In cavity quantum electrodynamics (CQED), cavity size, dipole position and
dipole orientation are the main factors controlling cavity effects, for exa
mple, suppression and enhancement of spontaneous emission. However, the dyn
amical effects of dipole orientation in CQED have, to date, remained largel
y unexplored, with most treatments simply concentrating on two (or three) o
rthogonal directions to illustrate the variations of CQED effects with dipo
le orientation. No mechanism which determines the evolution of the dipole o
rientation has been put forward to describe typical situations where atoms
move in the field of an excited cavity mode. We emphasize here that in the
presence of a cavity mode, the average dipole orientation is automatically
determined at every point to be parallel to the direction of the electric f
ield vector of the cavity mode. Besides giving rise to a single value for t
he spontaneous emission rate, this mechanism is shown to have important con
sequences for the dynamics of atoms in atom guides. In particular, it deter
mines the average trapping potential distributions and the average radiatio
n forces which guide the atoms along a cylindrical cavity. The effects of t
he field-dipole orientation mechanism are illustrated with reference to typ
ical situations involving sodium atoms in sub-micron cylindrical guides. Th
e role of a higher order cavity mode of the cylinder in the dynamics is hig
hlighted in terms of its influence on the rotational and vibrational motion
s in such guides.