# Planck law near absolute zero

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 $T\to 0$?

• Are you asking for experimental confirmation, or some kind of intuition about the theory? – DanielSank Jul 20 '18 at 18:41
• At first I asked for some kind of intuition about the theory. But if there are convincing experimental confirmations that would also be good. – kaffeeauf Jul 20 '18 at 18:51