ABSTRACT
Interest in a free-field theory is by no means purely academic. Before and after the particle collision in the process of scattering (known as the key experiment in the microworld) the distance between these particles is so great that their interaction is practically zero. Because of this, we can consider the particles at the initial and final states to be free. To describe such a physical situation, an adequate mathematical operation-the representation of interaction-may be used for going from equations of interacting fields to the free-field equations. Of course, we have to do with a certain idealization; there is no possibility to exclude interactions all together, while interactions between the scattered particles are excluded. In other words, exact equations for particles at the initial and final states have to include the terms determining the interactions with the environment.
