Mass–energy equivalence

Before special relativity, mass and energy were each assumed to be conserved on their own in a reaction. Einstein showed that mass is itself a form of energy, and can be converted into other forms, such as kinetic energy, and back.

The mass–energy relation is E = mc² (Eq. 13.6), with c ≈ 3 × 10⁸ m s⁻¹ the speed of light in vacuum.

Example 13.2: the energy equivalent of 1 g is 10⁻³ × (3 × 10⁸)² = 9 × 10¹³ J, an enormous amount for one gram.

The relation has been verified in reactions between nucleons, nuclei, electrons and other particles. Energy is conserved in a reaction only when the energy tied up in mass is counted too, so there is one combined law of conservation of mass and energy.