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.