Where is the extremal Kerr ISCO?

Although the circular photon orbit and ISCO for the Kerr black hole remain distinct from each other and from the horizon in the extremal spin limit on a constant Boyer-Lindquist time slice, on a horizon-crossing slice they both coincide with the null generators of the horizon.Comment: 6 pages; v2: minor cosmetic changes; coincides with published versio

Einstein-aether gravity: theory and observational constraints

Einstein-aether theory is general relativity coupled to a dynamical unit timelike vector field. A brief review of current theoretical understanding and observational constraints on the four coupling parameters of the theory is given.Comment: 8 pages, for proceedings of 4th Meeting on CPT and Lorentz Symmetry (CPT 07), Bloomington, Indiana, 8-11 Aug 200

On the nature of black hole entropy

I argue that black hole entropy counts only those states of a black hole that can influence the outside, and attempt (with only partial success) to defend this claim against various objections, all but one coming from string theory. Implications for the nature of the Bekenstein bound are discussed, and in particular the case for a holographic principle is challenged. Finally, a generalization of black hole thermodynamics to "partial event horizons" in general spacetimes without black holes is proposed.Comment: 13 pages, 2 figures; references added, almost same as version published in General Relativity And Relativistic Astrophysics: Eighth Canadian Conference, AIP Conference Proceedings 493, eds. C.P. Burgess and R.C. Myers (AIP, 1999