【Read between lines】Peskin, Shcroeder "An Introduction to Quantum Field Theory" pp. 28-29 (Continuous Lorentz transformation for spacelike x-y to y-x)

Key wordsLorentz transformation spacelike light cone Relevant sectionWhen $${(x-y)^2<0}$$, we can perform a Lorentz transformation on the second term (since each term is it separately Lorentz invariant), taking $${(x -y)\to-(x-y)}$$, as