Solitons and instantons in string theory

In this thesis we construct higher loop instanton solutions in string theory and study the dynamics of analogous soliton solutions.; In chapter two we extend to O({dollar}alphaspprime{dollar}) in bosonic string theory the tree-level exterior heterotic instanton solution of S. J. Rey and present several related non-supersymmetric solutions. For the spherically symmetric solution, the corresponding Weyl invariant sigma model is the product of a Wess-Zumino-Witten model and a Feigin-Fuks Coulomb gas. An exact single instanton solution is obtained by fixing the metric and antisymmetric tensor field in lowest order form and rescaling the dilaton order by order in {dollar}alphaspprime{dollar}. An exact multi-instanton solution for the heterotic string can be obtained by combining generic properties of the heterotic string with a simple extension of the methods used to construct the bosonic solution.; In chapter three we adopt a test string approach to study the dynamics of soliton solutions of string theory found by Dabholkar et al. Like BPS magnetic monopoles, these solutions exert zero static force on each other and can be superposed to form multi-soliton solutions. We show that, in contradistinction to the BPS case, the velocity-dependent forces between these strings also vanish (i.e. the scattering is trivial) in the test-string approximation. A similar result is found for fivebrane solutions of the supergravity equations. The trivial scattering result is consistent with a Veneziano amplitude calculation for macroscopic fundamental strings in the limit of large winding radius. These results provide dynamical evidence for the identification of the string solitons with the fundamental string and support the conjecture of duality between strings and fivebranes.; In chapter four we calculate the Manton metric on moduli space for the scattering of a class of heterotic string solitons with the structure of fivebranes. The geodesics of this metric represent the trajectories of multi-soliton solutions in the static solution manifold, and give a low-velocity approximation to the dynamics of the solitons. From the metric, we determine the scattering angle of the two-soliton interaction to leading order in the impact parameter.