  Van der Waals interactions, as a result of the exchange of photons between
particles, can be altered by modifying the environment through which these
photons propagate. As a consequence, phenomena such as the Rydberg blockade
mechanism between highly excited atoms or excitons, can be controlled by adding
reflecting surfaces. We provide the quantum electrodynamic framework for the
van der Waals interaction in the nonretarded limit that is relevant for
long-wavelength transitions such as those between Rydberg systems, and show its
intimate connection with common static dipole-dipole interactions, thereby
providing a generalization to include macroscopic bodies.
