  Non-local boundary conditions have been considered in theoretical high-energy
physics with emphasis on one-loop quantum cosmology, one-loop conformal
anomalies, Bose-Einstein condensation models and spectral branes. We have
therefore studied the Wightman function, the vacuum expectation value of the
field square and the energy-momentum tensor for a massive scalar field
satisfying non-local boundary conditions on a single and two parallel plates.
Interestingly, we find that suitable choices of the kernel in the non-local
boundary conditions lead to forces acting on the plates that can be repulsive
for intermediate distances.
