogously, for each quantum particle we find a local direction uniquely
determined by the many-particle wave function, which for each particle defines
the proper foliation of spacetime. This can be used to formulate a
relativistic-covariant version of Bohmian mechanics, with equivariant
probability density on proper hypersurfaces.
