  The length gauge uses a scalar potential to describe a laser field, thus
treating it as a longitudinal field rather than as a transverse field. This
distinction is revealed in the fact that the Maxwell equations that relate to
the length gauge are not the same as those for transverse fields. In
particular, a source term is necessary in the length-gauge Maxwell equations,
whereas the Coulomb-gauge description of plane waves possesses the basic
property of transverse fields that they propagate with no source terms at all.
This difference is shown to be importantly consequential in some previously
unremarked circumstances; and it explains why the G\"oppert-Mayer gauge
transformation does not provide the security that might be expected of full
gauge equivalence.
