Emcee

Meet Emcee, the host of the most famous equation in physics. Her mass is a store of energy.

In 1905 Albert Einstein showed, as a consequence of special relativity, that a body's energy and its mass are linked. Even at rest, an object has a rest energy E₀ equal to its mass times the speed of light squared. If a body gives off energy E, its mass falls by E/c².

Because c is about 3×10⁸ metres per second, c² is about 9×10¹⁶ square metres per second squared. So one gram of mass corresponds to about 9×10¹³ joules, roughly the energy of a 20-kiloton explosion. Everyday energy changes, like a battery charging, change mass by amounts far too small to weigh.

The relation is visible in nuclear physics: a nucleus weighs less than its separate protons and neutrons, and the difference is the binding energy. Fission reactors and the fusion that powers the Sun release energy matching measured mass differences.

E₀ = mc² is the rest-energy part. A moving body has more total energy, and for particles with momentum the full relation is E² = (pc)² + (mc²)².