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Concept

Equivalence principle

A floating globe symbolizes the equivalence principle, illustrating that an object's inertial mass is equal to its gravitational mass, as proposed by Einstein.

Einstein's premise that an object's inertial mass — its resistance to being pushed — is exactly equal to its gravitational mass, the property that makes it respond to gravity. The consequence is that all bodies, regardless of composition, fall identically in a gravitational field. The principle is the conceptual seed of general relativity and is now tested to roughly one part in 10^15 by torsion balances and by lunar laser ranging.

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