I already have gray hair but I belong to a generation which grew up in physics calculating Feynman graphs and using the CPT invariance of Quantum Field Theory. The world would look very different if we could reverse the flow of time (an operation denoted by T), inverse all directions in space (an operation denoted by P) and change all particles into their antiparticle (an operation denoted by C). Yet the laws of physics would remain the same and all phenomena would occur in the same way. Our present understanding of physics implies the existence of antimatter, and all the properties of antimatter are predictable from the known properties of matter. All this looked so powerful, so beautiful and almost so natural to us, as we were learning modern physics in the late 1950s and early 1960s. The two ways to read the same simple Feynman graph, using it to describe, for instance, either the exchange of a photon between two electrons, or electron–positron annihilation and formation through one photon, looked like an obvious part of the calculation rules. This is shown in Figure 2.1. One can read it horizontally. This is scattering. One can also read it vertically. This is annihilation and pair formation. The same term can be used to describe both processes.