ABSTRACT
If the forces between the scalar particles are assumed to have a finite range, then at very early times and at very late times the particles involved are free (non-interacting). This chapter finds the prescription for writing down the transition amplitude (S-matrix element) for states involving scalar particles. It considers the probability amplitude for a transition between eigenstates. The generalisation to an arbitrary initial state and an arbitrary final state is immediate, using the completeness of the eigenstates.
