#### JK423

Gold Member

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- 7

Yep! And by the way, i don't say that Feynman had such intentions, but unwillingly caused a lot of trouble (when at the same time made perturbation theory more approachable).The 'Explantations' from Peskin is unacceptable.

Yeah, multiparticle states do appear! You want to talk about them, teach them and explain them? Begin with the electromagnetic vacuum, put an interaction andI wouldn't give so much significance to that introductory paragraph, when he writes "multiparticle states" he is simply justifying the necessity of dealing with more than a single particle in relativistic QM. The example is admittedly not very fortunate.

It is true that it might be misleading, but I don't know many textbooks on complex mathematical or physical matters that are not completely misleading at one point or another. Although it shouldn't be used as an excuse let's agree that writing/teaching is hard.

[itex]\hat U\left( t \right)\left| {vac} \right\rangle = \sum\limits_n {\left\langle n \right|} \hat U\left( t \right)\left| {vac} \right\rangle \,\,\underbrace {\left| n \right\rangle }_{} [/itex]

the multiparticle states [itex]\left\{ {\left| n \right\rangle } \right\}[/itex] popped out from the vacuum, at finite time t of the interaction. At large times, none may survive

[itex]\left\langle n \right|\hat U\left( {t \to \infty } \right)\left| {vac} \right\rangle = 0\,\,\forall n \ne vac\,[/itex],

but still here you can see there is indeed an exchange of energy between the two fields, and real particles popped out from the vacuum and disappeared. Why don't we describe QFT like that? Virtual particles have nothing to do with these real excitations that do take place, so there is no need to talk about them afterall!

(Note: In the equations above i have omitted the states of the other field, e.g. electrons)