Einstein explained · 1907
Equivalence Principle
The equivalence principle says that, in a small enough region, gravity and acceleration can produce the same physical effects.
free fall ≈ no local gravity
ELEVATOR
What it means
A person in free fall feels weightless. In a closed laboratory, the effects of constant acceleration can also look like the effects of gravity.
Einstein's contribution
Einstein recognised this idea in 1907 and used it as the starting point for a new theory of gravity. It led him to think about how gravity affects clocks and light.
Why it matters
The equivalence principle is one of the foundations of general relativity and remains an important subject of precision experiments.
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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 →1905Mass–Energy EquivalenceWhy mass and energy are connected by Einstein's famous relation E = mc².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 →