Quantum field propagator for extended-objects in the microcanonical ensemble and the S-matrix formulation

Starting with the well-known Nambu-Goto action for an N-extended body system the propagator in the microcanonical ensemble is explicitly computed. This propagator is independent of the temperature and, in contrast with the previous references, takes account on all the non-local effects produced by the extended objects (e.g., strings) in interaction. The relation between relativistic quantum field theories in the microcanonical approach and the pure S-matrix formulation is established and analyzed.