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Equivalence principle
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Gravity and acceleration are locally indistinguishable.
In a small freely falling lab you cannot detect gravity, and in an accelerating rocket you feel a force identical to gravity. Einstein built general relativity on this.
A light beam bends in an accelerating lift, so also in gravity.
Applies locally.
In more detail
The equivalence principle states that gravity and acceleration are locally indistinguishable. It is the foundation of general relativity, which describes gravity as curved spacetime.
Real-world uses
- GPS corrections use the equivalence principle to account for gravitational time dilation.
- General relativity predicts the bending of light and the precession of orbits.
- An astronaut in a rocket accelerating at 9.8 m/s² cannot tell whether they are on the ground or in a region of gravity.
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Frequently asked questions
What does it mean in practice?
An observer in a closed lift cannot tell if they are accelerating or in a gravitational field.
Who proposed it?
Einstein, as a key idea leading to general relativity.
Does the equivalence principle explain gravity?
It leads to general relativity, where gravity is the curvature of spacetime rather than a force.
Related topics in Gravitation & Orbits
Newton's shell theorem · Gravitational self-energy · Inertial vs gravitational mass · Orbital precession · Galaxy and cosmic structure · Space elevators and tethers