Helmholtz Reciprocity and DLR/Backradiation

Helmholtz Reciprocity Principle (HRP) states that absorption and emission of light can be viewed as reversed processes arising by reversal of time. Absorption of a light ray by a (black) body is simply emission of a light ray running backwards in time, or the other way around.

Downwelling Longwave Radiation (DLR) and backradiation seek justification by HRP. Kirchhoff's Radiation Law stating that emissivity and absorptivity of a radiating body are equal,

was justified by Planck with reference to HRP.

But is HRP a valid physical principle? What is the relation between HRP and the 2nd Law?

HRP describes reversible physics while the 2nd Law described irreversible physics, and so HRP and the 2nd Law describe different physics.

Is the real physics of absorption and emission the time reversal of each other? Probably not.

The process of absorption is like reading and the process of emission like writing. To state

that reading and writing are the reverse of each other would miss the difference between the active constructive aspect of reading and the passive consumption aspect of reading.

The analysis in Mathematical Physics of Blackbody Radiation shows that absorption and emission are different processes satisfying a 2nd law which does not allow time reversal, and thus indicates that HRP is not valid.

Without HRP the support of DLR/backradiation evaporates.

Note that HRP conforms to a corpuscular theory of light as photon particles for which time reversal is no problem. But such a theory is capable of describing only simple ray tracing physics and not real physics.