  The usual computation of the spontaneous emission uses a mixture of classical
and quantum postulates. A purely classical computation shows that a source of
electromagnetic field absorbs light in the eigenmode it is able to emit. Thus
in an excitation by an other mode, the component of this mode on the eigenmode
is absorbed, while the remainder is scattered. This loss of energy does not
apply to the zero point field which has its regular energy in the eigenmode, so
that the zero point field seems more effective than the other fields for the
stimulation of light emission.
