I was reading Peskin and Schroeder's quantum field theory and going through the book mathematically. Then I got stuck at one equation.
Consider a single, non-interacting real scalar field. The book shows that
⟨0|ϕ(x)|p⟩=eip⋅x
Which can be interpreted as the position space wavefunction of a single particle state with momentum p (page 24)
and ϕ(x) equals ϕ(x)=∫d3p(2π)31√2wp(ap+a†p)eip⋅x
and when ϕ(x) acts on |0⟩
ϕ(x)|0⟩=∫d3p(2π)312Epe−ip⋅x|p⟩
How can the following be mathematically shown? ⟨0|ϕ(x)|p⟩=eip⋅x
Answer
Given ϕ(x)=∫d3p(2π)31√2Ep(ap+a†p)eip⋅x ,
Important: note this state is not fully localized in space, as you might well slip into assuming. Dotting it onto itself it yields a sharply peaked function, but not quite a delta function, as detailed in the linked question.
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