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In this paper we revisit the classical fluctuation-dissipation theorem withderivations and interpretations based on quantum electrodynamics (QED). As astarting point we take the widely cited semiclassical expression of the theoremconnecting the absorption coefficient with the correlation spectra of aradiating molecular dipole. The literature is suggesting how this connectioncan be derived in terms of quantum mechanical statistical averages, but thecorresponding results in terms of QED seems to be very difficult to trace indetail. The problem is therefore addressed here based on first principles.Interestingly, it turns out that the QED approach applied to the aforementionedstatistical averages does not only prove the validity of thefluctuation-dissipation theorem, but it also provides a derivation and aquantum mechanical interpretation of Schwarzschild's equation for radiativetransfer. In particular, it is found that the classical Beer-Bouguer-Lambertlaw is due to absorption as well as of stimulated emission, and furthermorethat the source term in Schwarzschild's equation (Kirchhoff's law) is duesolely to spontaneous emission. The significance of the fluctuation-dissipationtheorem is finally elaborated on in terms of the appropriate scaling of linestrength parameters (including line mixing) which is relevant in far infraredand millimeter wave broadband applications.

On the interpretation and significance of the fluctuation-dissipation theorem in infrared spectroscopy