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a few lines below equation (3.1.5) Weinberg writes:

"The transformation rule (3.1.1) is only possible for particles that for one reason or another are not interacting."

I thought a lot about it, but dont see any possible reason. Can you help please?

After a few lines he defines the states [tex] \Phi_{\alpha} [/tex]

then these one should transform like (3.1.1). But why?

Thanks a lot

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# Weinberg QFT I: Lorentz Transformation with interaction

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