  Vacuum energy in quantum field theory, being the sum of zero-point energies
of all field modes, is formally infinite but yet, after regularization or
renormalization, can give rise to finite observable effects. One way of
understanding how these effects arise is to compute the vacuum energy in an
idealized system such as a large cavity divided into disjoint regions by
pistons. In this paper, this type of calculation is carried out for situations
where the potential affecting a field is not the same in all regions of the
cavity. It is shown that the observable parts of the vacuum energy in such
potentials do not fall off to zero as the region where the potential is
nontrivial becomes large. This unusual behavior might be interesting for tests
involving quantum vacuum effects and for studies on the relation between vacuum
energy in quantum field theory and geometry.

High Energy Physics - Theory;General Relativity and Quantum Cosmology;Quantum Physics