We consider the the van der Waals force involving excited atoms in general
environments, constituted by magnetodielectric bodies. We develop a dynamical
approach studying the dynamics of the atoms and the field, mutually coupled.
When only one atom is excited, our dynamical theory suggests that for large
distances the van der Waals force acting on the ground-state atom is monotonic,
while the force acting in the excited atom is spatially oscillating. We show
how this latter force can be related to the known oscillating Casimir--Polder
force on an excited atom near a (ground-state) body. Our force also reveals a
population-induced dynamics: for times much larger that the atomic lifetime the
atoms will decay to their ground-states leading to the van der Waals
interaction between ground-state atoms.Comment: 19 pages, 4 figure
