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.
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. 
