ABSTRACT
We consider the Hamiltonian for a free scalar particle coupled to a source. We will see that the source can be used to create particles from the vacuum in quantum theory, and it forms an important ingredient, like for the derivation of the classical perturbation theory of the previous chapter, in deriving scattering amplitudes and cross sections. Also the Green’s function will reappear, but now with a unique specification of the required boundary conditions following from the time ordering in the quantum evolution equations.
