 For perturbative purposes, we gather the configuration of all the fields of our theory on an entire three-dimensional horizontal spacelike cross section of "P" into one object (a Fock state), and then describe the "evolution" of this state over time using the interaction picture.The Fock space is spanned by the multi-particle eigenstates of the "unperturbed" or "non-interaction" portion formula_2 of the Hamiltonian formula_3.Hence the instantaneous description of any Fock state is a complex-amplitude-weighted sum of eigenstates of formula_2.