When an electron and positron annihilate, both their masses are destroyed, creating two equal energy photons to preserve momentum.
(a) Confirm that the annihilation equation \({e^ + } + {e^ - } \to \gamma + \gamma \) conserves charge, electron family number, and total number of nucleons. To do this, identify the values of each before and after the annihilation.
(b) Find the energy of eachγ ray, assuming the electron and positron are initially nearly at rest.
(c) Explain why the twoγ rays travel in exactly opposite directions if the centre of mass of the electron-positron system is initially at rest.