 According to the currently accepted Standard Model of particle physics, electrons and photons are both “fundamental particles,” by which physicists mean that such particles have no structure, i.e., such particles are not composed of other, smaller particles. Furthermore, the photons that are the products in the annihilation reaction have zero rest-mass. Thus, the rest-masses of the incoming electron and positron seems to “disappear” and an equivalent amount of energy “appears” as the energy of the outgoing photons. Of course, Einstein’s famous equation makes all of the correct predictions concerning the relevant masses and energies involved in this reaction. So, for example, the total energy of the two photons is equal to the sum of the kinetic energies of the electron and positron plus the sum of the rest-masses of the electron and positron multiplied by \(c^2\).