Equivalence principle
hypothesis that inertial and gravitational masses are equivalent
The equivalence principle is the hypothesis that the observed equivalence of gravitational and inertial mass is a consequence of nature. The weak form, known for centuries, relates to masses of any composition in free fall taking the same trajectories and landing at identical times. The extended form by Albert Einstein requires special relativity to also hold in free fall and requires the weak equivalence to be valid everywhere. This form was a critical input for the development of the theory of general relativity. The strong form requires Einstein's form to work for stellar objects. Highly precise experimental tests of the principle limit possible deviations from equivalence to be very small.
Concept
In classical mechanics, Newton's equation of motion in a gravitational field, written out in full, is:
inertial mass × acceleration = gravitational mass × gravitational acceleration
Careful experiments have shown that the inertial mass on the left side and gravitational mass on the right side are numerically equal and independent of the material composing the masses. The equivalence principle is the hypothesis that this numerical equality of inertial and gravitational mass is a consequence of their fundamental identity. The equivalence principle can be considered an extension of the principle of relativity, the principle that the laws of physics are invariant under uniform motion.
An observer in a windowless room cannot distinguish between being in a uniform gravitational field of 1g and being in a spaceship in deep space accelerating at 1g.
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Vocabulary and entity names are the principal evidence signals here, because they determine the precision of every later search. The source revision retrieved here is dated Aug 23, 2026. The linked authority identifier is Q210546. The Library of Congress control number is sh85044562. 1 of 1 selected statements include explicit references; 0 carry qualifiers and 0 use preferred rank.
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This entry incorporates text from “Equivalence principle” on English Wikipedia. Contributors are listed in the page history. Text is available under the Creative Commons Attribution-ShareAlike 4.0 License. Selected authority identifiers and statements are retrieved from Wikidata under CC0; their references and qualifiers remain part of the verification path.