Black Hole - String Transition and Rolling D-brane

We investigate the black hole string transition in the two-dimensional Lorentzian black hole system from the exact boundary states that describe the rolling D-brane falling down into the two-dimensional black hole. The black hole string phase transition is one of the fundamental properties of the non-supersymmetric black holes in string theory, and we will reveal the nature of the phase transition from the exactly solvable world-sheet conformal field theory viewpoint. Since the two-dimensional Lorentzian black hole system (SL(2;R)k/U(1) coset model at level k) typically appears as near-horizon geometries of various singularities such as NS5-branes in string theory, our results can be regarded as the probe of such singularities from the nonsupersymmetric probe rolling D-brane. The exact construction of boundary states for the rolling D0-brane falling down into the two-dimensional D-brane enables us to probe the phase transition at k = 1 directly in the physical amplitudes. During the study, we uncover three fundamental questions in string theory as a consistent theory of quantum gravity: small charge limit v.s. large charge limit of non-supersymmetric quantum black holes, analyticity v.s. non-analyticity in physical amplitudes and physical observables, and unitarity v.s. open closed duality in time-dependent string backgrounds. This work is based on the PhD thesis submitted to Department of Physics, Faculty of Science, University of Tokyo, which was defended on January 2007.