# A question from Weinberg QFT

I'm self-studying Weinberg QFT. I'm confused by his treatment of scattering theory .

I have the following question:

He introduces the free particle states $\Phi_{\alpha}$ but I'm not sure what is it good for? It has the same shape of In and out states so why introduce it? So this equation

$$\tag{3.1.12} \int d\alpha e^{-iE_{\alpha}t}g(\alpha)\Psi^{\pm}_{\alpha}~\longrightarrow~\int d\alpha e^{-iE_{\alpha}t}g(\alpha)\Phi_{\alpha}$$

at the asymptotic time (eq 3.1.12) is not clear.

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Hi @Nabil. To merge accounts, go here. –  Qmechanic Apr 23 '13 at 18:32
Another Phys.SE question that also involves eq. (3.1.12) in Weinberg: physics.stackexchange.com/q/43481/2451 –  Qmechanic Apr 23 '13 at 18:40