  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
non-supersymmetric 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.
