Black hole physics and the universalities of superradiance and of grey-body radiation

In this essay we compare the response of a black hole to incoming radiation to that one of a system consisting of a hot source hidden behind a semi-transparent mirror, and the two happen to agree. Then, we display a thermodynamical proof showing that this agreement is not incidental: it is a universal feature of an ideal grey body. As a by-product of this argument the universality of superradiance emerges: absorptive media in rotation, instead of damping incoming radiation, are responsible for its amplification for all superradiant modes. Our main conclusion here is that the black hole response to incoming radiation and superandiance are not features that arise because black holes are "exceptional" systems but, on the contrary, because they are very "ordinary" in the sense that they fall into the category of ideal grey bodies.