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In Birrel and Davies ch4 they write:

[tex] \langle \psi|:T_{ab}:|\psi \rangle =\langle \psi|T_{ab}|\psi \rangle -\langle 0|T_{ab}|0 \rangle [/tex]

this is for the usual Mink field modes and vac state. Why does normal ordering reduce to this expression, could anybody point me the way to deriving this?

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# Expectation value of Normal ordered Stress-energy tensor

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