# QFT basics for Klein-Gordon fields

I am teaching myself QFT from Peskin for next years maths course and I have two questions:

1. What is a c-number? Is it a complex number, and if so why does it mean, $[\hat{\phi}(x),\hat{\phi}(y)]~=~<0|[\hat{\phi}(x),\hat{\phi}(y)]|0> {\bf 1}$.

2. Is $[\hat{\phi}(x),\hat{\phi}(y)]|$ still equal to zero if $(x^{2}-y^{2})<0$ ?

-

## migrated from math.stackexchange.comApr 28 '13 at 5:14

This question came from our site for people studying math at any level and professionals in related fields.

c-number stands for classical number i.e. it is not an operator like $\hat{p}, \hat{x}$, but just the usual quantity in classical mechanics. –  ramanujan_dirac Apr 28 '13 at 6:07
$x^2-y^2<0$ does not always belong to space/light/time like interval, so it is not obviously to say. For $(x-y)^2<0$ under +--- metric, the commutator is zero. –  user26143 Jul 18 '13 at 18:59