# What equation describes virtual photons becoming real photons?

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?

• Virtual means not obeying energy mass relation. Nothing magical about it, just math. Commented Jul 29, 2015 at 13:37

• Followed the material from your answer, and did some Googling. Am I correct in assuming that I can derive this relationship using the Dirac Equation? $\left(\beta mc^2 + c(\alpha_1 p_1 + \alpha_2 p_2 + \alpha_3 p_3)\right) \psi (x,t) = i \hbar \frac{\partial\psi(x,t) }{\partial t}$ Commented Mar 24, 2015 at 3:29