E = mc^{2}. Energy equals mass times the speed of light squared.

We’ve all heard of this equation, but what does it mean? Mass-energy equivalence basically means energy and matter are interchangeable; matter can be converted into energy, and energy can be converted into matter. It’s a consequence of special relativity, a subset of Einstein’s theory of relativity. We see and utilize this equation in everyday life. For example, radioactivity is the emission of energetic particles from unstable atomic nuclei. Mass (atomic nuclei) is being transformed into energy (radiation). Similarly, in nuclear fission (the splitting of atomic nuclei) energy is released through particles and electromagnetic radiation. In nuclear fusion (the fusing of atomic nuclei), the missing mass of the fused nuclei is converted to photons.