 When Bekenstein first proposed that a black hole should possess entropy, and that it should be proportional to its area, difficulties that appeared insurmountable immediately appeared. In a colloquium given at Princeton at 1970, Geroch proposed a mechanism that seemed to show that, if one could attribute a temperature to a black hole at all, it should be absolute zero; an immediate consequence of the working of the mechanism showed that to do otherwise would seem to allow arbitrarily large violations of what was to become known as the Generalized Second  Law.[14]  Far away from a black hole, prepare an essentially massless box to be full of energetic radiation with a high entropy; then the mass of the radiation will be attracted by the black hole's gravitational force. One can use this weight to drive an engine to produce energy (e.g., to produce friction from the raising of a counter-weight) while slowly lowering the box towards the event horizon of the black hole. This process extracts energy, but not entropy, from the radiation in the box. One can then arrange for all the mass-energy of the radiation to have been exhausted when the box reaches the event horizon. If one then opens the box to let the radiation fall into the black hole, the size of the event horizon will not increase (because the mass-energy of the black hole does not increase), but the thermodynamic entropy outside the black hole has decreased. Thus we seem to have violated the Generalized Second Law. Many ways to try to defuse the problem have been mooted in the literature, from entropy bounds (discussed below in  section 5.4.3)  to the attribution of an effective buoyancy to the object being lowered due to its immersion in radiation generated by its acceleration (Unruh and Wald 1982), a consequence of the so-called Unruh effect (for an account of which, see  note 16).  None of them is completely satisfying. 