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**Gravity** or **gravitation** (from Latin * gravitas* 'weight'^{ [1] }) is a natural phenomenon by which all things with mass or energy—including planets, stars, galaxies, and even light ^{ [2] }—are brought toward (or *gravitate* toward) one another. Gravity is a physical connection between space and matter that is precisely described by Einstein’s geometric theory of gravity. In simple terms, the theory states that matter curves the space around it, and it moves with respect to the curvature of space, including curvature caused by other matter.^{ [3] }

- History of gravitational theory
- Ancient world
- Scientific revolution
- Newton's theory of gravitation
- Equivalence principle
- General relativity
- Gravity and quantum mechanics
- Specifics
- Earth's gravity
- Equations for a falling body near the surface of the Earth
- Gravity and astronomy
- Gravitational radiation
- Speed of gravity
- Anomalies and discrepancies
- Alternative theories
- Historical alternative theories
- Modern alternative theories
- See also
- Footnotes
- References
- Further reading
- External links

On Earth, gravity gives weight to physical objects, and the Moon's gravity causes the ocean tides. The gravitational attraction of the original gaseous matter present in the Universe caused it to begin coalescing and forming stars and caused the stars to group together into galaxies, so gravity is responsible for many of the large-scale structures in the Universe. Gravity has an infinite range, although its effects become increasingly weaker as objects get further away.

Gravity is most accurately described by the general theory of relativity (proposed by Albert Einstein in 1915), which describes gravity not as a force, but as a consequence of masses moving "straight ahead" in a curved spacetime caused by the uneven distribution of mass. The most extreme example of this curvature of spacetime is a black hole, from which nothing—not even light—can escape once past the black hole's event horizon.^{ [4] } However, for most applications, gravity is well approximated by Newton's law of universal gravitation, which describes gravity as a force causing any two bodies to be attracted toward each other, with magnitude proportional to the product of their masses and inversely proportional to the square of the distance between them.

Gravity is the weakest of the four fundamental interactions of physics, approximately 10^{38} times weaker than the strong interaction, 10^{36} times weaker than the electromagnetic force and 10^{29} times weaker than the weak interaction. As a consequence, it has no significant influence at the level of subatomic particles.^{ [5] } In contrast, it is the dominant interaction at the macroscopic scale, and is the cause of the formation, shape and trajectory (orbit) of astronomical bodies.

Current models of particle physics imply that the earliest instance of gravity in the Universe, possibly in the form of quantum gravity, supergravity or a gravitational singularity, along with ordinary space and time, developed during the Planck epoch (up to 10^{−43} seconds after the birth of the Universe), possibly from a primeval state, such as a false vacuum, quantum vacuum or virtual particle, in a currently unknown manner.^{ [6] } Attempts to develop a theory of gravity consistent with quantum mechanics, a quantum gravity theory, which would allow gravity to be united in a common mathematical framework (a theory of everything) with the other three fundamental interactions of physics, are a current area of research.

The ancient Greek philosopher Archimedes discovered the center of gravity of a triangle.^{ [7] } He also postulated that if two equal weights did not have the same center of gravity, the center of gravity of the two weights together would be in the middle of the line that joins their centers of gravity.^{ [8] }

The Roman architect and engineer Vitruvius in * De Architectura * postulated that gravity of an object did not depend on weight but its "nature".^{ [9] }

In ancient India, Aryabhata first identified the force to explain why objects are not thrown outward as the earth rotates. Brahmagupta described gravity as an attractive force and used the term "gurutvaakarshan" for gravity.^{ [10] }^{ [11] }

Modern work on gravitational theory began with the work of Galileo Galilei in the late 16th and early 17th centuries. In his famous (though possibly apocryphal ^{ [12] }) experiment dropping balls from the Tower of Pisa, and later with careful measurements of balls rolling down inclines, Galileo showed that gravitational acceleration is the same for all objects. This was a major departure from Aristotle's belief that heavier objects have a higher gravitational acceleration.^{ [13] } Galileo postulated air resistance as the reason that objects with less mass fall more slowly in an atmosphere. Galileo's work set the stage for the formulation of Newton's theory of gravity.^{ [14] }

In 1687, English mathematician Sir Isaac Newton published * Principia *, which hypothesizes the inverse-square law of universal gravitation. In his own words, "I deduced that the forces which keep the planets in their orbs must [be] reciprocally as the squares of their distances from the centers about which they revolve: and thereby compared the force requisite to keep the Moon in her Orb with the force of gravity at the surface of the Earth; and found them answer pretty nearly."^{ [15] } The equation is the following:

Where *F* is the force, *m _{1}* and

Newton's theory enjoyed its greatest success when it was used to predict the existence of Neptune based on motions of Uranus that could not be accounted for by the actions of the other planets. Calculations by both John Couch Adams and Urbain Le Verrier predicted the general position of the planet, and Le Verrier's calculations are what led Johann Gottfried Galle to the discovery of Neptune.

A discrepancy in Mercury's orbit pointed out flaws in Newton's theory. By the end of the 19th century, it was known that its orbit showed slight perturbations that could not be accounted for entirely under Newton's theory, but all searches for another perturbing body (such as a planet orbiting the Sun even closer than Mercury) had been fruitless. The issue was resolved in 1915 by Albert Einstein's new theory of general relativity, which accounted for the small discrepancy in Mercury's orbit. This discrepancy was the advance in the perihelion of Mercury of 42.98 arcseconds per century.^{ [16] }

Although Newton's theory has been superseded by Albert Einstein's general relativity, most modern non-relativistic gravitational calculations are still made using Newton's theory because it is simpler to work with and it gives sufficiently accurate results for most applications involving sufficiently small masses, speeds and energies.

The equivalence principle, explored by a succession of researchers including Galileo, Loránd Eötvös, and Einstein, expresses the idea that all objects fall in the same way, and that the effects of gravity are indistinguishable from certain aspects of acceleration and deceleration. The simplest way to test the weak equivalence principle is to drop two objects of different masses or compositions in a vacuum and see whether they hit the ground at the same time. Such experiments demonstrate that all objects fall at the same rate when other forces (such as air resistance and electromagnetic effects) are negligible. More sophisticated tests use a torsion balance of a type invented by Eötvös. Satellite experiments, for example STEP, are planned for more accurate experiments in space.^{ [17] }

Formulations of the equivalence principle include:

- The weak equivalence principle:
*The trajectory of a point mass in a gravitational field depends only on its initial position and velocity, and is independent of its composition.*^{ [18] } - The Einsteinian equivalence principle:
*The outcome of any local non-gravitational experiment in a freely falling laboratory is independent of the velocity of the laboratory and its location in spacetime.*^{ [19] } - The strong equivalence principle requiring both of the above.

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In general relativity, the effects of gravitation are ascribed to spacetime curvature instead of a force. The starting point for general relativity is the equivalence principle, which equates free fall with inertial motion and describes free-falling inertial objects as being accelerated relative to non-inertial observers on the ground.^{ [20] }^{ [21] } In Newtonian physics, however, no such acceleration can occur unless at least one of the objects is being operated on by a force.

Einstein proposed that spacetime is curved by matter, and that free-falling objects are moving along locally straight paths in curved spacetime. These straight paths are called geodesics. Like Newton's first law of motion, Einstein's theory states that if a force is applied on an object, it would deviate from a geodesic. For instance, we are no longer following geodesics while standing because the mechanical resistance of the Earth exerts an upward force on us, and we are non-inertial on the ground as a result. This explains why moving along the geodesics in spacetime is considered inertial.

Einstein discovered the field equations of general relativity, which relate the presence of matter and the curvature of spacetime and are named after him. The Einstein field equations are a set of 10 simultaneous, non-linear, differential equations. The solutions of the field equations are the components of the metric tensor of spacetime. A metric tensor describes a geometry of spacetime. The geodesic paths for a spacetime are calculated from the metric tensor.

Notable solutions of the Einstein field equations include:

- The Schwarzschild solution, which describes spacetime surrounding a spherically symmetric non-rotating uncharged massive object. For compact enough objects, this solution generated a black hole with a central singularity. For radial distances from the center which are much greater than the Schwarzschild radius, the accelerations predicted by the Schwarzschild solution are practically identical to those predicted by Newton's theory of gravity.
- The Reissner-Nordström solution, in which the central object has an electrical charge. For charges with a geometrized length which are less than the geometrized length of the mass of the object, this solution produces black holes with double event horizons.
- The Kerr solution for rotating massive objects. This solution also produces black holes with multiple event horizons.
- The Kerr-Newman solution for charged, rotating massive objects. This solution also produces black holes with multiple event horizons.
- The cosmological Friedmann-Lemaître-Robertson-Walker solution, which predicts the expansion of the Universe.

The tests of general relativity included the following:^{ [22] }

- General relativity accounts for the anomalous perihelion precession of Mercury.
^{ [23] } - The prediction that time runs slower at lower potentials (gravitational time dilation) has been confirmed by the Pound–Rebka experiment (1959), the Hafele–Keating experiment, and the GPS.
- The prediction of the deflection of light was first confirmed by Arthur Stanley Eddington from his observations during the Solar eclipse of 29 May 1919.
^{ [24] }^{ [25] }Eddington measured starlight deflections twice those predicted by Newtonian corpuscular theory, in accordance with the predictions of general relativity. However, his interpretation of the results was later disputed.^{ [26] }More recent tests using radio interferometric measurements of quasars passing behind the Sun have more accurately and consistently confirmed the deflection of light to the degree predicted by general relativity.^{ [27] }See also gravitational lens. - The time delay of light passing close to a massive object was first identified by Irwin I. Shapiro in 1964 in interplanetary spacecraft signals.
- Gravitational radiation has been indirectly confirmed through studies of binary pulsars. On 11 February 2016, the LIGO and Virgo collaborations announced the first observation of a gravitational wave.
- Alexander Friedmann in 1922 found that Einstein equations have non-stationary solutions (even in the presence of the cosmological constant). In 1927 Georges Lemaître showed that static solutions of the Einstein equations, which are possible in the presence of the cosmological constant, are unstable, and therefore the static Universe envisioned by Einstein could not exist. Later, in 1931, Einstein himself agreed with the results of Friedmann and Lemaître. Thus general relativity predicted that the Universe had to be non-static—it had to either expand or contract. The expansion of the Universe discovered by Edwin Hubble in 1929 confirmed this prediction.
^{ [28] } - The theory's prediction of frame dragging was consistent with the recent Gravity Probe B results.
^{ [29] } - General relativity predicts that light should lose its energy when traveling away from massive bodies through gravitational redshift. This was verified on earth and in the solar system around 1960.

An open question is whether it is possible to describe the small-scale interactions of gravity with the same framework as quantum mechanics. General relativity describes large-scale bulk properties whereas quantum mechanics is the framework to describe the smallest scale interactions of matter. Without modifications these frameworks are incompatible.^{ [30] }

One path is to describe gravity in the framework of quantum field theory, which has been successful to accurately describe the other fundamental interactions. The electromagnetic force arises from an exchange of virtual photons, where the QFT description of gravity is that there is an exchange of virtual gravitons.^{ [31] }^{ [32] } This description reproduces general relativity in the classical limit. However, this approach fails at short distances of the order of the Planck length,^{ [30] } where a more complete theory of quantum gravity (or a new approach to quantum mechanics) is required.

Every planetary body (including the Earth) is surrounded by its own gravitational field, which can be conceptualized with Newtonian physics as exerting an attractive force on all objects. Assuming a spherically symmetrical planet, the strength of this field at any given point above the surface is proportional to the planetary body's mass and inversely proportional to the square of the distance from the center of the body.

The strength of the gravitational field is numerically equal to the acceleration of objects under its influence.^{ [33] } The rate of acceleration of falling objects near the Earth's surface varies very slightly depending on latitude, surface features such as mountains and ridges, and perhaps unusually high or low sub-surface densities.^{ [34] } For purposes of weights and measures, a standard gravity value is defined by the International Bureau of Weights and Measures, under the International System of Units (SI).

That value, denoted *g*, is *g* = 9.80665 m/s^{2} (32.1740 ft/s^{2}).^{ [35] }^{ [36] }

The standard value of 9.80665 m/s^{2} is the one originally adopted by the International Committee on Weights and Measures in 1901 for 45° latitude, even though it has been shown to be too high by about five parts in ten thousand.^{ [37] } This value has persisted in meteorology and in some standard atmospheres as the value for 45° latitude even though it applies more precisely to latitude of 45°32'33".^{ [38] }

Assuming the standardized value for g and ignoring air resistance, this means that an object falling freely near the Earth's surface increases its velocity by 9.80665 m/s (32.1740 ft/s or 22 mph) for each second of its descent. Thus, an object starting from rest will attain a velocity of 9.80665 m/s (32.1740 ft/s) after one second, approximately 19.62 m/s (64.4 ft/s) after two seconds, and so on, adding 9.80665 m/s (32.1740 ft/s) to each resulting velocity. Also, again ignoring air resistance, any and all objects, when dropped from the same height, will hit the ground at the same time.

According to Newton's 3rd Law, the Earth itself experiences a force equal in magnitude and opposite in direction to that which it exerts on a falling object. This means that the Earth also accelerates towards the object until they collide. Because the mass of the Earth is huge, however, the acceleration imparted to the Earth by this opposite force is negligible in comparison to the object's. If the object does not bounce after it has collided with the Earth, each of them then exerts a repulsive contact force on the other which effectively balances the attractive force of gravity and prevents further acceleration.

The force of gravity on Earth is the resultant (vector sum) of two forces:^{ [39] } (a) The gravitational attraction in accordance with Newton's universal law of gravitation, and (b) the centrifugal force, which results from the choice of an earthbound, rotating frame of reference. The force of gravity is weakest at the equator because of the centrifugal force caused by the Earth's rotation and because points on the equator are furthest from the center of the Earth. The force of gravity varies with latitude and increases from about 9.780 m/s^{2} at the Equator to about 9.832 m/s^{2} at the poles.

Under an assumption of constant gravitational attraction, Newton's law of universal gravitation simplifies to *F* = *mg*, where *m* is the mass of the body and *g* is a constant vector with an average magnitude of 9.81 m/s^{2} on Earth. This resulting force is the object's weight. The acceleration due to gravity is equal to this *g*. An initially stationary object which is allowed to fall freely under gravity drops a distance which is proportional to the square of the elapsed time. The image on the right, spanning half a second, was captured with a stroboscopic flash at 20 flashes per second. During the first ^{1}⁄_{20} of a second the ball drops one unit of distance (here, a unit is about 12 mm); by ^{2}⁄_{20} it has dropped at total of 4 units; by ^{3}⁄_{20}, 9 units and so on.

Under the same constant gravity assumptions, the potential energy, *E*_{p}, of a body at height *h* is given by *E*_{p} = *mgh* (or *E*_{p} = *Wh*, with *W* meaning weight). This expression is valid only over small distances *h* from the surface of the Earth. Similarly, the expression for the maximum height reached by a vertically projected body with initial velocity *v* is useful for small heights and small initial velocities only.

The application of Newton's law of gravity has enabled the acquisition of much of the detailed information we have about the planets in the Solar System, the mass of the Sun, and details of quasars; even the existence of dark matter is inferred using Newton's law of gravity. Although we have not traveled to all the planets nor to the Sun, we know their masses. These masses are obtained by applying the laws of gravity to the measured characteristics of the orbit. In space an object maintains its orbit because of the force of gravity acting upon it. Planets orbit stars, stars orbit galactic centers, galaxies orbit a center of mass in clusters, and clusters orbit in superclusters. The force of gravity exerted on one object by another is directly proportional to the product of those objects' masses and inversely proportional to the square of the distance between them.

The earliest gravity (possibly in the form of quantum gravity, supergravity or a gravitational singularity), along with ordinary space and time, developed during the Planck epoch (up to 10^{−43} seconds after the birth of the Universe), possibly from a primeval state (such as a false vacuum, quantum vacuum or virtual particle), in a currently unknown manner.^{ [6] }

General relativity predicts that energy can be transported out of a system through gravitational radiation. Any accelerating matter can create curvatures in the space-time metric, which is how the gravitational radiation is transported away from the system. Co-orbiting objects can generate curvatures in space-time such as the Earth-Sun system, pairs of neutron stars, and pairs of black holes. Another astrophysical system predicted to lose energy in the form of gravitational radiation are exploding supernovae.

The first indirect evidence for gravitational radiation was through measurements of the Hulse–Taylor binary in 1973. This system consists of a pulsar and neutron star in orbit around one another. Its orbital period has decreased since its initial discovery due to a loss of energy, which is consistent for the amount of energy loss due to gravitational radiation. This research was awarded the Nobel Prize in Physics in 1993.

The first direct evidence for gravitational radiation was measured on 14 September 2015 by the LIGO detectors. The gravitational waves emitted during the collision of two black holes 1.3 billion-light years from Earth were measured.^{ [41] }^{ [42] } This observation confirms the theoretical predictions of Einstein and others that such waves exist. It also opens the way for practical observation and understanding of the nature of gravity and events in the Universe including the Big Bang.^{ [43] } Neutron star and black hole formation also create detectable amounts of gravitational radiation.^{ [44] } This research was awarded the Nobel Prize in physics in 2017.^{ [45] }

As of 2020^{ [update] }, the gravitational radiation emitted by the Solar System is far too small to measure with current technology.

In December 2012, a research team in China announced that it had produced measurements of the phase lag of Earth tides during full and new moons which seem to prove that the speed of gravity is equal to the speed of light.^{ [46] } This means that if the Sun suddenly disappeared, the Earth would keep orbiting it normally for 8 minutes, which is the time light takes to travel that distance. The team's findings were released in the Chinese Science Bulletin in February 2013.^{ [47] }

In October 2017, the LIGO and Virgo detectors received gravitational wave signals within 2 seconds of gamma ray satellites and optical telescopes seeing signals from the same direction. This confirmed that the speed of gravitational waves was the same as the speed of light.^{ [48] }

There are some observations that are not adequately accounted for, which may point to the need for better theories of gravity or perhaps be explained in other ways.

**Extra-fast stars**: Stars in galaxies follow a distribution of velocities where stars on the outskirts are moving faster than they should according to the observed distributions of normal matter. Galaxies within galaxy clusters show a similar pattern. Dark matter, which would interact through gravitation but not electromagnetically, would account for the discrepancy. Various modifications to Newtonian dynamics have also been proposed.**Flyby anomaly**: Various spacecraft have experienced greater acceleration than expected during gravity assist maneuvers.**Accelerating expansion**: The metric expansion of space seems to be speeding up. Dark energy has been proposed to explain this. A recent alternative explanation is that the geometry of space is not homogeneous (due to clusters of galaxies) and that when the data are reinterpreted to take this into account, the expansion is not speeding up after all,^{ [49] }however this conclusion is disputed.^{ [50] }**Anomalous increase of the astronomical unit**: Recent measurements indicate that planetary orbits are widening faster than if this were solely through the Sun losing mass by radiating energy.**Extra energetic photons**: Photons travelling through galaxy clusters should gain energy and then lose it again on the way out. The accelerating expansion of the Universe should stop the photons returning all the energy, but even taking this into account photons from the cosmic microwave background radiation gain twice as much energy as expected. This may indicate that gravity falls off faster than inverse-squared at certain distance scales.^{ [51] }**Extra massive hydrogen clouds**: The spectral lines of the Lyman-alpha forest suggest that hydrogen clouds are more clumped together at certain scales than expected and, like dark flow, may indicate that gravity falls off slower than inverse-squared at certain distance scales.^{ [51] }

- Aristotelian theory of gravity
- Le Sage's theory of gravitation (1784) also called LeSage gravity, proposed by Georges-Louis Le Sage, based on a fluid-based explanation where a light gas fills the entire Universe.
- Ritz's theory of gravitation,
*Ann. Chem. Phys.*13, 145, (1908) pp. 267–271, Weber-Gauss electrodynamics applied to gravitation. Classical advancement of perihelia. - Nordström's theory of gravitation (1912, 1913), an early competitor of general relativity.
- Kaluza Klein theory (1921)
- Whitehead's theory of gravitation (1922), another early competitor of general relativity.

- Brans–Dicke theory of gravity (1961)
^{ [52] } - Induced gravity (1967), a proposal by Andrei Sakharov according to which general relativity might arise from quantum field theories of matter
- String theory (late 1960s)
- ƒ(R) gravity (1970)
- Horndeski theory (1974)
^{ [53] } - Supergravity (1976)
- In the modified Newtonian dynamics (MOND) (1981), Mordehai Milgrom proposes a modification of Newton's second law of motion for small accelerations
^{ [54] } - The self-creation cosmology theory of gravity (1982) by G.A. Barber in which the Brans-Dicke theory is modified to allow mass creation
- Loop quantum gravity (1988) by Carlo Rovelli, Lee Smolin, and Abhay Ashtekar
- Nonsymmetric gravitational theory (NGT) (1994) by John Moffat
- Tensor–vector–scalar gravity (TeVeS) (2004), a relativistic modification of MOND by Jacob Bekenstein
- Chameleon theory (2004) by Justin Khoury and Amanda Weltman.
- Pressuron theory (2013) by Olivier Minazzoli and Aurélien Hees.
- Conformal gravity
^{ [55] } - Gravity as an entropic force, gravity arising as an emergent phenomenon from the thermodynamic concept of entropy.
- In the superfluid vacuum theory the gravity and curved space-time arise as a collective excitation mode of non-relativistic background superfluid.

- Anti-gravity, the idea of neutralizing or repelling gravity
- Artificial gravity
- Gauss's law for gravity
- Gravitational potential
- Gravitational wave
- Kepler's third law of planetary motion
- Micro-g environment, also called microgravity
- Newton's laws of motion
- Standard gravitational parameter
- Weightlessness

- ↑ dict.cc dictionary :: gravitas :: English-Latin translation
- ↑ Comins, Neil F.; Kaufmann, William J. (2008).
*Discovering the Universe: From the Stars to the Planets*. MacMillan. p. 347. Bibcode:2009dufs.book.....C. ISBN 978-1429230421. - ↑ Misner; Thome; Wheeler (1973).
*Gravitation*. San Francisco: W. H. Freeman and Company. pp. ix, 5. ark:/13960/t4gn4m40j. - ↑ "HubbleSite: Black Holes: Gravity's Relentless Pull".
*hubblesite.org*. Retrieved 7 October 2016. - ↑ Krebs, Robert E. (1999).
*Scientific Development and Misconceptions Through the Ages: A Reference Guide*(illustrated ed.). Greenwood Publishing Group. p. 133. ISBN 978-0-313-30226-8. - 1 2 Staff. "Birth of the Universe".
*University of Oregon*. Retrieved 24 September 2016. – discusses "Planck time" and "Planck era" at the very beginning of the Universe - ↑ Reviel Neitz; William Noel (13 October 2011).
*The Archimedes Codex: Revealing The Secrets of the World's Greatest Palimpsest*. Hachette UK. p. 125. ISBN 978-1-78022-198-4.CS1 maint: multiple names: authors list (link) - ↑ CJ Tuplin, Lewis Wolpert (2002).
*Science and Mathematics in Ancient Greek Culture*. Hachette UK. p. xi. ISBN 978-0-19-815248-4. - ↑ Vitruvius, Marcus Pollio (1914). "7". In Alfred A. Howard (ed.).
*De Architectura libri decem*[*Ten Books on Architecture*].*VII*. Herbert Langford Warren, Nelson Robinson (illus), Morris Hicky Morgan. Harvard University, Cambridge: Harvard University Press. p. 215. - ↑ Pickover, Clifford (16 April 2008).
*Archimedes to Hawking: Laws of Science and the Great Minds Behind Them*. Oxford University Press. ISBN 9780199792689. - ↑
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*The Argumentative Indian*. Allen Lane. p. 29. ISBN 978-0-7139-9687-6.

- Sen, Amartya (2005).
- ↑ Ball, Phil (June 2005). "Tall Tales".
*Nature News*. doi:10.1038/news050613-10. - ↑ Galileo (1638),
*Two New Sciences*, First Day Salviati speaks: "If this were what Aristotle meant you would burden him with another error which would amount to a falsehood; because, since there is no such sheer height available on earth, it is clear that Aristotle could not have made the experiment; yet he wishes to give us the impression of his having performed it when he speaks of such an effect as one which we see." - ↑ Bongaarts, Peter (2014).
*Quantum Theory: A Mathematical Approach*(illustrated ed.). Springer. p. 11. ISBN 978-3-319-09561-5. - ↑
- Chandrasekhar, Subrahmanyan (2003).
*Newton's Principia for the common reader*. Oxford: Oxford University Press. (pp. 1–2). The quotation comes from a memorandum thought to have been written about 1714. As early as 1645 Ismaël Bullialdus had argued that any force exerted by the Sun on distant objects would have to follow an inverse-square law. However, he also dismissed the idea that any such force did exist. See, for example,

*From Eudoxus to Einstein – A History of Mathematical Astronomy*. Cambridge: Cambridge University Press. p. 225. ISBN 978-0-521-82750-8. - Chandrasekhar, Subrahmanyan (2003).
- ↑ Nobil, Anna M. (March 1986). "The real value of Mercury's perihelion advance".
*Nature*.**320**(6057): 39–41. Bibcode:1986Natur.320...39N. doi:10.1038/320039a0. S2CID 4325839. - ↑ M.C.W.Sandford (2008). "STEP: Satellite Test of the Equivalence Principle". Rutherford Appleton Laboratory. Archived from the original on 28 September 2011. Retrieved 14 October 2011.
- ↑ Paul S Wesson (2006).
*Five-dimensional Physics*. World Scientific. p. 82. ISBN 978-981-256-661-4. - ↑ Haugen, Mark P.; C. Lämmerzahl (2001), "Principles of Equivalence: Their Role in Gravitation Physics and Experiments that Test Them",
*Gyros*, Lecture Notes in Physics,**562**(562,*Gyros, Clocks, and Interferometers...: Testing Relativistic Gravity in Space*): 195–212, arXiv: gr-qc/0103067 , Bibcode:2001LNP...562..195H, doi:10.1007/3-540-40988-2_10, S2CID 15430387 - ↑ "Gravity and Warped Spacetime". black-holes.org. Archived from the original on 21 June 2011. Retrieved 16 October 2010.
- ↑ Dmitri Pogosyan. "Lecture 20: Black Holes – The Einstein Equivalence Principle". University of Alberta. Retrieved 14 October 2011.
- ↑ Pauli, Wolfgang Ernst (1958). "Part IV. General Theory of Relativity".
*Theory of Relativity*. Courier Dover Publications. ISBN 978-0-486-64152-2. - ↑ Max Born (1924),
*Einstein's Theory of Relativity*(The 1962 Dover edition, page 348 lists a table documenting the observed and calculated values for the precession of the perihelion of Mercury, Venus, and Earth.) - ↑ Dyson, F.W.; Eddington, A.S.; Davidson, C.R. (1920). "A Determination of the Deflection of Light by the Sun's Gravitational Field, from Observations Made at the Total Eclipse of May 29, 1919".
*Phil. Trans. Roy. Soc. A*.**220**(571–581): 291–333. Bibcode:1920RSPTA.220..291D. doi: 10.1098/rsta.1920.0009 .. Quote, p. 332: "Thus the results of the expeditions to Sobral and Principe can leave little doubt that a deflection of light takes place in the neighbourhood of the sun and that it is of the amount demanded by Einstein's generalised theory of relativity, as attributable to the sun's gravitational field." - ↑ Weinberg, Steven (1972).
*Gravitation and cosmology*. John Wiley & Sons.. Quote, p. 192: "About a dozen stars in all were studied, and yielded values 1.98 ± 0.11" and 1.61 ± 0.31", in substantial agreement with Einstein's prediction θ_{☉}= 1.75"." - ↑ Earman, John; Glymour, Clark (1980). "Relativity and Eclipses: The British eclipse expeditions of 1919 and their predecessors".
*Historical Studies in the Physical Sciences*.**11**(1): 49–85. doi:10.2307/27757471. JSTOR 27757471. S2CID 117096916. - ↑ Weinberg, Steven (1972).
*Gravitation and cosmology*. John Wiley & Sons. p. 194. - ↑ See W.Pauli, 1958, pp. 219–220
- ↑ "NASA's Gravity Probe B Confirms Two Einstein Space-Time Theories". Nasa.gov. Retrieved 23 July 2013.
- 1 2 Randall, Lisa (2005).
*Warped Passages: Unraveling the Universe's Hidden Dimensions*. Ecco. ISBN 978-0-06-053108-9. - ↑ Feynman, R.P.; Morinigo, F.B.; Wagner, W.G.; Hatfield, B. (1995).
*Feynman lectures on gravitation*. Addison-Wesley. ISBN 978-0-201-62734-3. - ↑ Zee, A. (2003).
*Quantum Field Theory in a Nutshell*. Princeton University Press. ISBN 978-0-691-01019-9. - ↑ Cantor, G.N.; Christie, J.R.R.; Hodge, M.J.S.; Olby, R.C. (2006).
*Companion to the History of Modern Science*. Routledge. p. 448. ISBN 978-1-134-97751-2. - ↑ Nemiroff, R.; Bonnell, J., eds. (15 December 2014). "The Potsdam Gravity Potato".
*Astronomy Picture of the Day*. NASA.Cite has empty unknown parameter:`|last-author-amp=`

(help) - ↑ Bureau International des Poids et Mesures (2006). "The International System of Units (SI)" (PDF) (8th ed.): 131.
Unit names are normally printed in Roman (upright) type ... Symbols for quantities are generally single letters set in an italic font, although they may be qualified by further information in subscripts or superscripts or in brackets.

Cite journal requires`|journal=`

(help) - ↑ "SI Unit rules and style conventions". National Institute For Standards and Technology (USA). September 2004.
Variables and quantity symbols are in italic type. Unit symbols are in Roman type.

- ↑ List, R.J. editor, 1968, Acceleration of Gravity,
*Smithsonian Meteorological Tables*, Sixth Ed. Smithsonian Institution, Washington, DC, p. 68. - ↑ U.S. Standard Atmosphere, 1976, U.S. Government Printing Office, Washington, D.C., 1976. (Linked file is very large.)
- ↑ Hofmann-Wellenhof, B.; Moritz, H. (2006).
*Physical Geodesy*(2nd ed.). Springer. ISBN 978-3-211-33544-4. § 2.1: "The total force acting on a body at rest on the earth's surface is the resultant of gravitational force and the centrifugal force of the earth's rotation and is called gravity". - ↑ "Milky Way Emerges as Sun Sets over Paranal".
*www.eso.org*. European Southern Obseevatory. Retrieved 29 April 2015. - ↑ Clark, Stuart (11 February 2016). "Gravitational waves: scientists announce 'we did it!' –live".
*the Guardian*. Retrieved 11 February 2016. - ↑ Castelvecchi, Davide; Witze, Witze (11 February 2016). "Einstein's gravitational waves found at last".
*Nature News*. doi:10.1038/nature.2016.19361. S2CID 182916902 . Retrieved 11 February 2016. - ↑ "WHAT ARE GRAVITATIONAL WAVES AND WHY DO THEY MATTER?". popsci.com. Retrieved 12 February 2016.
- ↑ Abbott, B. P.; et al. (LIGO Scientific Collaboration & Virgo Collaboration) (October 2017). "GW170817: Observation of Gravitational Waves from a Binary Neutron Star Inspiral" (PDF).
*Physical Review Letters*.**119**(16): 161101. arXiv: 1710.05832 . Bibcode:2017PhRvL.119p1101A. doi: 10.1103/PhysRevLett.119.161101 . PMID 29099225. - ↑ Devlin, Hanna (3 October 2017). "Nobel prize in physics awarded for discovery of gravitational waves".
*the Guardian*. Retrieved 3 October 2017. - ↑ Chinese scientists find evidence for speed of gravity, astrowatch.com, 12/28/12.
- ↑ TANG, Ke Yun; HUA ChangCai; WEN Wu; CHI ShunLiang; YOU QingYu; YU Dan (February 2013). "Observational evidences for the speed of the gravity based on the Earth tide".
*Chinese Science Bulletin*.**58**(4–5): 474–477. Bibcode:2013ChSBu..58..474T. doi: 10.1007/s11434-012-5603-3 . - ↑ "GW170817 Press Release".
*LIGO Lab – Caltech*. - ↑ Dark energy may just be a cosmic illusion,
*New Scientist*, issue 2646, 7 March 2008. - ↑ Swiss-cheese model of the cosmos is full of holes,
*New Scientist*, issue 2678, 18 October 2008. - 1 2 Chown, Marcus (16 March 2009). "Gravity may venture where matter fears to tread".
*New Scientist*. Retrieved 4 August 2013. - ↑ Brans, C.H. (March 2014). "Jordan-Brans-Dicke Theory".
*Scholarpedia*.**9**(4): 31358. arXiv: gr-qc/0207039 . Bibcode:2014Schpj...931358B. doi:10.4249/scholarpedia.31358. - ↑ Horndeski, G.W. (September 1974). "Second-Order Scalar-Tensor Field Equations in a Four-Dimensional Space".
*International Journal of Theoretical Physics*.**88**(10): 363–384. Bibcode:1974IJTP...10..363H. doi:10.1007/BF01807638. S2CID 122346086. - ↑ Milgrom, M. (June 2014). "The MOND paradigm of modified dynamics".
*Scholarpedia*.**9**(6): 31410. Bibcode:2014SchpJ...931410M. doi: 10.4249/scholarpedia.31410 . - ↑ Haugan, Mark P; Lämmerzahl, C (2011). "Einstein gravity from conformal gravity". arXiv: 1105.5632 [hep-th].

**General relativity**, also known as the **general theory of relativity**, is the geometric theory of gravitation published by Albert Einstein in 1915 and is the current description of gravitation in modern physics. General relativity generalizes special relativity and refines Newton's law of universal gravitation, providing a unified description of gravity as a geometric property of space and time or four-dimensional spacetime. In particular, the *curvature of spacetime* is directly related to the energy and momentum of whatever matter and radiation are present. The relation is specified by the Einstein field equations, a system of partial differential equations.

An **inertial frame of reference** in classical physics and special relativity possesses the property that in this frame of reference a body with zero net force acting upon it does not accelerate; that is, such a body is at rest or moving at a constant velocity. An inertial frame of reference can be defined in analytical terms as a frame of reference that describes time and space homogeneously, isotropically, and in a time-independent manner. Conceptually, the physics of a system in an inertial frame have no causes external to the system. An inertial frame of reference may also be called an **inertial reference frame**, **inertial frame**, **Galilean reference frame**, or **inertial space**.

**Mass** is both a property of a physical body and a measure of its resistance to acceleration when a net force is applied. An object's mass also determines the strength of its gravitational attraction to other bodies.

The **theory of relativity** usually encompasses two interrelated theories by Albert Einstein: special relativity and general relativity. Special relativity applies to all physical phenomena in the absence of gravity. General relativity explains the law of gravitation and its relation to other forces of nature. It applies to the cosmological and astrophysical realm, including astronomy.

A **gravitational singularity**, **spacetime singularity** or simply **singularity** is a location in spacetime where the gravitational field of a celestial body is predicted to become infinite by general relativity in a way that does not depend on the coordinate system. The quantities used to measure gravitational field strength are the scalar invariant curvatures of spacetime, which includes a measure of the density of matter. Since such quantities become infinite at the singularity, the laws of normal spacetime break down.

**Timeline of gravitational physics and general relativity**

* A Brief History of Time: From the Big Bang to Black Holes* is a popular-science book on cosmology by English physicist Stephen Hawking. It was first published in 1988. Hawking wrote the book for readers without prior knowledge of the universe and people who are just interested in learning something new.

In physics, a **gravitational field** is a model used to explain the influence that a massive body extends into the space around itself, producing a force on another massive body. Thus, a gravitational field is used to explain gravitational phenomena, and is measured in newtons per kilogram (N/kg). In its original concept, gravity was a force between point masses. Following Isaac Newton, Pierre-Simon Laplace attempted to model gravity as some kind of radiation field or fluid, and since the 19th century explanations for gravity have usually been taught in terms of a field model, rather than a point attraction.

In general relativity, a **white hole** is a hypothetical region of spacetime and singularity which cannot be entered from the outside, although energy-matter and light can escape from it. In this sense, it is the reverse of a black hole, which can be entered only from the outside and from which energy-matter and light cannot escape. White holes appear in the theory of eternal black holes. In addition to a black hole region in the future, such a solution of the Einstein field equations has a white hole region in its past. However, some believe this region does not exist for black holes that have formed through gravitational collapse, nor are there any known physical processes through which a white hole could be formed. Although information and evidence regarding white holes remains inconclusive, the 2006 GRB 060614 has been proposed as the first documented observance of a white hole.

**Gravity Probe A** (**GP-A**) was a space-based experiment to test the equivalence principle, a feature of Einstein's theory of relativity. It was performed jointly by the Smithsonian Astrophysical Observatory and the National Aeronautics and Space Administration. The experiment sent a hydrogen maser—a highly accurate frequency standard—into space to measure with high precision the rate at which time passes in a weaker gravitational field. Masses cause distortions in spacetime, which leads to the effects of length contraction and time dilation, both predicted results of Albert Einstein's theory of general relativity. Because of the bending of spacetime, an observer on Earth should measure a slower rate at which time passes than an observer that is higher in altitude. This effect is known as gravitational time dilation.

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 is the same as the *pseudo-force* experienced by an observer in a non-inertial (accelerated) frame of reference.

**General relativity** is a theory of gravitation that was developed by Albert Einstein between 1907 and 1915. According to general relativity, the observed gravitational effect between masses results from their warping of spacetime.

**Tests of general relativity** serve to establish observational evidence for the theory of general relativity. The first three tests, proposed by Albert Einstein in 1915, concerned the "anomalous" precession of the perihelion of Mercury, the bending of light in gravitational fields, and the gravitational redshift. The precession of Mercury was already known; experiments showing light bending in accordance with the predictions of general relativity were performed in 1919, with increasingly precise measurements made in subsequent tests; and scientists claimed to have measured the gravitational redshift in 1925, although measurements sensitive enough to actually confirm the theory were not made until 1954. A more accurate program starting in 1959 tested general relativity in the weak gravitational field limit, severely limiting possible deviations from the theory.

General relativity (GR) is a theory of gravitation that was developed by Albert Einstein between 1907 and 1915, with contributions by many others after 1915. According to general relativity, the observed gravitational attraction between masses results from the warping of space and time by those masses.

In physics, theories of gravitation postulate mechanisms of interaction governing the movements of bodies with mass. There have been numerous theories of gravitation since ancient times. The first extant sources discussing such theories are found in ancient Greek philosophy. This work was furthered by ancient Indian and medieval Islamic physicists, before gaining great strides during the Renaissance and Scientific Revolution, culminating in the formulation of Newton's law of gravity. This was superseded by Albert Einstein's theory of relativity in the early 20th century.

The **mathematics of general relativity** are complex. In Newton's theories of motion, an object's length and the rate at which time passes remain constant while the object accelerates, meaning that many problems in Newtonian mechanics may be solved by algebra alone. In relativity, however, an object's length and the rate at which time passes both change appreciably as the object's speed approaches the speed of light, meaning that more variables and more complicated mathematics are required to calculate the object's motion. As a result, relativity requires the use of concepts such as vectors, tensors, pseudotensors and curvilinear coordinates.

**Gravitational waves** are disturbances in the curvature of spacetime, generated by accelerated masses, that propagate as waves outward from their source at the speed of light. They were proposed by Henri Poincaré in 1905 and subsequently predicted in 1916 by Albert Einstein on the basis of his general theory of relativity. Gravitational waves transport energy as **gravitational radiation**, a form of radiant energy similar to electromagnetic radiation. Newton's law of universal gravitation, part of classical mechanics, does not provide for their existence, since that law is predicated on the assumption that physical interactions propagate instantaneously – showing one of the ways the methods of classical physics are unable to explain phenomena associated with relativity.

*This article will use the Einstein summation convention.*

The following outline is provided as an overview of and topical guide to black holes:

**Entropic gravity**, also known as **emergent gravity**, is a theory in modern physics that describes gravity as an *entropic force*—a force with macro-scale homogeneity but which is subject to quantum-level disorder—and not a fundamental interaction. The theory, based on string theory, black hole physics, and quantum information theory, describes gravity as an *emergent* phenomenon that springs from the quantum entanglement of small bits of spacetime information. As such, entropic gravity is said to abide by the second law of thermodynamics under which the entropy of a physical system tends to increase over time.

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*Physics v. 1*. New York: John Wiley & Sons. ISBN 978-0-471-32057-9. - Serway, Raymond A.; Jewett, John W. (2004).
*Physics for Scientists and Engineers*(6th ed.). Brooks/Cole. ISBN 978-0-534-40842-8. - Tipler, Paul (2004).
*Physics for Scientists and Engineers: Mechanics, Oscillations and Waves, Thermodynamics*(5th ed.). W.H. Freeman. ISBN 978-0-7167-0809-4.

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*Gravitation*. W.H. Freeman. ISBN 978-0-7167-0344-0. - Panek, Richard (2 August 2019). "Everything you thought you knew about gravity is wrong".
*Washington Post*.

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