  In this paper we discuss the force exerted by the field of an optical cavity
on a polarizable dipole. We show that the modification of the cavity modes due
to interaction with the dipole significantly alters the properties of the
force. In particular, all components of the force are found to be
non-conservative, and cannot, therefore, be derived from a potential energy. We
also suggest a simple generalization of the standard formulas for the optical
force on the dipole, which reproduces the results of calculations based on the
Maxwell stress tensor.
