Tests of cosmic censorship and a statistical description of the entropy of a black hole in string theory

The cosmic censorship conjecture is an important question in classical general relativity. Both weak and strong forms of the conjecture limit the extent to which the pervasive singularities of general relativity can destroy the predictive power of the theory. The weak form of the conjecture requires that Einstein's theory remain predictive for observers far from any matter and the strong form requires that the theory be predictive for all observers.;Tests of these conjectures are carried out using the Ernst spacetime, which describes two oppositely charged black holes accelerating in a background magnetic field. If one could adjust the background magnetic field in such a way that the event horizon of the black hole disappears, weak cosmic censorship would be violated. It is found that this can not be done; weak cosmic censorship holds for this spacetime. Strong cosmic censorship is also tested by investigating the stability of the inner horizon. If parameters of the solution can be chosen so that the inner horizon is stable, then observers inside the black hole could enter a region where general relativity is unpredictive. It is found that the inner horizon exhibits an infinite blue-shift instability, so that strong cosmic censorship is probably not violated.;Black holes are thermodynamic systems: they have energy, temperature and entropy. The search for a statistical description of these objects in terms of configurations of microstates requires a theory of quantum gravity. String theory is such a theory, and an embedding of extreme Reissner-Nordstrom into type IIA string theory is studied. The black hole solution presented carries 0- and 6-brane charge, is not supersymmetric, and is metastable to decay of both 0- and 6-branes. A statistical description of its entropy is found, where the microstates arise from configurations of light fermionic open string modes connecting 0-, 6-brane pairs.