Albert Einstein›Theories and contributions
Einstein explained
Key theories and contributions
Eight simple guides explain Einstein's best-known ideas, what he contributed and why each idea matters.
1905Special RelativityHow motion changes measurements of space and time while the speed of light stays constant.Read explainer →1905Mass–Energy EquivalenceWhy mass and energy are connected by Einstein's famous relation E = mc².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 →