Locality, Causality, and an Initial Value Formulation for Open String   Field Theory
  In this paper, we explore the questions of time, locality and causality in the framework of covariant open bosonic string field theory. We show that if an open string field is expressed as a certain local function on spacetime--in particular, a function of the lightcone component of the midpoint and the transverse center of mass degrees of freedom--that cubic string field theory is nonsingular and local in lightcone time. In particular, the theory has a well defined initial value formulation resembling that of an ordinary second order relativistic field theory in lightcone frame. This description can be achieved by a nonsingular unitary transformation on the Fock space, and we demonstrate explicitly that the theory is gauge invariant and the interaction vertex is local in this basis. With an initial value formulation at hand, we are able to construct an explicit second quantized operator formalism for the theory using the Hamiltonian BRST formalism. We also explore issues of causality by considering a singular limit of the theory where all spacetime coordinates are taken to the midpoint. At any stage in this limit, the theory is well-defined and arbitrarily close to being completely local and manifestly causal. We argue that the this limit must account for the macroscopic causality of the string S-matrix. 
Theodore G. Erler, David J. Gross