SPACE-TIME DUALITY AND DIRICHLET STRING THEORY

Bosonic open-string theory (with Neumann boundary conditions) compactified on a torus is related by spatial duality to a theory living on the dual torus and satisfying constant (Dirichlet) boundary conditions. Amplitudes are functions of the "positions" of the Dirichlet boundaries. A Minkowski-space Dirichlet string theory may be defined by summing insertions of zero-momentum Dirichlet boundaries in orientable world-sheets of arbitrary genus. This is a closed-string theory with a radically altered vacuum state, in which fixed-angle scattering is power behaved as a function of energy.