7 A New Theorem for Black Holes ∗

A new theorem for black holes is established. The mass of a black hole depends on where the observer is. The horizon mass theorem states that for all black holes: neutral, charged or rotating, the horizon mass is always twice the irreducible mass observed at infinity. The horizon mass theorem is crucial for understanding the occurrence of Hawking radiation. Without black hole radiation, the Second Law of Thermodynamics is lost. ∗Closing Lecture The XXVI International Colloquium on Group Theoretical Methods in Physics, New York, June 26-30, 2006