In this paper we revisit the classical fluctuation-dissipation theorem with
derivations and interpretations based on quantum electrodynamics (QED). As a
starting point we take the widely cited semiclassical expression of the theorem
connecting the absorption coefficient with the correlation spectra of a
radiating molecular dipole. The literature is suggesting how this connection
can be derived in terms of quantum mechanical statistical averages, but the
corresponding results in terms of QED seems to be very difficult to trace in
detail. The problem is therefore addressed here based on first principles.
Interestingly, it turns out that the QED approach applied to the aforementioned
statistical averages does not only prove the validity of the
fluctuation-dissipation theorem, but it also provides a derivation and a
quantum mechanical interpretation of Schwarzschild's equation for radiative
transfer. In particular, it is found that the classical Beer-Bouguer-Lambert
law is due to absorption as well as of stimulated emission, and furthermore
that the source term in Schwarzschild's equation (Kirchhoff's law) is due
solely to spontaneous emission. The significance of the fluctuation-dissipation
theorem is finally elaborated on in terms of the appropriate scaling of line
strength parameters (including line mixing) which is relevant in far infrared
and millimeter wave broadband applications