A Few Words on Entropy, Thermodynamics, and Horizons

We review recent progress in understanding certain aspects of the thermodynamics of black holes and other horizons. Our discussion centers on various “entropy bounds” which have been proposed in the literature and on the current understanding of how such bounds are not required for the semi-classical consistency of black hole thermodynamics. Instead, consistency under certain extreme circumstances is provided by two effects. The first is simply the exponential enhancement of the rate at which a macrostate with large entropy is emitted in any thermal process. The second is a new sense in which the entropy of an “object” depends on the observer making the measurement, so that observers crossing the horizon measure a different entropy flux across the horizon than do observers remaining outside. In addition to the review, some recent criticisms are addressed. In particular, additional arguments and detailed numerical calculations showing the observer dependence of entropy are presented in a simple model. This observer-dependence may have further interesting implications for the thermodynamics of black holes. For the Proceedings of the GR17 conference, Dublin, Ireland, July 2004.