  Five-dimensional (5D) space-time symmetry greatly facilitates how a 4D
observer perceives the propagation of a single spinless particle in a 5D
space-time. In particular, if the 5D geometry is independent of the fifth
coordinate then the 5D physics may be interpreted as 4D quantum mechanics. In
this work we address the case where symmetry is approximate, focusing on the
case where the 5D geometry depends weakly on the fifth coordinate. We show that
concepts developed for the case of exact symmetry approximately hold when other
concepts such as decaying quantum states, resonant quantum scattering and
Stokes drag are adopted, as well. We briefly comment on the optical model of
the nuclear interactions and Millikan's oil drop experiment.
