ABSTRACT
In time-dependent scattering theory, the collision problem is described by assuming that a particle was localized outside the interaction domain where V = 0. After collision, there exists a definite probability that the particle is deflected from its impact direction at a certain scattering angle. After collision, there exists a definite probability that the particle is deflected from its impact direction at a certain scattering angle. This probability cannot be directly observed in experiments but is indirectly accessible to measurements through differential cross sections. The unitarity of the operator S is one of the most important properties of the S-matrix theory of scattering. Of course, the study of the S-matrix will greatly be facilitated by the knowledge that the S-operator belongs to a narrower class of the unitary operators.
