Almost no computation in Quantum Field Theory can be done exactly. What physicists do is using perturbation theory which is very similar to doing Taylor series in introductory calculus. Feynman, in a genius inspiration, found a way to represent the various mathematical terms that occur in these types of calculations as pictures which could be described in words. In this pictorial language, one would say things like "this term correspond to the creation of a virtual pair of particles", etc." The perturbation expansion is a mathematical "trick" done so that we can do obtain approximate results. Each individual term in that expansion has no physical meaning - in spite of the pictorial language used.