We give an example of a geometry in which two metallic objects in vacuum
experience a repulsive Casimir force. The geometry consists of an elongated
metal particle centered above a metal plate with a hole. We prove that this
geometry has a repulsive regime using a symmetry argument and confirm it with
numerical calculations for both perfect and realistic metals. The system does
not support stable levitation, as the particle is unstable to displacements
away from the symmetry axis.Comment: 4 pages, 4 figures; added references, replaced Fig.
