Dear all, While I was reading chap2 of Peskin, I got some questions. (1) The vanishment of the commutator of fields [tex][\phi(x),\phi(y)]=0[/tex] means that the measurements at [tex]x[/tex] and [tex]y[/tex] do not interfere at all. Is this a postulate? Is this the so-called micro-causality? (2) How Peskin deform the contour of fig.2.3 ? Why the two contour integrals are the same? (3) How to prove if [tex]x,y[/tex] are space-like separated, there is a continuous Lorentz transformation take [tex]x-y[/tex] to [tex]-(x-y)[/tex]? i.e. I don't understand fig.2.4. Thanks for anyone.