ATOMS DRESSED AND PARTIALLY DRESSED BY THE ZERO-POINT FLUCTUATIONS OFTHE ELECTROMAGNETIC-FIELD

An atom or a molecule is constituted by a set of bound electric charge s with dynamics governed by the laws of quantum mechanics. These charg es are sources of the quantized electromagnetic field which also binds them together, and thus the effects of their interaction with the fie ld cannot be disregarded in principle. Consequently even overall neutr al atoms, on which we focus our attention, are driven by a dynamics wh ich is inextricably related to the dynamics of the quantized electroma gnetic field. One of the most prominent aspects of the atom-field inte raction is the existence of a cloud of virtual photons which dresses t he atom even in the lowest possible energy state of the system. In thi s paper we review the static as well as the dynamic aspects of the the ory of the virtual cloud around the neutral atoms. We begin by reviewi ng various forms of the atom-field coupling as well as various models of simplified atoms which will be used in the rest of the paper. The q uestion then arises as how to characterize quantitatively the shape of the virtual cloud, and we show that the energy density of the electro magnetic field is a physical quantity suitable for this purpose. First a perturbative approach to calculating this shape is developed and ap plied to several physical models of a ground-state dressed atom. The n ext step is to consider virtual clouds which are out of equilibrium an d examine their time development. This leads to the concept of half-dr essed sources, which are discussed in a different physical context bot h in the absence and in the presence of real photons. In particular th e role of the virtual cloud in ensuring causality of the held propagat ing in dressing and undressing processes is emphasized. Finally, the n ature of the virtual cloud is further discussed in the light of theori es concerning the dynamics of an atomic pair, the quantum theory of me asurement and the effects of a driving electromagnetic field.