The Faustian Deal of DLR/Backradiation by Planck

Max Planck just after having made the deal with statistics of quanta in 1900.

In our search for the origin of the idea of DLR or backradiation, let us see what Planck says

in the Preface to the 2nd edition of his Theory of Heat Radiation from 1914 when explaining the difference as compared to the 1st edition from 1907:

• The main fault of the original treatment was that it began with the classical electrodynamical laws of emission and absorption, whereas later on it became evident that, in order to meet the demand of experimental measurements, the assumption of finite energy elements must be introduced, an assumption which is in direct contradiction to the fundamental ideas of classical electrodynamics.
• It is true that this inconsistency is greatly reduced by the fact that, in reality, only mean values of energy are taken from classical electrodynamics, while, for the statistical calculation, the real values are used; nevertheless the treatment must, on the whole, have left the reader with the unsatisfactory feeling that it was not clearly to be seen, which of the assumptions made in the beginning could, and which could not, be finally retained.
• In contrast thereto I have now attempted to treat the subject from the very outset in such a way that none of the laws stated need, later on, be restricted or modified. This presents the advantage that the theory, so far as it is treated here, shows no contradiction in itself, though certainly I do not mean that it does not seem to call for improvements in many respects, as regards both its internal structure and its external form. To treat of the numerous applications, many of them .very important, which the hypothesis of quanta has already found in other parts of physics, I have not regarded as part of my task, still less to discuss all differing opinions.

In short, what Planck does here is to give up his analysis of blackbody radiation based on a classical deterministic continuum physics wave model in the form of Maxwell's equations (classical electrodynamics) for an analysis based on statistical particle physics model borrowed from thermodynamics.

Planck thus replaces deterministic continuum mechanics by statistical mechanics of quanta, an thus finally gives up his long struggle to save his rational soul from the statistics of quanta he had introduced in 1900 with the words:...

• ...the whole procedure was an act of despair because a theoretical interpretation had to be found at any price, no matter how high that might be

but which had propelled him to become the father of modern physics. Planck thus made a true Faustian deal, which is described in the upcoming book Dr Faustus of Modern Physics.

As concerns DLR/backradiation Planck states on page 1 of his 1914 book:

• All heat rays which at a given instant pass through
  the same point of the medium are perfectly independent of one
  another, and in order to specify completely the state of the
  radiation the intensity of radiation must be known in all the
  directions, infinite in number, which pass through the point in
  question; for this purpose two opposite directions must be
  considered as distinct, because the radiation in one of them is
  quite independent of the radiation in the other.
• ...heat rays are identical
  with light rays of the same wave length. The term "heat
  radiation" then, will be applied to all physical phenomena of the
  same nature as light rays.
• Every light ray is simultaneously a
  heat ray.
• As a further consequence of this law we shall
  apply to the radiation of heat all the well-known laws of
  experimental optics, especially those of reflection and refraction, as
  well as those relating to the propagation of light.

Planck speaks about "heat rays" as streams of "heat particles", in analogy with "light rays" as streams of "light particles" or "photons", in opposition to light as wave phenomenon.

Newton proposed a particle theory of light which was abandoned when Maxwell showed that light is an electromagnetic wave phenomenon. Planck and Einstein reintroduced the old particle theory because of certain perceived difficulties with Maxwell's wave model.

In Mathematical Physics of Blackbody Radiation I propose a way out of these difficulties

based on a new concept of finite precision computation within Maxwell's wave model and without any particle statistics.

A particle model of light must viewed as primitive, and as a primitive model it is misleading, in particular misleading into an unphysical concept of a two-way transfer of heat energy between bodies of different (or equal) temperature carried by "heat rays" traveling back and forth. Very primitive, indeed. But very popular among CO2 alarmists.

If Planck would have been alive today, we could have asked him about the rationality of his "heat rays" supporting CO2 alarmism. Since this is not the case we have to ask living physicists but they say nothing,

just refer to Planck in his grave.