Large Conformal Deformations and Symmetries of Strings

In this thesis, first a correct regularization for integrals of time ordered product of vertex operators is outlined. Applying this regularization I demonstrate in detail how the space of nearby two-dimensional quantum field theories can be parametrized by off-shell states of free closed string. The condition for the deformed theory to be conformally symmetrical induces a dynamic equation, which can be considered as a classical equation of motion in closed string field theory. This equation includes an infinite number of higher order terms. I give an exact procedure for their calculation and describe the symmetries that are equivalence relations of two-dimensional quantum field theories. In this framework I show how to perform a nonperturbative analysis in the low-energy limit and prove that it corresponds to the Brans-Dicke theory of gravity interacting with a skew symmetric tensor field.