In this publication (caption to Fig.5) you state that the sheer existence of (electromagetic) resonances in metallic cavities disproves the idea of such cavities containing black body radiation in thermal equilibrium.
I cannot follow this argument. Maybe this is due to the fact that I know little about how the experimental data in Fig.5 were acquired?
The legend to the ordinate indicates "Return loss". This suggests to me that what is measured is (in some way) the response to an external stimulus. Is this so?
If true, then this stimulus would immediately create a situation destroying thermal equilibrium at the driving frequency. therefore, the existence of resonances doesn't give any harm to those 'believing' Kirchhoff's conjecture. Remember that it was particularly the behavior of resonances in flames (of different temperatures) which contributed to paving the way towards its formulation.
In my opinion, what would be required to prove your point is the persistence of these resonances in the (thermal) noise spectrum of the device, i.e. without external stimulus. I have no idea to what extent this is technically possible.
(The evaluation of the outcome of such an experiment of course also depends critically on the correctness of the calculations you quote. I do not personally know this work. As you have correctly noted, the resonances you display in Fig.5 of your paper fall into the range of frequencies/wavelengths which Planck would have wanted to be excluded from his claim, at least for finite size cavities.)
Article Kirchhoff's Law of Thermal Emission: 150 Years
Dear Kai Fauth,
In order to answer your question, we might need to discuss the microscopic mechanism of thermal radiation.
Dear Guoliang Liu,
a first answer could be as simple as "yes" or "no". However, P.-M. R. choses to not discuss in the public.
In order to obtain an appreciation of the consequences of a "yes" or "no" in the first place, I agree that discussing the nature of thermal radiation and its generation is probably an appropriate topic.
Dear Kai Fauth,
I understand your frustration.
The emissivity of a matter is so complicated that it might be related to the density of a matter and eventually related to the temperature in case of gas, so we might only be able to apply Planck's law to condensed matter with constant density.
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