Einstein explained · 1905
Mass–Energy Equivalence
Mass and energy are two forms of the same physical quantity. The famous relation E = mc² shows how much energy is associated with mass.
E = mc²
E↔m
What it means
A system that gains energy also gains a tiny amount of mass, while a system that loses energy loses mass. Because c² is very large, even a small amount of mass represents a large amount of energy.
Einstein's contribution
In 1905 Einstein showed that if a body gives off energy as radiation, its mass must decrease by a corresponding amount. This result followed from special relativity.
Why it matters
Mass–energy equivalence helps explain nuclear reactions, the energy produced by stars, particle creation and many processes in modern physics.
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Einstein’s key theories and contributions
1905Special RelativityHow motion changes measurements of space and time while the speed of light stays constant.Read explainer →1907Equivalence PrincipleWhy free fall and acceleration gave Einstein the starting point for a new theory of gravity.Read explainer →1907Gravitational RedshiftHow gravity changes the frequency of light and the rate at which clocks run.Read explainer →1916–1918Gravitational WavesHow changing masses can send ripples through spacetime at the speed of light.Read explainer →1915General RelativityHow Einstein described gravity as curved spacetime shaped by matter and energy.Read explainer →1916–1917Quantum Theory of RadiationHow atoms absorb and emit light, including Einstein's prediction of stimulated emission.Read explainer →1917Cosmological ConstantWhy Einstein added Λ to his gravity equations and how the term returned in modern cosmology.Read explainer →