 Collisions among sub-atomic particles and their corresponding anti-particle are not quite so easily explained as merely involving the re-arrangement of particles and re-distribution of energy. The most extreme example of this sort, and one that is often used in the physics literature, is pair annihilation. Consequently, let us consider a collision between an electron \(e^-\) and a positron \(e^+\), which yields two photons \(\gamma\). Symbolically, this annihilation reaction is written as follows: