Einstein explained · 1905
Special Relativity
Special relativity explains how space and time behave when objects move at constant speed, especially at speeds close to the speed of light.
speed of light = constant
LIGHT
What it means
The speed of light is the same for all observers moving at constant velocity. Because of this, observers can measure time, distance and simultaneity differently.
Einstein's contribution
In 1905 Einstein built the theory from simple rules about motion, light and clocks. His approach made time dilation, length contraction and the relativity of simultaneity part of one clear framework.
Why it matters
Special relativity is essential in particle physics and precision timing. It also led directly to Einstein's work on mass–energy equivalence.
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Einstein’s key theories and contributions
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 →