Equivalence principle

The equivalence principle states that, within a sufficiently small region, gravitational effects are indistinguishable from effects of acceleration. It is a foundational idea behind general relativity.

Free fall: Motion under gravity alone, with no other forces acting on an object. In free fall, nearby objects share the same gravitational acceleration, masking its local effects.
Inertial mass: A measure of an object's resistance to acceleration under an applied force. Its equality with gravitational mass makes bodies fall alike regardless of composition.
Galileo's Leaning Tower of Pisa experiment: A traditional account of Galileo testing whether objects of different masses fall at the same rate. The story captures the universal-fall observation that motivated equivalence reasoning.
Newtonian gravity: A theory describing gravity as a force between masses in absolute space and time. It treats gravity as a force, unlike general relativity's local equivalence with acceleration.
Geodesic: A path that locally extremizes distance or spacetime interval in a given geometry. General relativity describes freely falling bodies as following paths through curved spacetime.
Gravitational mass: A measure of how strongly an object couples to a gravitational field. Its equality with inertial mass is the empirical basis for universal free fall.
Eötvös experiment: A torsion-balance test comparing gravitational and inertial effects for different materials. It measured the composition-independence of free fall with much greater precision.
Strong equivalence principle: A formulation requiring local gravitational experiments, including gravitational ones, to be independent of position and velocity. It extends beyond universal free fall to local laws and gravitational self-energy.
Gravitational time dilation: A difference in elapsed time between clocks at different gravitational potentials. The elevator argument predicts that acceleration changes clock rates, linking gravity to time.
Locality (physics): The idea that physical interactions or descriptions apply within bounded spacetime regions. The principle concerns small enough regions that tidal variations can be neglected.

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