On the quantum nature of the Casimir-Polder interaction

We compare the form of the Casimir-Polder interaction between a ground-stat e atom and a reflecting surface with the form obtained from a semiclassical calculation based on the method of images. The difference between these po tentials is attributable to operator ordering and to the fundamentally diff erent role prayed by the positive- and negative-frequency parts of the elec tric field in classical and quantum electrodynamics. Only with normal order ing does the images method lead to the fully quantum result.