  The one-century-old Stern-Gerlach setup is paradigmatic for a quantum
measurement. We visualize the electron trajectories following the Bohmian
zig-zag dynamics. This dynamics was developed in order to deal with the
fundamentally massless nature of particles (with mass emerging from the
Brout-Englert-Higgs mechanism). The corresponding trajectories exhibit a
stochastic zig-zagging, as the result of the coupling between left- and
right-handed chiral Weyl states. This zig-zagging persists in the
non-relativistic limit, which will be considered here, and which is described
the Pauli equation for a nonuniform external magnetic field. Our results
clarify the different meanings of "spin" as a property of the wave function and
as a random variable in the Stern-Gerlach setup, and they illustrate the notion
of effective collapse. We also examine the case of an EPR-pair. By letting one
of the entangled particles pass through a Stern-Gerlach device, the nonlocal
influence (action-at-a-distance) on the other particle is manifest in its
trajectory, e.g. by initiating its zig-zagging.

Quantum Physics