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Is the Planck law of radiation valid even for $T$ near absolute zero? Why can we be sure that the mean photon number inside a black body is zero for $T\to 0$?

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  • $\begingroup$ Are you asking for experimental confirmation, or some kind of intuition about the theory? $\endgroup$ – DanielSank Jul 20 '18 at 18:41
  • $\begingroup$ At first I asked for some kind of intuition about the theory. But if there are convincing experimental confirmations that would also be good. $\endgroup$ – kaffeeauf Jul 20 '18 at 18:51
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There are no photons but harmonic oscillators inside a blackbody. At T=0, or rather at T=0.2K due to zero point energy, no photons are emitted since all of these harmonic oscillators are in the ground state. The number of photons emitted is zero.

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  • $\begingroup$ If a harmonic oscillators from inside the black body emits a photon, how does it reach the exterior without to cross the interior of the body? $\endgroup$ – kaffeeauf Jul 20 '18 at 18:57
  • $\begingroup$ Photons are emitted from the surface, not from inside the black body. Remember we are discussing an ideal black body here. $\endgroup$ – my2cts Jul 20 '18 at 19:55
  • $\begingroup$ As far as I understand, you say that the harmonic oscillators are at the surface of the blackbody too? $\endgroup$ – kaffeeauf Jul 21 '18 at 6:58

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