Claim: The Virtual particle's concept arises in the perturbation theory of quantum field theory, where the calculations are often performed using schematic representations known as Feynman diagrams.
Evidence: The concept of virtual particles arises in perturbation theory of quantum field theory where interactions between ordinary particles are described in terms of exchanges of virtual particles. A process involving virtual particles can be described by a schematic representation known as a Feynman diagram, in which virtual particles are represented by internal lines.