ABSTRACT
We have seen in the π – K scattering example that in ordinary second-order perturbation theory the separate contributions to the amplitude were not relativistically invariant, but that their sum was invariant. Two Lorentz frames A and B are related by a rotation in space time. For example, the time order of the two successive interactions would be different in A and B in the situation in Fig. 18-1. This is true when the second vertex does not lie within the light cone of the first (otherwise the time order could not be reversed by a Lorentz transformation). https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780429492501/e6a0c676-1f8d-4684-97a0-6bf3353f7ce0/content/fig18_1.tif"/>
