ABSTRACT
This chapter describes how Quantum Field Theory puts together quantum mechanics and relativity. The shift from atomic states and transitions towards particle reactions is an important aspect of this development. The chapter discusses what quantum fields are, how they move and interact. The prototype of a relativistically moving particle is the photon. Consequently, the classical Maxwell equations are already covariant under Lorentz transformations. The senseless negative probabilities may well have motivated the British theoretician Paul Dirac to search for an equation of motion linear in time. Since the relativistic energy conservation law has roots of negative energy due to its quadratic form, one may suspect that the negative probabilities in the Klein-Gordon equation are due to its quadratic form. Knowing how quantum fields move and how potentials are created by charges and currents, it remains to understand how they interact with each other.
