 One expects that such a framework would find its most natural application in the treatment of problems in which, in some sense or other, the curvature of spacetime is well above the Planck length, in so far as there are some theoretical grounds for suspecting that in this regime one can safely ignore any quantum properties of the spacetime geometry itself. (Hence, the framework is often called ‘the semi-classical approximation’ or ‘semi-classical gravity’.) In this vein, its most popular and successful applications have been to problems involving particle creation in the early universe and in the vicinity of black holes. Now, according to general relativity a black hole ought to be a perfect sink for energy, mass and radiation, in so far as it absorbs everything (including light), and emits nothing (including light). It was therefore more than shocking when Hawking (1974, 1975) predicted that, when quantum effects are taken into account, a black hole ought to behave rather like a perfect black body, in the sense of ordinary statistical thermodynamics: a stationary black hole should emit thermal radiation with the Planckian power spectrum characteristic of a perfect blackbody at a fixed temperature. It glows like a lump of smoldering coal even though light should not be able to escape from  it![16] 