therefore, the quantity "m0" used in einstein's equation is sometimes called the "rest mass." (the "0" reminds us that we are talking about the energy and mass when the speed is 0.) this famous "mass-energy relation" formula (usually written without the "0"s) suggests that mass has a large amount of energy, so maybe we could convert some mass to a more useful form of energy. the nuclear power industry is based on that idea.