  We use a general formulation of nonrelativistic quantum electrodynamics in
which the gauge freedom is carried by the arbitrary transverse component of the
the Green's function for the divergence operator to calculate the natural
lineshape of spontaneous emission, thus discerning the full dependence of the
result on the choice of gauge. We also use a representation of the Hamiltonian
in which the virtual field associated with the atomic ground state is
explicitly absent. We consider two processes by which the atom is excited; the
first is resonant absorption of incident radiation with a sharp line. This
treatment is then adapted to derive a resonance fluorescence rate associated
with the Lamb line in atomic hydrogen. Second we consider the atom's excitation
due to irradiation with a laser pulse treated semi-classically. An experiment
could be used to reveal which of the calculated lineshape distributions is
closest to the measured one. This would provide an answer to a question of
fundamental importance; how does one best describe atom-radiation interactions
with the canonical formalism?
