Black hole thermodynamics: a crisis of identity

Abstract

Do black holes expand for the same reason that cups of tea cool down? There is a striking similarity between the laws of black hole mechanics and the laws of thermodynamics; so striking, that some have gone as far as —indeed the orthodoxy in theoretical physics is— to say it is an identity. But others point out differences between the quantities associated to black holes and the quantities of ordinary thermal systems, like cups of tea and boxes of gas. How can we say black hole entropy is thermodynamic entropy when there are these differences? In this paper, I show to what extent the increasingly popular tool of functionalism can be used to understand the claim that S_BH is S_TD.