  We provide support for the claim that momentum is conserved for individual
events in the electron double slit experiment. The natural consequence is that
a physical mechanism is responsible for this momentum exchange, but that even
if the fundamental mechanism is known for electron crystal diffraction and the
Kapitza-Dirac effect, it is unknown for electron diffraction from
nano-fabricated double slits. Work towards a proposed explanation in terms of
particle trajectories affected by a vacuum field is discussed. The contentious
use of trajectories is discussed within the context of oil droplet analogues of
double slit diffraction.
