AQA A-Level Physics/Mass and energy

The mass-energy equivalence principle theorised by Albert Einstein describes the relationship between mass and energy through the equation $E=mc^{2}$. As $c^{2}$ is constant, $E\ \alpha \ m$. For example, 1kg of matter is equivalent to around $3\times 10^{8}$J of energy. This principle can be used to calculate the rest mass of a pair of particles which are produced through pair production, or the energy of a gamma photon produced through annihilation.