Thank you very much that does help. Could you possibly point me in the direction as to why
<0|\textbf{p};\textbf{q}> = 0
where |\textbf{p}, \textbf{q}> is a two particle state. I remember this but can't recall why. Also how do I mark this as answered?
[SOLVED] Klein-Gordon Causality calculation
Homework Statement
In Peskin and Schroeder on page 27 it is stated that when we compute the Klien-Gordon propagator in terms of creation and annihilation operators the only term that survived the expansion is...