BLACK-HOLE CONDENSATION AND THE UNIFICATION OF STRING VACUA

It is argued that black hole condensation can occur at conifold singul arities in the moduli space of type II Calabi-Yau string vacua. The co ndensate signals a smooth transition to a new Calabi-Yau space with di fferent Euler characteristic and Hedge numbers. In this manner string theory unifies the moduli spaces of many or possibly all Calabi-Yau va cua. Elementary string states and black holes are smoothly interchange d under the transitions, and therefore cannot be invariantly distingui shed. Furthermore, the transitions establish the existence of mirror s ymmetry for many or possibly all Calabi-Yau manifolds.