Physical Aspects of Quasi-local Black Hole Horizons

There are a number of physical effects expected to be associated with black holes. Black holes are now a standard paradigm for explaining a large number of astrophysical phenomena such as quasars, gamma-ray bursts and X-ray binaries. Such astrophysical black holes are expected to be very simple, described to a good approximation by the Kerr stationary vacuum solution. Much smaller black holes may also be produced in high energy particle collisions, in cosmic rays or in particle colliders. From a theoretical viewpoint black holes have provided the background for a great deal of speculation about extensions to the known laws of physics. First, they are expected to contain singularities, or at least regions where the usual description of gravity in terms of general relativity breaks down. Second, the similarities between the classical laws of thermodynamics and the laws of black hole mechanics have led to the conjecture that black holes carry a type of entropy related to their surface area. This idea has motivated the search for the microscopic gravitational degrees of freedom that can explain this entropy through a statistical mechanical state counting argument. Third, black holes are expected to be unstable due to the emission of Hawking radiation. This Hawking radiation should transport energy away from the black hole and lead to the mass of the black hole diminishing. This effect may even lead to the black hole disappearing entirely and this poses a challenge to the accepted description of the universe in terms of unitary quantum evolution.