# Ax 4 in page 19 of Ticciati's QFT textbook.

1. Mar 11, 2012

### MathematicalPhysicist

The axiom says:
"The interaction between two observations is constrained by causality
$$|x-y|^2 <0 \Rightarrow [\phi(x),\phi(y)]=0$$
"

But |x-y|^2<0 is always false that means the if the condition applies then by simple logic the consequnet of the condition can occur or not occur, I don't understand this.

2. Mar 11, 2012

### Polyrhythmic

The statement is about a timelike distance four-vector in Minkowski space. Its squared length is negative in the case of a -+++ metric.