Principle of general relativity stating that inertial and gravitational masses are equivalent
In the theory of general relativity, the equivalence principle is the equivalence of gravitational and inertial mass, and Albert Einstein's observation that the gravitational "force" as experienced locally while standing on a massive body (such as the Earth) is the same as the pseudoforce experienced by an observer in a noninertial (accelerated) frame of reference.
Einstein's statement of the equality of inertial and gravitational mass
A little reflection will show that the law of the equality of the inertial and gravitational mass is equivalent to the assertion that the acceleration imparted to a body by a gravitational field is independent of the nature of the body. For Newton's equation of motion in a gravitational field, written out in full, it is:
 (Inertial mass) $\cdot$ (Acceleration) $=$ (Intensity of the gravitational field) $\cdot$ (Gravitational mass).
It is only when there is numerical equality between the inertial and gravitational mass that the acceleration is independent of the nature of the body.^{[1]}^{[2]}
Development of gravitational theory
During the
Apollo 15 mission in 1971, astronaut
David Scott showed that Galileo was right: acceleration is the same for all bodies subject to gravity on the Moon, even for a hammer and a feather.
Something like the equivalence principle emerged in the early 17th century, when Galileo expressed experimentally that the acceleration of a test mass due to gravitation is independent of the amount of mass being accelerated.
Johannes Kepler, using Galileo's discoveries, showed knowledge of the equivalence principle by accurately describing what would occur if the moon were stopped in its orbit and dropped towards Earth. This can be deduced without knowing if or in what manner gravity decreases with distance, but requires assuming the equivalency between gravity and inertia.
If two stones were placed in any part of the world near each other, and beyond the sphere of influence of a third cognate body, these stones, like two magnetic needles, would come together in the intermediate point, each approaching the other by a space proportional to the comparative mass of the other. If the moon and earth were not retained in their orbits by their animal force or some other equivalent, the earth would mount to the moon by a fiftyfourth part of their distance, and the moon fall towards the earth through the other fiftythree parts, and they would there meet, assuming, however, that the substance of both is of the same density.
— Johannes Kepler, "Astronomia Nova", 1609^{[3]}
The 1/54 ratio is Kepler's estimate of the Moon–Earth mass ratio, based on their diameters. The accuracy of his statement can be deduced by using Newton's inertia law F=ma and Galileo's gravitational observation that distance $D=(1/2)at^{2}$. Setting these accelerations equal for a mass is the equivalence principle. Noting the time to collision for each mass is the same gives Kepler's statement that D_{moon}/D_{Earth}=M_{Earth}/M_{moon}, without knowing the time to collision or how or if the acceleration force from gravity is a function of distance.
Newton's gravitational theory simplified and formalized Galileo's and Kepler's ideas by recognizing Kepler's "animal force or some other equivalent" beyond gravity and inertia were not needed, deducing from Kepler's planetary laws how gravity reduces with distance.
The equivalence principle was properly introduced by Albert Einstein in 1907, when he observed that the acceleration of bodies towards the center of the Earth at a rate of 1g (g = 9.81 m/s^{2} being a standard reference of gravitational acceleration at the Earth's surface) is equivalent to the acceleration of an inertially moving body that would be observed on a rocket in free space being accelerated at a rate of 1g. Einstein stated it thus:
we ... assume the complete physical equivalence of a gravitational field and a corresponding acceleration of the reference system.
— Einstein, 1907
That is, being on the surface of the Earth is equivalent to being inside a spaceship (far from any sources of gravity) that is being accelerated by its engines. The direction or vector of acceleration equivalence on the surface of the earth is "up" or directly opposite the center of the planet while the vector of acceleration in a spaceship is directly opposite from the mass ejected by its thrusters. From this principle, Einstein deduced that freefall is inertial motion. Objects in freefall do not experience being accelerated downward (e.g. toward the earth or other massive body) but rather weightlessness and no acceleration. In an inertial frame of reference bodies (and photons, or light) obey Newton's first law, moving at constant velocity in straight lines. Analogously, in a curved spacetime the world line of an inertial particle or pulse of light is as straight as possible (in space and time).^{[4]} Such a world line is called a geodesic and from the point of view of the inertial frame is a straight line. This is why an accelerometer in freefall doesn't register any acceleration; there isn't any between the internal test mass and the accelerometer's body.
As an example: an inertial body moving along a geodesic through space can be trapped into an orbit around a large gravitational mass without ever experiencing acceleration. This is possible because spacetime is radically curved in close vicinity to a large gravitational mass. In such a situation the geodesic lines bend inward around the center of the mass and a freefloating (weightless) inertial body will simply follow those curved geodesics into an elliptical orbit. An accelerometer onboard would never record any acceleration.
By contrast, in Newtonian mechanics, gravity is assumed to be a force. This force draws objects having mass towards the center of any massive body. At the Earth's surface, the force of gravity is counteracted by the mechanical (physical) resistance of the Earth's surface. So in Newtonian physics, a person at rest on the surface of a (nonrotating) massive object is in an inertial frame of reference. These considerations suggest the following corollary to the equivalence principle, which Einstein formulated precisely in 1911:
Whenever an observer detects the local presence of a force that acts on all objects in direct proportion to the inertial mass of each object, that observer is in an accelerated frame of reference.
Einstein also referred to two reference frames, K and K'. K is a uniform gravitational field, whereas K' has no gravitational field but is uniformly accelerated such that objects in the two frames experience identical forces:
We arrive at a very satisfactory interpretation of this law of experience, if we assume that the systems K and K' are physically exactly equivalent, that is, if we assume that we may just as well regard the system K as being in a space free from gravitational fields, if we then regard K as uniformly accelerated. This assumption of exact physical equivalence makes it impossible for us to speak of the absolute acceleration of the system of reference, just as the usual theory of relativity forbids us to talk of the absolute velocity of a system; and it makes the equal falling of all bodies in a gravitational field seem a matter of course.
— Einstein, 1911
This observation was the start of a process that culminated in general relativity. Einstein suggested that it should be elevated to the status of a general principle, which he called the "principle of equivalence" when constructing his theory of relativity:
As long as we restrict ourselves to purely mechanical processes in the realm where Newton's mechanics holds sway, we are certain of the equivalence of the systems K and K'. But this view of ours will not have any deeper significance unless the systems K and K' are equivalent with respect to all physical processes, that is, unless the laws of nature with respect to K are in entire agreement with those with respect to K'. By assuming this to be so, we arrive at a principle which, if it is really true, has great heuristic importance. For by theoretical consideration of processes which take place relatively to a system of reference with uniform acceleration, we obtain information as to the career of processes in a homogeneous gravitational field.
— Einstein, 1911
Einstein combined (postulated) the equivalence principle with special relativity to predict that clocks run at different rates in a gravitational potential, and light rays bend in a gravitational field, even before he developed the concept of curved spacetime.
So the original equivalence principle, as described by Einstein, concluded that freefall and inertial motion were physically equivalent. This form of the equivalence principle can be stated as follows. An observer in a windowless room cannot distinguish between being on the surface of the Earth, and being in a spaceship in deep space accelerating at 1g. This is not strictly true, because massive bodies give rise to tidal effects (caused by variations in the strength and direction of the gravitational field) which are absent from an accelerating spaceship in deep space. The room, therefore, should be small enough that tidal effects can be neglected.
Although the equivalence principle guided the development of general relativity, it is not a founding principle of relativity but rather a simple consequence of the geometrical nature of the theory. In general relativity, objects in freefall follow geodesics of spacetime, and what we perceive as the force of gravity is instead a result of our being unable to follow those geodesics of spacetime, because the mechanical resistance of Earth's matter or surface prevents us from doing so.
Since Einstein developed general relativity, there was a need to develop a framework to test the theory against other possible theories of gravity compatible with special relativity. This was developed by Robert Dicke as part of his program to test general relativity. Two new principles were suggested, the socalled Einstein equivalence principle and the strong equivalence principle, each of which assumes the weak equivalence principle as a starting point. They only differ in whether or not they apply to gravitational experiments.
Another clarification needed is that the equivalence principle assumes a constant acceleration of 1g without considering the mechanics of generating 1g. If we do consider the mechanics of it, then we must assume the aforementioned windowless room has a fixed mass. Accelerating it at 1g means there is a constant force being applied, which = m*g where m is the mass of the windowless room along with its contents (including the observer). Now, if the observer jumps inside the room, an object lying freely on the floor will decrease in weight momentarily because the acceleration is going to decrease momentarily due to the observer pushing back against the floor in order to jump. The object will then gain weight while the observer is in the air and the resulting decreased mass of the windowless room allows greater acceleration; it will lose weight again when the observer lands and pushes once more against the floor; and it will finally return to its initial weight afterwards. To make all these effects equal those we would measure on a planet producing 1g, the windowless room must be assumed to have the same mass as that planet. Additionally, the windowless room must not cause its own gravity, otherwise the scenario changes even further. These are technicalities, clearly, but practical ones if we wish the experiment to demonstrate more or less precisely the equivalence of 1g gravity and 1g acceleration.
Modern usage
Three forms of the equivalence principle are in current use: weak (Galilean), Einsteinian, and strong.
The weak equivalence principle
The weak equivalence principle, also known as the universality of free fall or the Galilean equivalence principle can be stated in many ways. The strong EP, a generalization of the weak EP, includes astronomic bodies with gravitational selfbinding energy^{[5]} (e.g., 1.74 solarmass pulsar PSR J1903+0327, 15.3% of whose separated mass is absent as gravitational binding energy^{[6]}^{[failed verification]}). Instead, the weak EP assumes falling bodies are selfbound by nongravitational forces only (e.g. a stone). Either way:
 The trajectory of a point mass in a gravitational field depends only on its initial position and velocity, and is independent of its composition and structure.
 All test particles at the alike spacetime point, in a given gravitational field, will undergo the same acceleration, independent of their properties, including their rest mass.^{[7]}
 All local centers of mass freefall (in vacuum) along identical (paralleldisplaced, same speed) minimum action trajectories independent of all observable properties.
 The vacuum worldline of a body immersed in a gravitational field is independent of all observable properties.
 The local effects of motion in a curved spacetime (gravitation) are indistinguishable from those of an accelerated observer in flat spacetime, without exception.
 Mass (measured with a balance) and weight (measured with a scale) are locally in identical ratio for all bodies (the opening page to Newton's Philosophiæ Naturalis Principia Mathematica, 1687).
Locality eliminates measurable tidal forces originating from a radial divergent gravitational field (e.g., the Earth) upon finite sized physical bodies. The "falling" equivalence principle embraces Galileo's, Newton's, and Einstein's conceptualization. The equivalence principle does not deny the existence of measurable effects caused by a rotating gravitating mass (frame dragging), or bear on the measurements of light deflection and gravitational time delay made by nonlocal observers.
Active, passive, and inertial masses
By definition of active and passive gravitational mass, the force on $M_{1}$ due to the gravitational field of $M_{0}$ is:
 $F_{1}={\frac {M_{0}^{\mathrm {act} }M_{1}^{\mathrm {pass} }}{r^{2}}}$
Likewise the force on a second object of arbitrary mass_{2} due to the gravitational field of mass_{0} is:
 $F_{2}={\frac {M_{0}^{\mathrm {act} }M_{2}^{\mathrm {pass} }}{r^{2}}}$
By definition of inertial mass:
 $F=m^{\mathrm {inert} }a$
If $m_{1}$ and $m_{2}$ are the same distance $r$ from $m_{0}$ then, by the weak equivalence principle, they fall at the same rate (i.e. their accelerations are the same)
 $a_{1}={\frac {F_{1}}{m_{1}^{\mathrm {inert} }}}=a_{2}={\frac {F_{2}}{m_{2}^{\mathrm {inert} }}}$
Hence:
 ${\frac {M_{0}^{\mathrm {act} }M_{1}^{\mathrm {pass} }}{r^{2}m_{1}^{\mathrm {inert} }}}={\frac {M_{0}^{\mathrm {act} }M_{2}^{\mathrm {pass} }}{r^{2}m_{2}^{\mathrm {inert} }}}$
Therefore:
 ${\frac {M_{1}^{\mathrm {pass} }}{m_{1}^{\mathrm {inert} }}}={\frac {M_{2}^{\mathrm {pass} }}{m_{2}^{\mathrm {inert} }}}$
In other words, passive gravitational mass must be proportional to inertial mass for all objects.
Furthermore, by Newton's third law of motion:
 $F_{1}={\frac {M_{0}^{\mathrm {act} }M_{1}^{\mathrm {pass} }}{r^{2}}}$
must be equal and opposite to
 $F_{0}={\frac {M_{1}^{\mathrm {act} }M_{0}^{\mathrm {pass} }}{r^{2}}}$
It follows that:
 ${\frac {M_{0}^{\mathrm {act} }}{M_{0}^{\mathrm {pass} }}}={\frac {M_{1}^{\mathrm {act} }}{M_{1}^{\mathrm {pass} }}}$
In other words, passive gravitational mass must be proportional to active gravitational mass for all objects.
The dimensionless Eötvösparameter $\eta (A,B)$ is the difference of the ratios of gravitational and inertial masses divided by their average for the two sets of test masses "A" and "B."
 $\eta (A,B)=2{\frac {\left({\frac {m_{g}}{m_{i}}}\right)_{A}\left({\frac {m_{g}}{m_{i}}}\right)_{B}}{\left({\frac {m_{g}}{m_{i}}}\right)_{A}+\left({\frac {m_{g}}{m_{i}}}\right)_{B}}}$
Tests of the weak equivalence principle
Tests of the weak equivalence principle are those that verify the equivalence of gravitational mass and inertial mass. An obvious test is dropping different objects, ideally in a vacuum environment, e.g., inside the Fallturm Bremen drop tower.
Researcher

Year

Method

Result

John Philoponus

6th century

Said that by observation, two balls of very different weights will fall at nearly the same speed

no detectable difference

Simon Stevin^{[8]}

~1586

Dropped lead balls of different masses off the Delft churchtower

no detectable difference

Galileo Galilei

~1610

Rolling balls of varying weight down inclined planes to slow the speed so that it was measurable

no detectable difference

Isaac Newton

~1680

Measure the period of pendulums of different mass but identical length

difference is less than 1 part in 10^{3}

Friedrich Wilhelm Bessel

1832

Measure the period of pendulums of different mass but identical length

no measurable difference

Loránd Eötvös

1908

Measure the torsion on a wire, suspending a balance beam, between two nearly identical masses under the acceleration of gravity and the rotation of the Earth

difference is 10±2 part in 10^{9} (H_{2}O/Cu)^{[9]}

Roll, Krotkov and Dicke

1964

Torsion balance experiment, dropping aluminum and gold test masses

$\eta (\mathrm {Al} ,\mathrm {Au} )=(1.3\pm 1.0)\times 10^{11}$^{[10]}

David Scott

1971

Dropped a falcon feather and a hammer at the same time on the Moon

no detectable difference (not a rigorous experiment, but very dramatic being the first lunar one^{[11]})

Braginsky and Panov

1971

Torsion balance, aluminum and platinum test masses, measuring acceleration towards the Sun

difference is less than 1 part in 10^{12}

EötWash group

1987–

Torsion balance, measuring acceleration of different masses towards the Earth, Sun and galactic center, using several different kinds of masses

$\eta ({\text{Earth}},{\text{BeTi}})=(0.3\pm 1.8)\times 10^{13}$^{[12]}

See:^{[13]}
Year

Investigator

Sensitivity

Method

500?

Philoponus^{[14]}

"small"

Drop tower

1585

Stevin^{[15]}

5×10^{−2}

Drop tower

1590?

Galileo^{[16]}

2×10^{−2}

Pendulum, drop tower

1686

Newton^{[17]}

10^{−3}

Pendulum

1832

Bessel^{[18]}

2×10^{−5}

Pendulum

1908 (1922)

Eötvös^{[19]}

2×10^{−9}

Torsion balance

1910

Southerns^{[20]}

5×10^{−6}

Pendulum

1918

Zeeman^{[21]}

3×10^{−8}

Torsion balance

1923

Potter^{[22]}

3×10^{−6}

Pendulum

1935

Renner^{[23]}

2×10^{−9}

Torsion balance

1964

Dicke, Roll, Krotkov^{[10]}

3x10^{−11}

Torsion balance

1972

Braginsky, Panov^{[24]}

10^{−12}

Torsion balance

1976

Shapiro, et al.^{[25]}

10^{−12}

Lunar laser ranging

1981

Keiser, Faller^{[26]}

4×10^{−11}

Fluid support

1987

Niebauer, et al.^{[27]}

10^{−10}

Drop tower

1989

Stubbs, et al.^{[28]}

10^{−11}

Torsion balance

1990

Adelberger, Eric G.; et al.^{[29]}

10^{−12}

Torsion balance

1999

Baessler, et al.^{[30]}^{[31]}

5x10^{−14}

Torsion balance

2017

MICROSCOPE^{[32]}

10^{−15}

Earth orbit

Experiments are still being performed at the University of Washington which have placed limits on the differential acceleration of objects towards the Earth, the Sun and towards dark matter in the galactic center. Future satellite experiments^{[33]} – STEP (Satellite Test of the Equivalence Principle), and Galileo Galilei – will test the weak equivalence principle in space, to much higher accuracy.
With the first successful production of antimatter, in particular antihydrogen, a new approach to test the weak equivalence principle has been proposed. Experiments to compare the gravitational behavior of matter and antimatter are currently being developed.^{[34]}
Proposals that may lead to a quantum theory of gravity such as string theory and loop quantum gravity predict violations of the weak equivalence principle because they contain many light scalar fields with long Compton wavelengths, which should generate fifth forces and variation of the fundamental constants. Heuristic arguments suggest that the magnitude of these equivalence principle violations could be in the 10^{−13} to 10^{−18} range.^{[35]} Currently envisioned tests of the weak equivalence principle are approaching a degree of sensitivity such that nondiscovery of a violation would be just as profound a result as discovery of a violation. Nondiscovery of equivalence principle violation in this range would suggest that gravity is so fundamentally different from other forces as to require a major reevaluation of current attempts to unify gravity with the other forces of nature. A positive detection, on the other hand, would provide a major guidepost towards unification.^{[35]}
The Einstein equivalence principle
What is now called the "Einstein equivalence principle" states that the weak equivalence principle holds, and that:^{[36]}
 The outcome of any local nongravitational experiment in a freely falling laboratory is independent of the velocity of the laboratory and its location in spacetime.
Here "local" has a very special meaning: not only must the experiment not look outside the laboratory, but it must also be small compared to variations in the gravitational field, tidal forces, so that the entire laboratory is freely falling. It also implies the absence of interactions with "external" fields other than the gravitational field.^{[citation needed]}
The principle of relativity implies that the outcome of local experiments must be independent of the velocity of the apparatus, so the most important consequence of this principle is the Copernican idea that dimensionless physical values such as the finestructure constant and electrontoproton mass ratio must not depend on where in space or time we measure them. Many physicists believe that any Lorentz invariant theory that satisfies the weak equivalence principle also satisfies the Einstein equivalence principle.
Schiff's conjecture suggests that the weak equivalence principle implies the Einstein equivalence principle, but it has not been proven. Nonetheless, the two principles are tested with very different kinds of experiments. The Einstein equivalence principle has been criticized as imprecise, because there is no universally accepted way to distinguish gravitational from nongravitational experiments (see for instance Hadley^{[37]} and Durand^{[38]}).
Tests of the Einstein equivalence principle
In addition to the tests of the weak equivalence principle, the Einstein equivalence principle can be tested by searching for variation of dimensionless constants and mass ratios. The present best limits on the variation of the fundamental constants have mainly been set by studying the naturally occurring Oklo natural nuclear fission reactor, where nuclear reactions similar to ones we observe today have been shown to have occurred underground approximately two billion years ago. These reactions are extremely sensitive to the values of the fundamental constants.
There have been a number of controversial attempts to constrain the variation of the strong interaction constant. There have been several suggestions that "constants" do vary on cosmological scales. The best known is the reported detection of variation (at the 10^{−5} level) of the finestructure constant from measurements of distant quasars, see Webb et al.^{[39]} Other researchers^{[who?]} dispute these findings. Other tests of the Einstein equivalence principle are gravitational redshift experiments, such as the Pound–Rebka experiment which test the position independence of experiments.
The strong equivalence principle
The strong equivalence principle suggests the laws of gravitation are independent of velocity and location. In particular,
 The gravitational motion of a small test body depends only on its initial position in spacetime and velocity, and not on its constitution.
and
 The outcome of any local experiment (gravitational or not) in a freely falling laboratory is independent of the velocity of the laboratory and its location in spacetime.
The first part is a version of the weak equivalence principle that applies to objects that exert a gravitational force on themselves, such as stars, planets, black holes or Cavendish experiments. The second part is the Einstein equivalence principle (with the same definition of "local"), restated to allow gravitational experiments and selfgravitating bodies. The freelyfalling object or laboratory, however, must still be small, so that tidal forces may be neglected (hence "local experiment").
This is the only form of the equivalence principle that applies to selfgravitating objects (such as stars), which have substantial internal gravitational interactions. It requires that the gravitational constant be the same everywhere in the universe and is incompatible with a fifth force. It is much more restrictive than the Einstein equivalence principle.
The strong equivalence principle suggests that gravity is entirely geometrical by nature (that is, the metric alone determines the effect of gravity) and does not have any extra fields associated with it. If an observer measures a patch of space to be flat, then the strong equivalence principle suggests that it is absolutely equivalent to any other patch of flat space elsewhere in the universe. Einstein's theory of general relativity (including the cosmological constant) is thought to be the only theory of gravity that satisfies the strong equivalence principle. A number of alternative theories, such as Brans–Dicke theory, satisfy only the Einstein equivalence principle.
Tests of the strong equivalence principle
The strong equivalence principle can be tested by searching for a variation of Newton's gravitational constant G over the life of the universe, or equivalently, variation in the masses of the fundamental particles. A number of independent constraints, from orbits in the Solar System and studies of Big Bang nucleosynthesis have shown that G cannot have varied by more than 10%.
Thus, the strong equivalence principle can be tested by searching for fifth forces (deviations from the gravitational forcelaw predicted by general relativity). These experiments typically look for failures of the inversesquare law (specifically Yukawa forces or failures of Birkhoff's theorem) behavior of gravity in the laboratory. The most accurate tests over short distances have been performed by the Eöt–Wash group. A future satellite experiment, SEE (Satellite Energy Exchange), will search for fifth forces in space and should be able to further constrain violations of the strong equivalence principle. Other limits, looking for much longerrange forces, have been placed by searching for the Nordtvedt effect, a "polarization" of solar system orbits that would be caused by gravitational selfenergy accelerating at a different rate from normal matter. This effect has been sensitively tested by the Lunar Laser Ranging Experiment. Other tests include studying the deflection of radiation from distant radio sources by the sun, which can be accurately measured by very long baseline interferometry. Another sensitive test comes from measurements of the frequency shift of signals to and from the Cassini spacecraft. Together, these measurements have put tight limits on Brans–Dicke theory and other alternative theories of gravity.
In 2014, astronomers discovered a stellar triple system containing a millisecond pulsar PSR J0337+1715 and two white dwarfs orbiting it. The system provided them a chance to test the strong equivalence principle in a strong gravitational field with high accuracy.^{[40]}^{[41]}^{[42]}
In 2020, a group of astronomers analyzing data from the Spitzer Photometry and Accurate Rotation Curves (SPARC) sample together with estimates of the largescale external gravitational field from an allsky galaxy catalog, concluded that there was highly statistically significant evidence of violations of the strong equivalence principle in weak gravitational fields in the vicinity of rotationally supported galaxies.^{[43]} They observed an effect consistent with the external field effect of Modified Newtonian dynamics (MOND), a hypothesis that proposes a modified gravity theory beyond General Relativity, and inconsistent with tidal effects in the LambdaCDM model paradigm, commonly known as the Standard Model of Cosmology.
Challenges
One challenge to the equivalence principle is the Brans–Dicke theory. Selfcreation cosmology is a modification of the Brans–Dicke theory.
In August 2010, researchers from the University of New South Wales, Swinburne University of Technology, and Cambridge University published a paper titled "Evidence for spatial variation of the fine structure constant", whose tentative conclusion is that, "qualitatively, [the] results suggest a violation of the Einstein Equivalence Principle, and could infer a very large or infinite universe, within which our 'local' Hubble volume represents a tiny fraction."^{[44]}
Explanations
Dutch physicist and string theorist Erik Verlinde has generated a selfcontained, logical derivation of the equivalence principle based on the starting assumption of a holographic universe. Given this situation, gravity would not be a true fundamental force as is currently thought but instead an "emergent property" related to entropy. Verlinde's entropic gravity theory apparently leads naturally to the correct observed strength of dark energy; previous failures to explain its incredibly small magnitude have been called by such people as cosmologist Michael Turner (who is credited as having coined the term "dark energy") as "the greatest embarrassment in the history of theoretical physics".^{[45]} These ideas are far from settled and still very controversial.
Experiments
 University of Washington^{[46]}
 Lunar Laser Ranging^{[47]}^{[48]}
 GalileoGalilei satellite experiment^{[49]}
 Satellite Test of the Equivalence Principle (STEP)^{[50]}
 MICROSCOPE^{[51]}
 Satellite Energy Exchange (SEE)^{[52]}
 "...Physicists in Germany have used an atomic interferometer to perform the most accurate ever test of the equivalence principle at the level of atoms..."^{[53]}
See also
Notes
 ^ Einstein, Albert, How I Constructed the Theory of Relativity, translated by Masahiro Morikawa from the text recorded in Japanese by Jun Ishiwara, Association of Asia Pacific Physical Societies (AAPPS) Bulletin, Vol. 15, No. 2, pp. 17–19, April 2005. Einstein recalls events of 1907 in a talk in Japan on 14 December 1922.
 ^ Einstein, Albert (2003). The Meaning of Relativity. Routledge. p. 59. ISBN 9781134449798.
 ^ http://quotes.yourdictionary.com/orbits/quote/71225/
 ^ Macdonald, Alan (15 September 2012). "General Relativity in a Nutshell" (PDF). Luther College. p. 32.
 ^ Wagner, Todd A.; Schlamminger, Stephan; Gundlach, Jens H.; Adelberger, Eric G. (2012). "Torsionbalance tests of the weak equivalence principle". Classical and Quantum Gravity. 29 (18): 184002. arXiv:1207.2442. Bibcode:2012CQGra..29r4002W. doi:10.1088/02649381/29/18/184002. S2CID 59141292.
 ^ Champion, David J.; Ransom, Scott M.; Lazarus, Patrick; Camilo, Fernando; et al. (2008). ""; "Title". Science. 320 (5881): 1309–12. arXiv:0805.2396. doi:10.1126/science.1157580. PMID 18483399. S2CID 6070830.
 ^ Wesson, Paul S. (2006). Fivedimensional Physics. World Scientific. p. 82. ISBN 9789812566614.
 ^ Devreese, Jozef T.; Vanden Berghe, Guido (2008). 'Magic Is No Magic': The Wonderful World of Simon Stevin. p. 154. ISBN 9781845643911.
 ^ Eötvös, Loránd; Annalen der Physik (Leipzig) 68 11 (1922)
 ^ ^{a} ^{b} Roll, Peter G.; Krotkov, Robert; Dicke, Robert H.; The equivalence of inertial and passive gravitational mass, Annals of Physics, Volume 26, Issue 3, 20 February 1964, pp. 442–517
 ^ "Weak Equivalence Principle test on the moon".
 ^ Schlamminger, Stephan; Choi, KiYoung; Wagner, Todd A.; Gundlach, Jens H.; Adelberger, Eric G. (2008). "Test of the Equivalence Principle Using a Rotating Torsion Balance". Physical Review Letters. 100 (4): 041101. arXiv:0712.0607. Bibcode:2008PhRvL.100d1101S. doi:10.1103/PhysRevLett.100.041101. PMID 18352252. S2CID 18653407.
 ^ Ciufolini, Ignazio; Wheeler, John A.; "Gravitation and Inertia", Princeton, New Jersey: Princeton University Press, 1995, pp. 117–119
 ^ Philoponus, John; "Corollaries on Place and Void", translated by David Furley, Ithaca, New York: Cornell University Press, 1987
 ^ Stevin, Simon; De Beghinselen der Weeghconst ["Principles of the Art of Weighing"], Leyden, 1586; Dijksterhuis, Eduard J.; "The Principal Works of Simon Stevin", Amsterdam, 1955
 ^ Galilei, Galileo; "Discorsi e Dimostrazioni Matematiche Intorno a Due Nuove Scienze", Leida: Appresso gli Elsevirii, 1638; "Discourses and Mathematical Demonstrations Concerning Two New Sciences", Leiden: Elsevier Press, 1638
 ^ Newton, Isaac; "Philosophiae Naturalis Principia Mathematica" [Mathematical Principles of Natural Philosophy and his System of the World], translated by Andrew Motte, revised by Florian Cajori, Berkeley, California: University of California Press, 1934; Newton, Isaac; "The Principia: Mathematical Principles of Natural Philosophy", translated by I. Bernard Cohen and Anne Whitman, with the assistance of Julia Budenz, Berkeley, California: University of California Press, 1999
 ^ Bessel, Friedrich W.; "Versuche Uber die Kraft, mit welcher die Erde Körper von verschiedner Beschaffenhelt anzieht", Annalen der Physik und Chemie, Berlin: J. Ch. Poggendorff, 25 401–408 (1832)
 ^ R. v. Eötvös 1890 Mathematische und Naturwissenschaftliche Berichte aus Ungarn, 8, 65; Annalen der Physik (Leipzig) 68 11 (1922); Smith, G. L.; Hoyle, C. D.; Gundlach, J. H.; Adelberger, E. G.; Heckel, B. R.; Swanson, H. E. (1999). "Shortrange tests of the equivalence principle". Physical Review D. 61 (2). doi:10.1103/PhysRevD.61.022001.
 ^ Southerns, Leonard (1910). "A Determination of the Ratio of Mass to Weight for a Radioactive Substance". Proceedings of the Royal Society of London. 84 (571): 325–344. Bibcode:1910RSPSA..84..325S. doi:10.1098/rspa.1910.0078.
 ^ Zeeman, Pieter (1918) "Some experiments on gravitation: The ratio of mass to weight for crystals and radioactive substances", Proceedings of the Koninklijke Nederlandse Akademie van Wetenschappen, Amsterdam 20(4) 542–553
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References
 Dicke, Robert H.; "New Research on Old Gravitation", Science 129, 3349 (1959). This paper is the first to make the distinction between the strong and weak equivalence principles.
 Dicke, Robert H.; "Mach's Principle and Equivalence", in Evidence for gravitational theories: proceedings of course 20 of the International School of Physics "Enrico Fermi", ed. C. Møller (Academic Press, New York, 1962). This article outlines the approach to precisely testing general relativity advocated by Dicke and pursued from 1959 onwards.
 Einstein, Albert; "Über das Relativitätsprinzip und die aus demselben gezogene Folgerungen", Jahrbuch der Radioaktivitaet und Elektronik 4 (1907); translated "On the relativity principle and the conclusions drawn from it", in The collected papers of Albert Einstein. Vol. 2 : The Swiss years: writings, 1900–1909 (Princeton University Press, Princeton, New Jersey, 1989), Anna Beck translator. This is Einstein's first statement of the equivalence principle.
 Einstein, Albert; "Über den Einfluß der Schwerkraft auf die Ausbreitung des Lichtes", Annalen der Physik 35 (1911); translated "On the Influence of Gravitation on the Propagation of Light" in The collected papers of Albert Einstein. Vol. 3 : The Swiss years: writings, 1909–1911 (Princeton University Press, Princeton, New Jersey, 1994), Anna Beck translator, and in The Principle of Relativity, (Dover, 1924), pp 99–108, W. Perrett and G. B. Jeffery translators, ISBN 0486600815. The two Einstein papers are discussed online at The Genesis of General Relativity.
 Brans, Carl H.; "The roots of scalartensor theory: an approximate history", arXiv:grqc/0506063. Discusses the history of attempts to construct gravity theories with a scalar field and the relation to the equivalence principle and Mach's principle.
 Misner, Charles W.; Thorne, Kip S.; and Wheeler, John A.; Gravitation, New York: W. H. Freeman and Company, 1973, Chapter 16 discusses the equivalence principle.
 Ohanian, Hans; and Ruffini, Remo; Gravitation and Spacetime 2nd edition, New York: Norton, 1994, ISBN 0393965015 Chapter 1 discusses the equivalence principle, but incorrectly, according to modern usage, states that the strong equivalence principle is wrong.
 Uzan, JeanPhilippe; "The fundamental constants and their variation: Observational status and theoretical motivations", Reviews of Modern Physics 75, 403 (2003). arXiv:hepph/0205340 This technical article reviews the best constraints on the variation of the fundamental constants.
 Will, Clifford M.; Theory and experiment in gravitational physics, Cambridge, UK: Cambridge University Press, 1993. This is the standard technical reference for tests of general relativity.
 Will, Clifford M.; Was Einstein Right?: Putting General Relativity to the Test, Basic Books (1993). This is a popular account of tests of general relativity.
 Will, Clifford M.; The Confrontation between General Relativity and Experiment, Living Reviews in Relativity (2006). An online, technical review, covering much of the material in Theory and experiment in gravitational physics. The Einstein and strong variants of the equivalence principles are discussed in sections 2.1 and 3.1, respectively.
 Friedman, Michael; Foundations of SpaceTime Theories, Princeton, New Jersey: Princeton University Press, 1983. Chapter V discusses the equivalence principle.
 Ghins, Michel; Budden, Tim (2001), "The Principle of Equivalence", Stud. Hist. Phil. Mod. Phys., 32 (1): 33–51, Bibcode:2001SHPMP..32...33G, doi:10.1016/S13552198(00)000381
 Ohanian, Hans C. (1977), "What is the Principle of Equivalence?", American Journal of Physics, 45 (10): 903–909, Bibcode:1977AmJPh..45..903O, doi:10.1119/1.10744
 Di Casola, E.; Liberati, S.; Sonego, S. (2015), "Nonequivalence of equivalence principles", American Journal of Physics, 83 (1): 39, arXiv:1310.7426, Bibcode:2015AmJPh..83...39D, doi:10.1119/1.4895342, S2CID 119110646