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The '''equivalence principle''' is the statement that the mass which resists acceleration (inertial mass) and the mass which responds to gravity (gravitational mass) are the same thing, so that all bodies fall alike in a gravitational field and a uniformly accelerated laboratory cannot be distinguished locally from one at rest in a gravitational field.

==The standard account==

The observation that bodies of different weight fall at the same rate goes back to Galileo, and [[Isaac Newton|Newton]] tested it with pendulums of different composition in the ''Principia''. The empirical statement — that the ratio of gravitational to inertial mass is the same for all substances — is called the '''weak equivalence principle''' or the universality of free fall. It was tested to high precision by Loránd Eötvös with a torsion balance from 1889 onwards, and the Eötvös–Pekár–Fekete experiments published in 1922 remain a reference point in the literature. Modern torsion-balance work and lunar laser ranging constrain the Eötvös parameter to roughly one part in 10<sup>13</sup>, and the French MICROSCOPE satellite mission (2016–2018, final analysis 2022) tightened this to about one part in 10<sup>15</sup> for titanium and platinum test masses. No violation has been detected.

[[Albert Einstein|Einstein]] took a further step in 1907, in what he later called the happiest thought of his life: he proposed that a freely falling observer feels no gravity at all, and that ''all'' local physics — not just falling bodies — is the same in a freely falling frame as in an inertial frame far from any mass. This stronger statement, usually called the '''Einstein equivalence principle''', adds local Lorentz invariance and local position invariance to the universality of free fall, and it is the foundation on which [[General Relativity]] is built: gravity is geometrised precisely because it can be locally transformed away. The '''strong equivalence principle''' extends the claim to bodies whose own gravitational binding energy is significant. A key prediction, gravitational redshift, was measured by Pound and Rebka in 1959 in the 22.5-metre tower at Harvard and refined by Pound and Snider in 1964.

Even within mainstream physics the principle is known to be delicate: it is a statement about ''local'' physics, and how local is a matter of the tidal precision demanded. Whether the strong form survives in a quantum theory of gravity is an open question, and most proposed extensions of the Standard Model predict violations somewhere below current experimental limits.

==On this wiki==

The equivalence principle is one of the most heavily contested items in this wiki's collection, and the objections fall into several distinct lines.

The most technical comes from [[Ronald R Hatch]], a GPS engineer, who argues in ''[[Using GPS to Refute the Equivalence Principle]]'' (2010) that operational GPS data, correctly read, contradicts the principle, and takes apart the standard arguments of Einstein, Feynman and Clifford Will in turn. He develops the same case in ''[[Clocks and the Equivalence Principle]]'' (''[[Foundations of Physics]]'', 2004) and at book length in ''[[Escape from Einstein]]'' (1992).

A second line questions the ''evidence''. [[Hector A Munera]] re-examines the Eötvös–Pekár–Fekete data itself in ''[[The empirical basis for the equivalence principle: the EPF revisited--Once again]]'' (2013), arguing that the conventional reading of the classic experiment is not the only one available. [[James Carter]], in ''[[Just Which Equivalence Principle Do You Believe In?]]'' (1998), makes a parallel argument about the Pound–Rebka experiment: he identifies five distinct interpretations compatible with the measured result, each implying a different physical meaning for the principle, and asks which one the reader actually believes. [[Stewart Ian Wells]] argues in ''[[Galileo Revisited: a True Test of General Relativity]]'' (2006) that the famous tests of general relativity do not in fact test the equivalence principle at all, and proposes a falling-body experiment involving electric charge that would.

A third line is historical and conceptual. [[Robert J Heaston]] asks ''[[Why Did Einstein Put So Much Emphasis on the Equivalence Principle?]]'' (2008), tracing the 1907 epiphany and what Einstein subsequently made of it. [[Jaroslav J Kopernicky]] works through the closed-box thought experiment directly in ''[[The Equivalence Principle]]'' (2012). [[Mitch Emery]] connects the question back to inertia itself in ''[[Re-Examination of Newton's First Law]]'' (2008).

Finally, some contributors accept an equivalence principle but not Einstein's. [[David F Roscoe]] derives it as a consequence of Newton's third law in ''[[The Equivalence Principle as a Consequence of the Third Law]]'' (''[[Apeiron]]'', 1992), while [[Edward Kapuscik]] and [[Andrzej Horzela]] propose a version valid for all fundamental interactions, not gravitation alone, in ''[[A Non-Einsteinian Equivalence Principle]]'' (''[[Galilean Electrodynamics]]'', 1993) — a principle which, they show, does not always coincide with Einstein's.

Related threads on this wiki include [[Mach's Principle]], where the origin of inertial mass is at issue, and the [[Dark Matter]] debate, since a violation of equivalence at galactic scales is one of the standard alternatives to unseen matter.

==See also==

* [[General Relativity]]
* [[Mach's Principle]]
* [[Gravity]]
* [[Special Relativity]]
* [[Dark Matter]]
* [[Albert Einstein]]

[[Category:Gravity]]
[[Category:Relativity]]
[[Category:Theory & Models]]

Equivalence Principle - Revision history

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