Optical theorem and effective action: new proofs of old results in QFT

A new proof of the optical theorem at all orders is presented. Although the theorem is a well-known result in Quantum Field Theory, our proof is interesting because it is particularly simple. Indeed, the theorem is a direct consequence of the pole-ology formalism discussed in Weinberg’s Cambridge books. We also discuss a new proof of the standard result concerning the effective action as generating functional of 1PI contributions.