Physical Society Colloquium

The Mystery of Black Hole Entropy

Werner Israel

Department of Physics and Astronomy
University of Victoria

The Bekenstein-Hawking relation S=A/4 between black hole entropy and area is one of the most beautiful equations in physics. It is also one of the most mysterious. What exactly is the nature of this entropy and where is it located — on the surface or deep inside the hole? There are many views. Gerard 'tHooft's brick wall model pictures S as entropy of a thermal atmosphere extending about a Planck length above the horizon. Another popular interpretation is that it is entanglement entropy, associated with lack of information about degrees of freedom hidden beneath the horizon. But what becomes of this information when the black hole finally evaporates? Is it recoverable or lost forever? It has been hailed as a triumph of string theory that it can reproduce the Bekenstein-Hawking formula in special instances by counting degrees of freedom of strings attached to D-branes.

The talk will review these and other questions, and conclude with some provocative speculations.