At the beginning of section 3.3, he says lorentz invariance of S-matrix means the same unitary operator acts on both in and out states. I feel a bit blur about this since he doesn't give any concrete example. Say the eletron-positron annihilation process, we have 1 electron, 1 positron, 0 photon as in state, and 0 electron, 0 positron, 2 photons as out state, we know lorentz group has different representations on electrons and photons, then how shall we have the same unitary operator of both in and out states in such process?(adsbygoogle = window.adsbygoogle || []).push({});

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# Confusion (5) from Weinberg's QFT.(invariance of S-matrix)

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