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Equivalence principle
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Gravity locally equals acceleration.
Locally, a uniformly accelerating frame is indistinguishable from a uniform gravitational field. This implies light bends and clocks slow in gravity.
Dropping a ball inside a lift in free fall.
Holds only for small regions.
In more detail
The equivalence principle states that gravity and acceleration are locally indistinguishable. Einstein used it as the foundation of general relativity, linking gravity to the geometry of spacetime.
Real-world uses
- It explains why all objects fall with the same acceleration.
- It is the basis for GPS corrections for gravitational time effects.
- Engineers designing inertial navigation use the equivalence principle to explain why accelerometers cannot tell gravity from acceleration.
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Frequently asked questions
Is the principle exact?
It holds locally, and tests show equality of gravitational and inertial mass to very high precision.
What does it imply?
Gravity is not a force in the usual sense, but a curvature of spacetime.
Why does the equivalence principle matter?
It is the foundation of general relativity, which explains gravity as the curvature of spacetime.
Related topics in Relativity
Mass–energy in nuclear reactions · Relativistic electromagnetism · Tachyons · Gravitational time dilation · Gravitational redshift · GPS and relativity