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In physics,gravity(fromLatingravitas'weight'[1]) is afundamental interactionprimarily observed as mutual attraction between all things that havemass.Gravity is, by far, the weakest of the four fundamental interactions, approximately 1038times weaker than thestrong interaction,1036times weaker than theelectromagnetic forceand 1029times weaker than theweak interaction.As a result, it has no significant influence at the level ofsubatomic particles.[2]However, gravity is the most significant interaction between objects at themacroscopic scale,and it determines the motion ofplanets,stars,galaxies,and evenlight.

On Earth,gravity givesweighttophysical objects,and theMoon's gravityis responsible for sublunartidesin the oceans. The corresponding antipodal tide is caused by the inertia of the Earth and Moon orbiting one another. Gravity also has many important biological functions, helping to guide the growth of plants through the process ofgravitropismand influencing thecirculationof fluids inmulticellular organisms.

The gravitational attraction between the original gaseous matter in theuniversecaused it tocoalesceandform starswhich eventually condensed 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 weaker as objects get farther away.

Gravity is most accurately described by thegeneral theory of relativity,proposed byAlbert Einsteinin 1915, which describes gravity not as a force, but as thecurvatureofspacetime,caused by the uneven distribution of mass, and causing masses to move alonggeodesiclines. The most extreme example of this curvature of spacetime is ablack hole,from which nothing—not even light—can escape once past the black hole'sevent horizon.[3]However, for most applications, gravity is well approximated byNewton's law of universal gravitation,which describes gravity as aforcecausing any two bodies to be attracted toward each other, with magnitudeproportionalto the product of their masses andinversely proportionalto thesquareof thedistancebetween them.

Current models ofparticle physicsimply that the earliest instance of gravity in the universe, possibly in the form ofquantum gravity,supergravityor agravitational singularity,along with ordinaryspaceandtime,developed during thePlanck epoch(up to 10−43seconds after thebirthof the universe), possibly from a primeval state, such as afalse vacuum,quantum vacuumorvirtual particle,in a currently unknown manner.[4]Scientists are currently working to develop a theory of gravity consistent withquantum mechanics,a quantum gravity theory,[5]which would allow gravity to be united in a common mathematical framework (atheory of everything) with the other three fundamental interactions of physics.

Definitions

Gravitation,also known as gravitational attraction, is the mutual attraction between all masses in the universe. Gravity is the gravitational attraction at the surface of a planet or other celestial body;[6]gravity may also include, in addition to gravitation, thecentrifugal forceresulting from the planet's rotation(see§ Earth's gravity).[7]

History

Main article:History of gravitational theory

Ancient world

The nature and mechanism of gravity were explored by a wide range of ancient scholars. InGreece,Aristotlebelieved that objects fell towards the Earth because the Earth was the center of the Universe and attracted all of the mass in the Universe towards it. He also thought that the speed of a falling object should increase with its weight, a conclusion that was later shown to be false.[8]While Aristotle's view was widely accepted throughout Ancient Greece, there were other thinkers such asPlutarchwho correctly predicted that the attraction of gravity was not unique to the Earth.[9]

Although he did not understand gravity as a force, the ancient Greek philosopherArchimedesdiscovered thecenter of gravityof a triangle.[10]He 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.[11]Two centuries later, the Roman engineer and architect Vitruvius contended in hisDe architecturathat gravity is not dependent on a substance's weight but rather on its "nature".[12] In the 6th century CE, the Byzantine Alexandrian scholar John Philoponus proposed the theory of impetus, which modifies Aristotle's theory that "continuation of motion depends on continued action of a force" by incorporating a causative force that diminishes over time.[13]

In the seventh century CE, theIndianmathematician and astronomerBrahmaguptaproposed the idea that gravity is an attractive force that draws objects to the Earth and used the termgurutvākarṣaṇto describe it.[14][15][16]

In the ancientMiddle East,gravity was a topic of fierce debate. ThePersianintellectualAl-Birunibelieved that the force of gravity was not unique to the Earth, and he correctly assumed that otherheavenly bodiesshould exert a gravitational attraction as well.[17]In contrast,Al-Khaziniheld the same position as Aristotle that all matter in the Universe is attracted to the center of the Earth.[18]

TheLeaning Tower of Pisa,where according to legend Galileo performed an experiment about the speed of falling objects

Scientific revolution

Main article:Scientific revolution

In the mid-16th century, various European scientists experimentally disproved theAristoteliannotion that heavier objectsfallat a faster rate.[19]In particular, theSpanishDominican priestDomingo de Sotowrote in 1551 that bodies infree falluniformly accelerate.[19]De Soto may have been influenced by earlier experiments conducted by otherDominicanpriests in Italy, including those byBenedetto Varchi,Francesco Beato,Luca Ghini,andGiovan Bellasowhich contradicted Aristotle's teachings on the fall of bodies.[19]

The mid-16th century Italian physicistGiambattista Benedettipublished papers claiming that, due tospecific gravity,objects made of the same material but with different masses would fall at the same speed.[20]With the 1586Delft tower experiment,theFlemishphysicistSimon Stevinobserved that two cannonballs of differing sizes and weights fell at the same rate when dropped from a tower.[21]In the late 16th century,Galileo Galilei's careful measurements of balls rolling downinclinesallowed him to firmly establish that gravitational acceleration is the same for all objects.[22]Galileo postulated thatair resistanceis the reason that objects with a low density and highsurface areafall more slowly in an atmosphere.

In 1604, Galileo correctly hypothesized that the distance of a falling object is proportional to thesquareof the time elapsed.[23]This was later confirmed by Italian scientistsJesuitsGrimaldiandRicciolibetween 1640 and 1650. They also calculated the magnitude ofthe Earth's gravityby measuring the oscillations of a pendulum.[24]

Newton's theory of gravitation

Main articles:Newton's law of universal gravitationandNewton-Hooke priority controversy for the inverse square law

In 1657,Robert Hookepublished hisMicrographia,in which he hypothesised that the Moon must have its own gravity.[25]In 1666, he added two further principles: that all bodies move in straight lines until deflected by some force and that the attractive force is stronger for closer bodies. In a communication to the Royal Society in 1666, Hooke wrote[26]

I will explain a system of the world very different from any yet received. It is founded on the following positions. 1. That all the heavenly bodies have not only a gravitation of their parts to their own proper centre, but that they also mutually attract each other within their spheres of action. 2. That all bodies having a simple motion, will continue to move in a straight line, unless continually deflected from it by some extraneous force, causing them to describe a circle, an ellipse, or some other curve. 3. That this attraction is so much the greater as the bodies are nearer. As to the proportion in which those forces diminish by an increase of distance, I own I have not discovered it....

Hooke's 1674 Gresham lecture,An Attempt to prove the Annual Motion of the Earth,explained that gravitation applied to "all celestial bodies"[27]

English physicist and mathematician, SirIsaac Newton(1642–1727)

In 1684, Newton sent a manuscript toEdmond HalleytitledDe motu corporum in gyrum('On the motion of bodies in an orbit'),which provided a physical justification forKepler's laws of planetary motion.[28]Halley was impressed by the manuscript and urged Newton to expand on it, and a few years later Newton published a groundbreaking book calledPhilosophiæ Naturalis Principia Mathematica(Mathematical Principles of Natural Philosophy). In this book, Newton described gravitation as a universal force, and claimed 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." This statement was later condensed into the following inverse-square law:

whereFis the force,m1andm2are the masses of the objects interacting,ris the distance between the centers of the masses andGis thegravitational constant6.674×10−11m3⋅kg−1⋅s−2.[29]

Newton'sPrincipiawas well received by the scientific community, and his law of gravitation quickly spread across the European world.[30]More than a century later, in 1821, his theory of gravitation rose to even greater prominence when it was used to predict the existence ofNeptune.In that year, the French astronomerAlexis Bouvardused this theory to create a table modeling the orbit ofUranus,which was shown to differ significantly from the planet's actual trajectory. In order to explain this discrepancy, many astronomers speculated that there might be a large object beyond the orbit of Uranus which was disrupting its orbit. In 1846, the astronomersJohn Couch AdamsandUrbain Le Verrierindependently used Newton's law to predict Neptune's location in the night sky, and the planet was discovered there within a day.[31]

General relativity

See also:Introduction to general relativity
General relativity
Spacetime curvature schematic
Fundamental concepts
Phenomena
Spacetime
  • Equations
  • Formalisms
Equations
Formalisms
Advanced theory
Solutions
Scientists

Eventually, astronomers noticed an eccentricity in the orbit of the planetMercurywhich could not be explained by Newton's theory: theperihelionof the orbit was increasing by about 42.98arcsecondsper century. The most obvious explanation for this discrepancy was an as-yet-undiscovered celestial body, such as a planet orbiting the Sun even closer than Mercury, but all efforts to find such a body turned out to be fruitless. In 1915,Albert Einsteindeveloped a theory ofgeneral relativitywhich was able to accurately model Mercury's orbit.[32]

In general relativity, the effects of gravitation are ascribed to spacetimecurvatureinstead of a force. Einstein began to toy with this idea in the form of theequivalence principle,a discovery which he later described as "the happiest thought of my life."[33]In this theory, free fall is considered to be equivalent to inertial motion, meaning that free-falling inertial objects are accelerated relative to non-inertial observers on the ground.[34][35]In contrast toNewtonian physics,Einstein believed that it was possible for this acceleration to occur without any force being applied to the object.

Einstein proposed thatspacetimeis curved by matter, and that free-falling objects are moving along locally straight paths in curved spacetime. These straight paths are calledgeodesics.As in Newton's first law of motion, Einstein believed that a force applied to an object would cause it to deviate from a geodesic. For instance, people standing on the surface of the Earth are prevented from following a geodesic path because the mechanical resistance of the Earth exerts an upward force on them. This explains why moving along the geodesics in spacetime is considered inertial.

Einstein's description of gravity was quickly accepted by the majority of physicists, as it was able to explain a wide variety of previously baffling experimental results.[36]In the coming years, a wide range of experiments provided additional support for the idea of general relativity.[37]: p.1-9 [38][39][40][41]Today, Einstein's theory of relativity is used for all gravitational calculations where absolute precision is desired, although Newton's inverse-square law is accurate enough for virtually all ordinary calculations.[37]: p.79 [42]

Modern research

Inmodern physics,general relativity remains the framework for the understanding of gravity.[43]Physicists continue to work to findsolutionsto theEinstein field equationsthat form the basis of general relativity and continue to test the theory, finding excellent agreement in all cases.[44][45][37]: p.9 

Einstein field equations

The Einstein field equations are asystemof 10partial differential equationswhich describe how matter affects the curvature of spacetime. The system is often expressed in the form whereGμνis theEinstein tensor,gμνis themetric tensor,Tμνis thestress–energy tensor,Λis thecosmological constant,is the Newtonian constant of gravitation andis thespeed of light.[46]The constantis referred to as the Einstein gravitational constant.[47]

An illustration of theSchwarzschild metric,which describes spacetime around a spherical, uncharged, and nonrotating object with mass

A major area of research is the discovery ofexact solutionsto the Einstein field equations. Solving these equations amounts to calculating a precise value for the metric tensor (which defines the curvature and geometry of spacetime) under certain physical conditions. There is no formal definition for what constitutes such solutions, but most scientists agree that they should be expressable usingelementary functionsorlinear differential equations.[48]Some of the most notable solutions of the equations include:

Today, there remain many important situations in which the Einstein field equations have not been solved. Chief among these is thetwo-body problem,which concerns the geometry of spacetime around two mutually interacting massive objects, such as the Sun and the Earth, or the two stars in abinary star system.The situation gets even more complicated when considering the interactions of three or more massive bodies (the "n-body problem "), and some scientists suspect that the Einstein field equations will never be solved in this context.[57]However, it is still possible to construct an approximate solution to the field equations in then-body problem by using the technique ofpost-Newtonian expansion.[58]In general, the extreme nonlinearity of the Einstein field equations makes it difficult to solve them in all but the most specific cases.[59]

Gravity and quantum mechanics

Main articles:GravitonandQuantum gravity

Despite its success in predicting the effects of gravity at large scales, general relativity is ultimately incompatible withquantum mechanics.This is because general relativity describes gravity as a smooth, continuous distortion of spacetime, while quantum mechanics holds that all forces arise from the exchange of discrete particles known asquanta.This contradiction is especially vexing to physicists because the other three fundamental forces (strong force, weak force and electromagnetism) were reconciled with a quantum framework decades ago.[60]As a result, modern researchers have begun to search for a theory that could unite both gravity and quantum mechanics under a more general framework.[61]

One path is to describe gravity in the framework ofquantum field theory,which has been successful to accurately describe the otherfundamental interactions.The electromagnetic force arises from an exchange of virtualphotons,where the QFT description of gravity is that there is an exchange ofvirtualgravitons.[62][63]This description reproduces general relativity in theclassical limit.However, this approach fails at short distances of the order of thePlanck length,[64]where a more complete theory ofquantum gravity(or a new approach to quantum mechanics) is required.

Tests of general relativity

Main article:Tests of general relativity

Testing the predictions of general relativity has historically been difficult, because they are almost identical to the predictions of Newtonian gravity for small energies and masses.[65]Still, since its development, an ongoing series of experimental results have provided support for the theory:[65]

The 1919total solar eclipseprovided one of the first opportunities to test the predictions of general relativity.
  • In 1919, the British astrophysicistArthur Eddingtonwas able to confirm the predictedgravitational lensingof light duringthat year's solar eclipse.[66][67]Eddington measured starlight deflections twice those predicted by Newtonian corpuscular theory, in accordance with the predictions of general relativity. Although Eddington's analysis was later disputed, this experiment made Einstein famous almost overnight and caused general relativity to become widely accepted in the scientific community.[68]
  • In 1959, American physicistsRobert PoundandGlen Rebkaperformedan experimentin which they usedgamma raysto confirm the prediction ofgravitational time dilation.By sending the rays down a 74-foot tower and measuring their frequency at the bottom, the scientists confirmed that light isredshiftedas it moves towards a source of gravity. The observed redshift also supported the idea that time runs more slowly in the presence of a gravitational field.[69]
  • Thetime delay of lightpassing close to a massive object was first identified byIrwin I. Shapiroin 1964 in interplanetary spacecraft signals.[70]
  • In 1971, scientists discovered the first-ever black hole in the galaxyCygnus.The black hole was detected because it was emitting bursts ofx-raysas it consumed a smaller star, and it came to be known asCygnus X-1.[71]This discovery confirmed yet another prediction of general relativity, because Einstein's equations implied that light could not escape from a sufficiently large and compact object.[72]
  • General relativity states that gravity acts on light and matter equally, meaning that a sufficiently massive object could warp light around it and create agravitational lens.This phenomenon was first confirmed by observation in 1979 using the 2.1 meter telescope atKitt Peak National Observatoryin Arizona, which saw two mirror images of the same quasar whose light had been bent around the galaxyYGKOW G1.[73][74]
  • Frame dragging,the idea that a rotating massive object should twist spacetime around it, was confirmed byGravity Probe Bresults in 2011.[75][76]
  • In 2015, theLIGOobservatory detected faintgravitational waves,the existence of which had been predicted by general relativity. Scientists believe that the waves emanated from ablack hole mergerthat occurred 1.5 billionlight-yearsaway.[77]

Specifics

Earth's gravity

An initially-stationary object that is allowed to fall freely under gravity drops a distance that is proportional to the square of the elapsed time. This image spans half a second and was captured at 20 flashes per second.
Main article:Gravity of Earth

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.

If an object with comparable mass to that of the Earth were to fall towards it, then the corresponding acceleration of the Earth would be observable.

The strength of the gravitational field is numerically equal to the acceleration of objects under its influence.[78]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.[79]For purposes of weights and measures, astandard gravityvalue is defined by theInternational Bureau of Weights and Measures,under theInternational System of Units(SI).

The force of gravity on Earth is the resultant (vector sum) of two forces:[7](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 thecentrifugal forcecaused 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/s2at the Equator to about 9.832 m/s2at the poles.[80][81]

Gravitational radiation

LIGO Hanford Observatory
TheLIGOHanford Observatory located in Washington, United States, where gravitational waves were first observed in September 2015
Main article:Gravitational wave

General relativity predicts that energy can be transported out of a system through gravitational radiation. The first indirect evidence for gravitational radiation was through measurements of theHulse–Taylor binaryin 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 theNobel Prize in Physicsin 1993.[82]

The first direct evidence for gravitational radiation was measured on 14 September 2015 by theLIGOdetectors. The gravitational waves emitted during the collision of two black holes 1.3 billion light years from Earth were measured.[83][84]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.[85]Neutron starandblack holeformation also create detectable amounts of gravitational radiation.[86]This research was awarded the Nobel Prize in Physics in 2017.[87]

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 ofEarth tidesduring full and new moons which seem to prove that the speed of gravity is equal to the speed of light.[88]This means that if the Sun suddenly disappeared, the Earth would keep orbiting the vacant point normally for 8 minutes, which is the time light takes to travel that distance. The team's findings were released inScience Bulletinin February 2013.[89]

In October 2017, theLIGOand 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.[90]

Anomalies and discrepancies

Not to be confused withGravity anomaly.

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.

Rotation curve of a typical spiral galaxy: predicted (A) and observed (B). The discrepancy between the curves is attributed todark matter.

Alternative theories

Main article:Alternatives to general relativity

Historical alternative theories

Modern alternative theories

See also

References

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