I read several textbooks of QFT and found that there are two kinds of definition of S operator (or S matrix).
First kind:
Define ˆS is map from out space to in space ˆS|β,out⟩:=|β,in⟩, so that Sβα:=⟨β,out|α,in⟩=⟨β,out|ˆS|α,out⟩=⟨β,in|ˆS|α,in⟩. I understand that all these vectors are defined in the Heisenberg picture.
Second definition: Sβα:=I⟨β|ˆS|α⟩I where subscript I means vector are in interacting picture. In this definition, then, ˆS=UI(+∞,−∞), where UI(+∞,−∞) is the evolution operator in interacting picture.
Are these two definitions equivalent? I am confused about it.
Remark: I konw that the matrix element Sβα is the same in these two pictures, what I want to ask is whether the operator ˆS is same in these two definitions. Thanks!
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