# Perturbation theory, Intermediate states, Virtual particles

1. Jan 23, 2016

### spaghetti3451

The following is taken from page 13 of Peskin and Schroeder.

Any relativistic process cannot be assumed to be explained in terms of a single particle, since $E=mc^{2}$ allows for the creation of particle-antiparticle pairs. Even when there is not enough energy for pair creation, multiparticle states appear, for example, as intermediate states in second-order perturbation theory. We can think of such states as existing only for a very short time, according to $\Delta E \cdot{\Delta t}=\hbar$. As one goes to higher orders in perturbation theory, arbitrarily many such virtual particles can be created.

I have the following questions regarding the above paragraph:
1. What is meant by a multiparticle state?
2. What is meant by an intermediate state in second-order perturbation theory?
3. Why do multiparticle states appear as intermediate states in second-order perturbation theory
4. In what other ways do multiparticle states appear?
5. What is the range of energies over which multiparticle states appear?

Last edited: Jan 23, 2016
2. Jan 23, 2016