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{{redirect-multi|2|Gravitation|Law of Gravity}} {{redirect-multi|2|Gravitation|Law of Gravity}}
{{Use dmy dates|date=December 2014}}{{Classical mechanics}} {{Use dmy dates|date=December 2014}}{{Classical mechanics}}
before the discovery of the gravity people could fly
] ] ] on the Moon enacting the legend of Galileo's gravity experiment. (1.38 ], ]/] format).]]
after newton found it they couldnt fly anymore

and becose the needation is the mother of invention
'''Gravity''', or '''gravitation''', is a ] by which all things with ] are brought toward (or ''gravitate'' toward) one another, including ]s, ]s and ]. Since ], all forms of ], including ], also cause gravitation and are under the influence of it. On ], gravity gives ] to physical objects and causes the ocean ]. The gravitational attraction of the original gaseous matter present in the ] caused it to begin coalescing, forming ]s — and the stars to group together into ] — 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 on farther objects.
people invented the plains so the recould refly angain

touti touti story ended
Gravity is most accurately described by the ] (proposed by ] in 1915) which describes gravity not as a ], but as a consequence of the curvature of ] caused by the uneven distribution of ]/]. The most extreme example of this curvature of spacetime is a ], from which nothing can escape once past its event horizon, not even light.<ref>{{Cite web|url=http://hubblesite.org/explore_astronomy/black_holes/home.html|title=HubbleSite: Black Holes: Gravity's Relentless Pull|website=hubblesite.org|access-date=2016-10-07}}</ref> More gravity results in ], where time lapses more slowly at a lower (stronger) ]. However, for most applications, gravity is well approximated by ], which postulates that gravity causes a ] where two bodies of mass are directly drawn (or 'attracted') to each other according to a mathematical relationship, where the attractive force is ] to the product of their masses and ] to the ] of the ] between them.<!---This is considered{{by whom|date=February 2016}} to occur over an infinite range, such that all bodies (with mass) in the Universe are drawn to each other no matter how far they are apart.{{citation needed|date=February 2016|reason=infinite range claim is not supported in the body of this article}}--->

Gravity is the weakest of the four ]s of nature. The gravitational attraction is approximately 10<sup>38</sup> times weaker than the ], 10<sup>36</sup> times weaker than the ] and 10<sup>29</sup> times weaker than the ]. As a consequence, gravity has a negligible influence on the behavior of subatomic particles, and plays no role in determining the internal properties of everyday matter (but see ]). On the other hand, gravity is the dominant interaction at the ], and is the cause of the formation, shape and ] (]) of astronomical bodies. It is responsible for various phenomena observed on Earth and throughout the Universe; for example, it causes the Earth and the other planets to orbit the Sun, the ] to orbit the Earth, the formation of ]s, the ], ] and ].

The earliest instance of gravity in the Universe, possibly in the form of ], ] or a ], along with ordinary ] and ], developed during the ] (up to 10<sup>−43</sup> seconds after the ] of the ]), possibly from a primeval state, such as a ], ] or ], in a currently unknown manner.<ref name="Planck-UOregon">{{cite web |author=Staff |title=Birth of the Universe |url=http://abyss.uoregon.edu/~js/cosmo/lectures/lec20.html |date= |work=] |accessdate=September 24, 2016 }} - discusses "]" and "]" at the ] of the ]</ref> For this reason, in part, pursuit of a ], the merging of the general theory of relativity and ] (or ]) into ], has become an area of research.

==History of gravitational theory==
{{main article|History of gravitational theory}}
===Earlier Concepts of Gravity===

While the modern European thinkers are rightly credited with development of gravitational theory, there were pre-existing ideas which had identified the force of gravity.
Some of the earliest descriptions came from early mathematician-astronomers, such as ], who had identified the force of gravity to explain why objects do not fall out when the Earth rotates.<ref>*{{cite book
| first= Amartya
| last= Sen
| title= The Argumentative Indian
| date= 2005
| page= 29
| publisher= Allen Lane
| isbn= 978-0-7139-9687-6}} </ref>

Later, the works of ] referred to the presence of this force.{{cn|date=October 2016}}

===Scientific revolution===
Modern work on gravitational theory began with the work of ] in the late 16th and early 17th centuries. In his famous (though possibly ]l<ref name=Ball_Piza>{{cite journal |last=Ball |first=Phil |date=June 2005 |title=Tall Tales |journal=Nature News |doi=10.1038/news050613-10 }}</ref>) experiment dropping balls from the ], and later with careful measurements of balls rolling down ], Galileo showed that gravitational acceleration is the same for all objects. This was a major departure from ]'s belief that heavier objects have a higher gravitational acceleration.<ref>] (1638), '']'', 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."</ref> Galileo postulated ] as the reason that objects with less mass may fall slower in an atmosphere. Galileo's work set the stage for the formulation of Newton's theory of gravity.<ref>{{cite book |title=Quantum Theory: A Mathematical Approach |edition=illustrated |first1=Peter |last1=Bongaarts |publisher=Springer |year=2014 |isbn=978-3-319-09561-5 |page=11 |url=https://books.google.com/books?id=Cc6lBQAAQBAJ}} </ref>

===Newton's theory of gravitation===
{{main article|Newton's law of universal gravitation}}
], an English physicist who lived from 1642 to 1727]]
In 1687, English mathematician Sir ] published '']'', which hypothesizes the ] of universal gravitation. In his own words, "I deduced that the forces which keep the planets in their orbs must 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."<ref>*{{cite book
| first= Subrahmanyan
| last= Chandrasekhar
| authorlink= Subrahmanyan Chandrasekhar
| title= Newton's Principia for the common reader
| date= 2003
| publisher= Oxford University Press
| location= Oxford}} (pp.1–2). The quotation comes from a memorandum thought to have been written about 1714. As early as 1645 ] 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,
{{cite book | title= From Eudoxus to Einstein—A History of Mathematical Astronomy
| author= Linton, Christopher M.
| publisher= Cambridge University Press
| date= 2004
| location= Cambridge
| page= 225
| isbn= 978-0-521-82750-8
| ref= Linton-2004}}
</ref> The equation is the following:

<math>F = G \frac{m_1 m_2}{r^2}\ </math>

Where ''F'' is the force, ''m<sub>1</sub>'' and ''m<sub>2</sub>'' are the masses of the objects interacting, ''r'' is the distance between the centers of the masses and ''G'' is the ].

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

A discrepancy in ]'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 ] even closer than Mercury) had been fruitless. The issue was resolved in 1915 by ]'s new theory of ], which accounted for the small discrepancy in Mercury's orbit.

Although Newton's theory has been superseded by the ]'s general relativity, most modern ] 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.

===Equivalence principle===
The ], explored by a succession of researchers including Galileo, ], 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 ]es 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 ], are planned for more accurate experiments in space.<ref>{{cite web |author=M.C.W.Sandford |publisher=] |title=STEP: Satellite Test of the Equivalence Principle |url=http://www.sstd.rl.ac.uk/fundphys/step/ |date=2008 |accessdate=2011-10-14}}</ref>

Formulations of the equivalence principle include:
* The weak equivalence principle: ''The trajectory of a point mass in a ] depends only on its initial position and velocity, and is independent of its composition.''<ref name=Wesson>{{cite book |title=Five-dimensional Physics |author= Paul S Wesson |page=82 |url=https://books.google.com/?id=dSv8ksxHR0oC&printsec=frontcover&dq=intitle:Five+intitle:Dimensional+intitle:Physics |isbn=981-256-661-9 |publisher=World Scientific |date=2006}}</ref>
* 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.''<ref name="Lāmmerzahl">{{cite book |last=Haugen | first=Mark P. | author2=C. Lämmerzahl |title=Principles of Equivalence: Their Role in Gravitation Physics and Experiments that Test Them |date=2001 |publisher=Springer |isbn=978-3-540-41236-6 |arxiv=gr-qc/0103067}}</ref>
* The strong equivalence principle requiring both of the above.

===General relativity===
{{see also|Introduction to general relativity}}
] imposed on the curved spacetime, which would be ] in a flat spacetime.]]
{{General relativity sidebar}}
In ], the effects of gravitation are ascribed to ] ] instead of a force. The starting point for general relativity is the ], which equates free fall with inertial motion and describes free-falling inertial objects as being accelerated relative to non-inertial observers on the ground.<ref>{{cite web|url=http://www.black-holes.org/relativity6.html |title=Gravity and Warped Spacetime |publisher=black-holes.org |accessdate=2010-10-16}}</ref><ref>{{cite web |title=Lecture 20: Black Holes—The Einstein Equivalence Principle |author=Dmitri Pogosyan |url=http://www.ualberta.ca/~pogosyan/teaching/ASTRO_122/lect20/lecture20.html |publisher=University of Alberta |accessdate=2011-10-14}}</ref> In ], 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 ]. 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 ]s of general relativity, which relate the presence of matter and the curvature of spacetime and are named after him. The ] are a set of 10 ], ], ]s. The solutions of the field equations are the components of the ] of spacetime. A metric tensor describes a geometry of spacetime. The geodesic paths for a spacetime are calculated from the metric tensor.

====Solutions====
Notable solutions of the Einstein field equations include:
* The ], which describes spacetime surrounding a ] non-] uncharged massive object. For compact enough objects, this solution generated a ] with a central ]. For radial distances from the center which are much greater than the ], the accelerations predicted by the Schwarzschild solution are practically identical to those predicted by Newton's theory of gravity.
* The ], in which the central object has an electrical charge. For charges with a ] length which are less than the geometrized length of the mass of the object, this solution produces black holes with double ]s.
* The ] for rotating massive objects. This solution also produces black holes with multiple event horizons.
* The ] for charged, rotating massive objects. This solution also produces black holes with multiple event horizons.
* The ] ], which predicts the expansion of the ].

====Tests====
The ] included the following:<ref name=Pauli1958>{{cite book|last=Pauli|first=Wolfgang Ernst|title=Theory of Relativity|date=1958|isbn=978-0-486-64152-2|publisher=Courier Dover Publications|chapter=Part IV. General Theory of Relativity}}</ref>
* General relativity accounts for the anomalous ].<ref>] (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.)</ref>
* The prediction that time runs slower at lower potentials (]) has been confirmed by the ] (1959), the ], and the ].
* The prediction of the deflection of light was first confirmed by ] from his observations during the ].<ref>{{cite journal|last1=Dyson|first1=F.W.|authorlink1=Frank Watson Dyson|last2= Eddington|first2=A.S.|authorlink2=Arthur Eddington|last3=Davidson|first3=C.R. |date=1920 |title=A Determination of the Deflection of Light by the Sun's Gravitational Field, from Observations Made at the Total Eclipse of May 29, 1919|journal= ]|volume=220|issue=571–581|pages= 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."</ref><ref>{{cite book|first=Steven|last=Weinberg|authorlink=Steven Weinberg|title=Gravitation and cosmology|publisher=John Wiley &amp; Sons|date=1972}}. 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<sub>☉</sub> = 1.75"."</ref> 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.<ref>{{cite journal| last1=Earman |first1=John |last2=Glymour |first2=Clark |title=Relativity and Eclipses: The British eclipse expeditions of 1919 and their predecessors |date=1980 |journal=Historical Studies in the Physical Sciences |volume=11 |pages=49–85| doi=10.2307/27757471}}</ref> More recent tests using radio interferometric measurements of ]s passing behind the ] have more accurately and consistently confirmed the deflection of light to the degree predicted by general relativity.<ref>{{cite book|first=Steven|last=Weinberg|authorlink=Steven Weinberg|title=Gravitation and cosmology|publisher=John Wiley &amp; Sons|date=1972|page=194}}</ref> See also ].
* The ] passing close to a massive object was first identified by ] in 1964 in interplanetary spacecraft signals.
* ] has been indirectly confirmed through studies of binary ]s. On 11 February 2016, the ] and ] collaborations announced the first observation of a gravitational wave.
* ] in 1922 found that Einstein equations have non-stationary solutions (even in the presence of the ]). In 1927 ] 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 ] in 1929 confirmed this prediction.<ref name=Pauli1>See W.Pauli, 1958, pp.219–220</ref>
*The theory's prediction of ] was consistent with the recent ] results.<ref>{{cite web|url=http://www.nasa.gov/home/hqnews/2011/may/HQ_11-134_Gravity_Probe_B.html |title=NASA's Gravity Probe B Confirms Two Einstein Space-Time Theories |publisher=Nasa.gov |accessdate=2013-07-23}}</ref>
*General relativity predicts that light should lose its energy when traveling away from massive bodies through ]. This was verified on earth and in the solar system around 1960.

===Gravity and quantum mechanics===
{{main article|Graviton|Quantum gravity}}
In the decades after the discovery of general relativity, it was realized that general relativity is incompatible with ].<ref name="Randall, Lisa 2005">{{cite book | author=Randall, Lisa | title=Warped Passages: Unraveling the Universe's Hidden Dimensions | publisher=Ecco | date=2005 | isbn=0-06-053108-8}}</ref> It is possible to describe gravity in the framework of ] like the other ], such that the attractive force of gravity arises due to exchange of ] gravitons, in the same way as the electromagnetic force arises from exchange of virtual ]s.<ref>{{cite book |last= Feynman |first= R. P. |author2=Morinigo, F. B. |author3=Wagner, W. G. |author4=Hatfield, B. |title= Feynman lectures on gravitation |publisher= Addison-Wesley |date= 1995 |isbn=0-201-62734-5 }}</ref><ref>{{cite book | author=Zee, A. |title=Quantum Field Theory in a Nutshell | publisher = Princeton University Press | date=2003 | isbn=0-691-01019-6}}</ref> This reproduces general relativity in the ]. However, this approach fails at short distances of the order of the ],<ref name="Randall, Lisa 2005"/> where a more complete theory of ] (or a new approach to quantum mechanics) is required.

==Specifics==

===Earth's gravity===
]
{{main article|Earth's gravity}}
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.{{citation needed|date=March 2015}} 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.<ref>{{Cite APOD|date = 15 December 2014|title = The Potsdam Gravity Potato|access-date = }}</ref> For purposes of weights and measures, a ] value is defined by the ], under the ] (SI).

That value, denoted ''g'', is ''g'' = 9.80665&nbsp;m/s<sup>2</sup> (32.1740&nbsp;ft/s<sup>2</sup>).<ref>{{cite journal
|author=Bureau International des Poids et Mesures
|date=2006
|url=http://www.bipm.org/utils/common/pdf/si_brochure_8_en.pdf
|title=The International System of Units (SI)
|page=131
|edition=8th
|accessdate=2009-11-25
|quote=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.}}</ref><ref>{{cite web
|url=http://physics.nist.gov/cuu/Units/checklist.html
|quote=Variables and quantity symbols are in italic type. Unit symbols are in Roman type.
|title=SI Unit rules and style conventions
|date=September 2004
|publisher=National Institute For Standards and Technology (USA)
|accessdate=2009-11-25}}</ref>

The standard value of 9.80665&nbsp;m/s<sup>2</sup> 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.<ref name=List>List, R. J. editor, 1968, Acceleration of Gravity, ''Smithsonian Meteorological Tables'', Sixth Ed. Smithsonian Institution, Washington, D.C., p. 68.</ref> 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".<ref name=USSA1976>, 1976, U.S. Government Printing Office, Washington, D.C., 1976. (Linked file is very large.)</ref>

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&nbsp;m/s (32.1740&nbsp;ft/s or 22&nbsp;mph) for each second of its descent. Thus, an object starting from rest will attain a velocity of 9.80665&nbsp;m/s (32.1740&nbsp;ft/s) after one second, approximately 19.62&nbsp;m/s (64.4&nbsp;ft/s) after two seconds, and so on, adding 9.80665&nbsp;m/s (32.1740&nbsp;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 ], the Earth itself experiences a ] 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 doesn't bounce after it has collided with the Earth, each of them then exerts a repulsive ] 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:<ref>{{cite book
|last1 = Hofmann-Wellenhof |first1 = B.
|last2 = Moritz |first2 = H.
|title = Physical Geodesy
|publisher = Springer
|edition = 2nd
|isbn = 978-3-211-33544-4
|year = 2006
|postscript = . &sect; 2.1: &ldquo;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.&rdquo;
}}</ref> (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 the 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&nbsp;m/s<sup>2</sup> at the Equator to about 9.832&nbsp;m/s<sup>2</sup> at the poles.

===Equations for a falling body near the surface of the Earth===
{{main article|Equations for a falling body}}
Under an assumption of constant gravitational attraction, ] simplifies to ''F'' = ''mg'', where ''m'' is the ] of the body and ''g'' is a constant vector with an average magnitude of 9.81&nbsp;m/s<sup>2</sup> on Earth. This resulting force is the object's ]. 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 {{frac|20}} of a second the ball drops one unit of distance (here, a unit is about 12&nbsp;mm); by {{frac|2|20}} it has dropped at total of 4 units; by {{frac|3|20}}, 9 units and so on.

Under the same constant gravity assumptions, the ], ''E<sub>p</sub>'', of a body at height ''h'' is given by ''E<sub>p</sub>'' = ''mgh'' (or ''E<sub>p</sub>'' = ''Wh'', with ''W'' meaning weight). This expression is valid only over small distances ''h'' from the surface of the Earth. Similarly, the expression <math>h = \tfrac{v^2}{2g}</math> for the maximum height reached by a vertically projected body with initial velocity ''v'' is useful for small heights and small initial velocities only.

===Gravity and astronomy===
{{Nature timeline}}
].<ref>{{cite web|title=Milky Way Emerges as Sun Sets over Paranal|url=http://www.eso.org/public/images/potw1517a/|website=www.eso.org|publisher=European Southern Obseevatory|accessdate=29 April 2015}}</ref>]]
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 ]s; even the existence of ] 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 ] because of the force of gravity acting upon it. Planets orbit stars, stars orbit ]s, ] orbit a center of mass in clusters, and clusters orbit in ]s. 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 ], ] or a ]), along with ordinary ] and ], developed during the ] (up to 10<sup>−43</sup> seconds after the ] of the ]), possibly from a primeval state (such as a ], ] or ]), in a currently unknown manner.<ref name="Planck-UOregon"/>

===Gravitational radiation===
{{main article|Gravitational wave}}
According to general relativity, gravitational radiation is generated in situations where the curvature of ] is oscillating, such as is the case with co-orbiting objects. The gravitational radiation emitted by the ] is far too small to measure. However, gravitational radiation has been indirectly observed as an energy loss over time in binary pulsar systems such as ]. It is believed that ] mergers and ] formation may create detectable amounts of gravitational radiation. Gravitational radiation observatories such as the Laser Interferometer Gravitational Wave Observatory (]) have been created to study the problem. In February 2016, the Advanced LIGO team announced that they had detected gravitational waves from a black hole collision. On September 14, 2015 LIGO registered gravitational waves for the first time, as a result of the collision of two black holes 1.3 billion light-years from Earth.<ref name='Clark 2016'>{{Cite web|title = Gravitational waves: scientists announce 'we did it!' – live|url = https://www.theguardian.com/science/across-the-universe/live/2016/feb/11/gravitational-wave-announcement-latest-physics-einstein-ligo-black-holes-live|website = the Guardian|date=2016-02-11|access-date = 2016-02-11|first = Stuart|last = Clark}}</ref><ref name="Discovery 2016">{{cite journal |title=Einstein's gravitational waves found at last |journal=Nature News|url=http://www.nature.com/news/einstein-s-gravitational-waves-found-at-last-1.19361 |date=February 11, 2016 |last=Castelvecchi |first=Davide |last2=Witze |first2=Witze |doi=10.1038/nature.2016.19361 |accessdate=2016-02-11 }}</ref> This observation confirms the theoretical predictions of Einstein and others that such waves exist. The event confirms that ]s 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 and what happened after it.<ref name="WorldBreakingNews">{{cite news|title=Scientists announce finding Gravitational Waves confirming Einstein's theory|url=https://www.youtube.com/watch?v=n5Ycv2yYNG8#t=12|publisher=WorldBreakingNews}}</ref><ref>{{cite web|title=WHAT ARE GRAVITATIONAL WAVES AND WHY DO THEY MATTER?|url=http://www.popsci.com/whats-so-important-about-gravitational-waves|publisher=popsci.com|accessdate=12 February 2016}}</ref>

===Speed of gravity===
{{main article|Speed of gravity}}
In December 2012, a research team in China announced that it had produced measurements of the phase lag of ]s during full and new moons which seem to prove that the speed of gravity is equal to the speed of light.<ref>, astrowatch.com, 12/28/12.</ref> 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 ] in February 2013.<ref>{{cite journal|last=TANG|first=Ke Yun|author2=HUA ChangCai |author3=WEN Wu |author4=CHI ShunLiang |author5=YOU QingYu |author6=YU Dan |title=Observational evidences for the speed of the gravity based on the Earth tide|journal=Chinese Science Bulletin|date=February 2013|volume=58|issue=4-5|pages=474–477|doi=10.1007/s11434-012-5603-3|url=http://link.springer.com/content/pdf/10.1007%2Fs11434-012-5603-3.pdf|accessdate=12 June 2013}}</ref>

==Anomalies and discrepancies==
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 ] where stars on the outskirts are moving faster than they should according to the observed distributions of normal matter. Galaxies within ] show a similar pattern. ], which would interact through gravitation but not electromagnetically, would account for the discrepancy. Various ] have also been proposed.
* ''']''': Various spacecraft have experienced greater acceleration than expected during ] maneuvers.
* '''Accelerating expansion''': The ] seems to be speeding up. ] 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,<ref>, ''New Scientist'', issue 2646, 7 March 2008.</ref> however this conclusion is disputed.<ref>, ''New Scientist'', issue 2678, 18 October 2008.</ref>
* '''Anomalous increase of the ]''': Recent measurements indicate that ] 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 ] gain twice as much energy as expected. This may indicate that gravity falls off ''faster'' than inverse-squared at certain distance scales.<ref name=newsci2699>{{cite web|last=Chown|first=Marcus|title=Gravity may venture where matter fears to tread|url=http://www.newscientist.com/article/mg20126990.400-gravity-may-venture-where-matter-fears-to-tread.html|work=New Scientist|accessdate=4 August 2013|date=16 March 2009|issue=2699}}</ref>
* '''Extra massive hydrogen clouds''': The spectral lines of the ] suggest that hydrogen clouds are more clumped together at certain scales than expected and, like ], may indicate that gravity falls off ''slower'' than inverse-squared at certain distance scales.<ref name=newsci2699/>
* '''Power''': Proposed ] could explain why the gravity force is so weak.<ref>{{cite web|url=http://home.web.cern.ch/about/physics/extra-dimensions-gravitons-and-tiny-black-holes|title=Extra dimensions, gravitons, and tiny black holes|date=20 January 2012|author=CERN}}</ref>

==Alternative theories==
{{main article|Alternatives to general relativity}}

===Historical alternative theories===
* ]
* ] (1784) also called LeSage gravity, proposed by ], based on a fluid-based explanation where a light gas fills the entire Universe.
* ], ''Ann. Chem. Phys.'' 13, 145, (1908) pp.&nbsp;267–271, Weber-Gauss electrodynamics applied to gravitation. Classical advancement of perihelia.
* ] (1912, 1913), an early competitor of general relativity.
* ] (1921)
* ] (1922), another early competitor of general relativity.

===Modern alternative theories===
* ] of gravity (1961) <ref name=2014Schpj...931358B>{{cite journal|author=Brans, C.H. |date=Mar 2014 |title= Jordan-Brans-Dicke Theory|journal=Scholarpedia |volume=9 |pages=31358 |doi= 10.4249/scholarpedia.31358|bibcode= 2014Schpj...931358B}}</ref>
* ] (1967), a proposal by ] according to which ] might arise from ] of matter
* ] (1970)
* ] (1974) <ref name=1974IJTP...10..363H>{{cite journal|author=Horndeski, G.W. |date=Sep 1974 |title= Second-Order Scalar-Tensor Field Equations in a Four-Dimensional Space |journal=International Journal of Theoretical Physics |volume=88 |issue= 10 |pages=363–384 |doi= 10.1007/BF01807638|bibcode= 1974IJTP...10..363H}}</ref>
*] (1976)
*]
* In the ] (MOND) (1981), ] proposes a modification of ] of motion for small accelerations <ref name=2014SchpJ...931410M>{{cite journal|author=Milgrom, M. |date=Jun 2014 |title= The MOND paradigm of modified dynamics|journal=Scholarpedia |volume=9 |pages=31410 |doi= 10.4249/scholarpedia.31410|bibcode= 2014SchpJ...931410M}}</ref>
* The ] theory of gravity (1982) by G.A. Barber in which the Brans-Dicke theory is modified to allow mass creation
* ] (1988) by ], ], and ]
* ] (NGT) (1994) by ]
* ]<ref></ref>
* ] (TeVeS) (2004), a relativistic modification of MOND by ]
* ], gravity arising as an emergent phenomenon from the thermodynamic concept of entropy.
* In the ] the gravity and curved space-time arise as a ] mode of non-relativistic background ].
* ] (2004) by ] and ].
* ] (2013) by ] and ].

==See also==
{{div col|3}}
* ]
* ], the idea of neutralizing or repelling gravity
* ]
* ]
* ]
* ]
* ], the minimum velocity needed to escape from a ]
* ], a measure of ]
* ]
* ]
* ]
* ]
* ]
* ]
* ]
* ]
* ]
* ]
* ]
* ]
* ], also called microgravity
* ]
* ]
* ]
* ]
* ]
* ]
* ]
* ]
* ]
{{div col end}}

==Footnotes==
{{Reflist|colwidth=30em}}

==References==
{{refbegin}}
*{{cite book | last = Halliday | first = David | author2 = Robert Resnick | author3 = Kenneth S. Krane | title = Physics v. 1 | location = New York | publisher = John Wiley & Sons | date = 2001 | isbn = 0-471-32057-9 }}
*{{cite book | last = Serway | first = Raymond A. | author2 = Jewett, John W. | title = Physics for Scientists and Engineers | edition = 6th | publisher = Brooks/Cole | date = 2004 | isbn = 0-534-40842-7 }}
*{{cite book | last = Tipler | first = Paul | title = Physics for Scientists and Engineers: Mechanics, Oscillations and Waves, Thermodynamics | edition = 5th | publisher = W. H. Freeman | date = 2004 | isbn = 0-7167-0809-4 }}
{{refend}}
<!--Unused ref: Proposition 75, Theorem 35: p.&nbsp;956 - I.Bernard Cohen and Anne Whitman, translators: Isaac Newton, ''The Principia'': Mathematical Principles of Natural Philosophy. Preceded by ''A Guide to Newton's Principia'', by I. Bernard Cohen. University of California Press 1999 ISBN 0-520-08816-6 ISBN 0-520-08817-4 -->

==Further reading==
* {{cite book |author=Thorne, Kip S. |author-link=Kip Thorne |author2=Misner, Charles W. |author3=Wheeler, John Archibald |title=Gravitation |publisher=W.H. Freeman |date=1973 |isbn=0-7167-0344-0}}

==External links==
{{wiktionary}}
{{Commons category|Gravitation}}
{{EB1911 poster|Gravitation}}
* {{springer|title=Gravitation|id=p/g045040}}
* {{springer|title=Gravitation, theory of|id=p/g045050}}
{{Fundamental interactions}}
{{Theories of gravitation}}
{{Authority control}}
{{portal bar|Astronomy|Cosmology|Gravitation|Physics|Space}}

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before the discovery of the gravity people could fly after newton found it they couldnt fly anymore and becose the needation is the mother of invention people invented the plains so the recould refly angain touti touti story ended