  We consider the dispersion energy of a pair of dipoles embedded in a metallic
waveguide with transverse dimension $a$ smaller than the characteristic dipolar
wavelength. We find that $a$ sets the scale that separates retarded,
Casimir-Polder-like, from quasistatic, van der Waals-like, interactions.
Whereas in the retarded regime, the energy decays exponentially with
inter-dipolar distance, typical of evanescent waves, in the van der Waals
regime, the known free-space result is obtained. This short-range scaling
implies that the additivity of the dispersion interactions inside a waveguide
extends to denser media, along with modifications to related Casimir effects in
such structures.
