Computing Black Hole Entropy at Infinity

Abstract: We show that the entropy of asymptotically flat, nonextremal black holes can be computed at infinity. We provide a prescription for transforming these black holes to $AdS_2$ black holes with the same entropy by dimensional reduction to 2D and a Weyl transformation. We apply our prescription to Schwarzschild, 4D Reissner--Nordstrom and generic nonextremal black holes. In the transformed coordinates, the asymptotic regions contain a global $AdS_2$ whose entropy can be computed either as the holographic entanglement entropy or as the entanglement entropy of a pair of Rindler $AdS_2$ spaces in the thermofield double state. This precisely reproduces the entropy of the original nonextremal black holes.