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 radi ation 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 disp lay a thermodynamical proof showing that this agreement is not inciden tal: 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 respons ible for its amplification for all superradiant modes. Our main conclu sion here is that the black hole response to incoming radiation and su perandiance are not features that arise because black holes are ''exce ptional'' systems but, on the contrary, because they are very ''ordina ry'' in the sense that they fall into the category of ideal grey bodie s.