The Casimir effect is a well known Physical phenomenon. Basically if we have
two highly flat plates separated by a small distance we can measure an
attractive force between the plates. This is due to the fact that virtual
particles whose wavelengths exceed the distance between the plates are
excluded from existing between them causing the vacuum energy to be lower
between the plates than outside the plates. Since we define the vacuum as a
zero point energy we can say we have negative energy density in the region
between the plates.
However it turns out that if we replace the plates with objects with a
highly curved surfaces we actually observe repulsive forces when these two
objects are brought close together. It would seem curved surfaces enhance the
number of virtual particles possible between the objects as compared to the
vacuum. Can anyone explain how this could be?
Also we also get repulsive Casimir effects if the dielectric constant of
the space separating the plates has a value greater than the dielectric
constant of one plate and less than the other plate. What could be the
explanation for this? Any help would be appreciated.