In my talk, I will discuss an interesting property of blackbody radation when it propagates through complex media, such as disordered materials. As it turns out, the average time spent by the radiation in such a system–or, equivalently, its average path length–is entirely independent of whether the system is strongly disordered (and therefore opaque) or very weakly disordered (and therefore transparent). This property is linked to the so-called "Weyl law", that was discovered in 1911 by Hermann Weyl and allows one to estimate the density of states of a system just based on its geometrical parameters. After introducing this "path length invariance"  and its experimental implementations [2,3], I will show how these insights can be connected to information theory  and to the forces of the quantum vacuum .
 R. Pierrat, P. Ambichl, S. Gigan, A. Haber, R. Carminati, and S. Rotter, PNAS 111, 17765 (2014)
 R. Savo, R. Pierrat, U. Najar, R. Carminati, S. Rotter, and S. Gigan, Science 358, 765 (2017)
 M. Davy, M. Kühmayer, S. Gigan, and S. Rotter, Communications Physics 4, 85 (2021)
 M. Horodynski, D. Bouchet, M. Kühmayer, and S. Rotter, Phys. Rev. Lett. 127, 233201 (2021)
 L. M. Rachbauer, D. Bouchet, U. Leonhardt, and S. Rotter (manuscript in preparation)