- #1
Silviu
- 624
- 11
Hello! I am reading Peskin's book on QFT and in the first chapter (pg. 30) he introduces this: ##<0|[\phi(x),\phi(y)]|0> = \int\frac{d^3p}{(2\pi)^3}\int\frac{dp^0}{2\pi i}\frac{-1}{p^2-m^2}e^{-ip(x-y)}## and then he spends 2 pages explaining the importance of choosing the right contour integral (i.e. the right prescription of going around the poles at ##\pm E_0##). However I asked a question here just to make sure and I have been told that the way you go around the poles has no influence on the final value of the integral. So now I am confused. If the way you do the integral give the same result in the end, why is Peskin talking about, exactly? Am I missing something? Thank you!