I am teaching myself QFT from Peskin for next years maths course and I have two questions:
What is a c-number? Is it a complex number, and if so why does it mean, $[\hat{\phi}(x),\hat{\phi}(y)]~=~<0|[\hat{\phi}(x),\hat{\phi}(y)]|0> {\bf 1}$.
Is $[\hat{\phi}(x),\hat{\phi}(y)]|$ still equal to zero if $(x^{2}-y^{2})<0 $ ?