## Abstract

The word “particle” and particle names: electron, positron, photon, have occasionally been mentioned in Parts I and II, although without much emphasis. The theory has been developed strictly as a field theory, a theory of a continuum. But the fields *Ψ̄* and i are never observed, electrons and positrons are. Therefore, in order to make contact with reality, particles must be incorporated into the theory. The prevalent view seems to be that particles and fields are complementary aspects of one and the same situation. This view is based on the Fock space description of free particles, which can be obtained from a free field theory as described in Sect. 5.1 for the free scalar field. As has been mentioned in the Introduction, this view is more problematic in the interacting case. We will therefore talk about particles only as approximate notions which are useful for the phenomenological description of situations which are close to the free case. More exactly, this means that the state under scrutiny is examined in a regime in which it behaves sensibly like a state of a free theory. Empirically one finds that the states that are successfully described by perturbative QED can be characterized as *“scattering states”*. These are states that at a sufficiently early time are experimentally indistinguish- able from states of a finite number of well-separated particles (electrons, positrons, photons) behaving classically, i.e. following well-defined straight orbits with constant velocity.

### Keywords

Hull Lution Conglomerate## Preview

Unable to display preview. Download preview PDF.

### References

- 1.The standard reference is still [YFS 61] . See also [W] , Chap. 13.Google Scholar