  In previous works Suppes and de Barros used a pure particle model to derive
interference effects, where individual photons have well-defined trajectories,
and hence no wave properties. In the present paper we extend that description
to account for the Casimir effect. We consider that the linear momentum
$\sum\frac{1}{2}\hbar {\bf k}$ of the vacuum state in quantum electrodynamics
corresponds to the linear momentum of virtual photons. The Casimir effect, in
the cases of two parallel plates and the solid ball, is explained in terms of
the pressure caused by the photons. Contrary to quantum electrodynamics, we
assume a finite number of virtual photons.
