Extremal black holes, gravitational entropy and nonstationary metric fields**Based on an essay selected for honorable mention in the 2010 Gravity Research Foundation Essay Competition. This work was partly completed at KITP, Santa Barbara, CA, USA.

Abstract

We show that extremal black holes have zero entropy by pointing out a simple fact: they are time independent throughout the spacetime and correspond to a single classical microstate. We show that nonextremal black holes, including the Schwarzschild black hole, contain a region hidden behind the event horizon where all their Killing vectors are spacelike. This region is nonstationary and the time t labels a continuous set of classical microstates, the phase space [hab(t), Pab(t)], where hab is a three-metric induced on a spacelike hypersurface Σt and Pab is its momentum conjugate. We determine explicitly the phase space in the interior region of the Schwarzschild black hole. We identify its entropy as a measure of an outside observer's ignorance of the classical microstates in the interior since the parameter t which labels the states lies anywhere between 0 and 2M. We provide numerical evidence from recent simulations of gravitational collapse in isotropic coordinates that the entropy of the Schwarzschild black hole stems from the region inside and near the event horizon where the metric fields are nonstationary; the rest of the spacetime, which is static, makes no contribution. Extremal black holes have an event horizon but in contrast to nonextremal black holes, their extended spacetimes do not possess a bifurcate Killing horizon. This is consistent with the fact that extremal black holes are time independent and therefore have no distinct time reverse.