From my understanding, virtual particles are actually "disturbances" between two elementary particle interactions and appear during cross sections of these interactions. What equation(s) describe this process? Is there a linear relationship?
For example, take the Casimir effect when two mirrors are aimed at each other. Since these virtual photons have the potential to become real particles, is there some sort of linear relationship that shows how these virtual photons become "disturbed" enough to develop into real photons? What is the equation that demonstrates this?