Perturbation theory, Intermediate states, Virtual particles

[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?

 

[A. Neumaier]

We can think of such states as existing only for a very short time

Not in any realistic sense, only as part of our imagination, hence ''virtual particles''. These appear in a pictorial representation of the formulas of perturbation theory; see P/S, Section 4.4-4.7. See also the thread
https://www.physicsforums.com/threads/are-virtual-particles-real-or-just-math-filler.850765/