The fundamental principle that gravitational acceleration is locally indistinguishable from acceleration due to other forces, forming the foundation of Einstein's general relativity. It states that all objects fall at the same rate in a gravitational field regardless of their mass or composition.
From Latin 'aequivalentia' (equal value) and 'principium' (beginning/foundation). Einstein elevated Galileo's observation about falling objects into this fundamental principle around 1907, calling it his 'happiest thought' that led to general relativity.
The equivalence principle means that if you were in a windowless elevator, you couldn't tell whether you were sitting on Earth's surface or accelerating through space at 9.8 m/s²—gravity and acceleration are fundamentally the same thing! This insight led Einstein to realize that gravity isn't a force but the curvature of spacetime itself.
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