  We formulate a symmetry principle on the basis of the duality of electric and
magnetic fields and apply it to dispersion forces. Within the context of
macroscopic quantum electrodynamics, we rigorously establish duality invariance
for the free electromagnetic field in the presence of causal magnetoelectrics.
Dispersion forces are given in terms of the Green tensor for the
electromagnetic field and the atomic response functions. After discussing the
behavior of the Green tensor under a duality transformation, we are able to
show that Casimir forces on bodies in free space as well as local-field
corrected Casimir-Polder and van der Waals forces are duality invariant.
