Mass–Energy Equivalence

Rest energy

A body at rest has an energy content proportional to its inertial mass. The modern statement $E=mc^2$ says that rest energy equals mass times the square of the speed of light.

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Physics Special relativity Mass–energy equivalence

\htmlData{term=energy}{E} = \htmlData{term=mass}{m} \htmlData{term=speed-of-light}{c}^{2}

Notes

Einstein did not write $E=mc^2$ in the 1905 paper. He concluded that if a body gives off energy $L$ as radiation, its mass diminishes by $L/c^2$ . The familiar rest-energy form is the same claim in modern notation.

In a frame where the system moves, the same algebra relates relativistic energy and relativistic mass. The seed entry treats the rest-energy statement.

Related equations

Schrödinger Equationrelated

Sources

Does the Inertia of a Body Depend Upon Its Energy-Content? — Albert Einstein (1905)
1905 derivation in words.
Mass–energy equivalence (2026)
Modern E = mc^2 statement.