  It is often argued that measurable predictions of Bohmian mechanics cannot be
distinguished from those of a theory with arbitrarily modified particle
velocities satisfying the same equivariance equation. By considering the wave
function of a closed system in a state with definite total energy, we argue
that a distinction in measurable predictions is possible. Even though such a
wave function is time-independent, the conditional wave function for a
subsystem depends on time through the time-dependent particle trajectories not
belonging to the subsystem. If these trajectories can be approximated by
classical trajectories, then the conditional wave function can be approximated
by a wave function which satisfies Schrodinger equation in a classical
time-dependent potential, which is in good agreement with observations.
However, such an approximation cannot be justified for particle velocities
significantly deviating from the Bohmian ones, implying that Bohmian velocities
are observationally preferred.
