rest energy (E₀ = mc²)

Rest energy is the energy a body possesses purely by virtue of having mass, even when it is sitting perfectly still. Einstein's relation E0 = m c^2 says that mass and energy are two faces of the same coin: a tiny amount of mass corresponds to an enormous amount of energy, because the speed of light squared, c^2, is a gigantic number. One gram holds about as much energy as burning thousands of tonnes of coal.

This is the energy that gets partly released in nuclear processes. When a heavy nucleus splits or light nuclei fuse, the products together weigh slightly less than the ingredients, and that missing mass reappears as energy according to E0 = m c^2. The Sun shines, and reactors run, by converting a small fraction of mass into light and heat in exactly this way.

Rest energy also underlies the creation and annihilation of particles. An electron and its antiparticle can vanish entirely, their combined rest energy turning into two photons; conversely, energetic light can materialize as a particle-antiparticle pair. In a collider, raw kinetic energy is routinely converted into brand-new massive particles, mass literally made from motion.