Wednesday, December 11, 2019

scattering - There are two definitions of S operator (or S matrix) in quantum field theory. Are they equivalent?


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:=Iβ|ˆS|αI 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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