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Gravity

In physics, gravity (from Latin gravitas 'weight'[1]) is a fundamental interaction which causes mutual attraction between all things that have mass. Gravity is, by far, the weakest of the four fundamental interactions, approximately 1038 times weaker than the strong interaction, 1036 times weaker than the electromagnetic force and 1029 times weaker than the weak interaction. As a result, it has no significant influence at the level of subatomic particles.[2] However, gravity is the most significant interaction between objects at the macroscopic scale, and it determines the motion of planets, stars, galaxies, and even light.

For other uses, see Gravity (disambiguation).

On Earth, gravity gives weight to physical objects, and the Moon's gravity is responsible for sublunar tides in 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 of gravitropism and influencing the circulation of fluids in multicellular organisms.


The gravitational attraction between the original gaseous matter in the universe caused it to coalesce and form stars which 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 the general theory of relativity, proposed by Albert Einstein in 1915, which describes gravity not as a force, but as the curvature of spacetime, caused by the uneven distribution of mass, and causing masses to move along geodesic lines. 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.[3] 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.


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.[4] Scientists are currently working to develop a theory of gravity consistent with quantum mechanics, a quantum gravity theory,[5] which would allow gravity to be united in a common mathematical framework (a theory 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, the centrifugal force resulting from the planet's rotation (see § Earth's gravity).[7]

The , 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.[46] At points far away from the central mass, the accelerations predicted by the Schwarzschild solution are practically identical to those predicted by Newton's theory of gravity.[47]

Schwarzschild solution

The , which analyzes a non-rotating spherically symmetric object with charge and was independently discovered by several different researchers between 1916 and 1921.[48] In some cases, this solution can predict the existence of black holes with double event horizons.[49]

Reissner–Nordström solution

The , which generalizes the Schwarzchild solution to rotating massive objects. Because of the difficulty of factoring in the effects of rotation into the Einstein field equations, this solution was not discovered until 1963.[50]

Kerr solution

The for charged, rotating massive objects. This solution was derived in 1964, using the same technique of complex coordinate transformation that was used for the Kerr solution.[51]

Kerr–Newman solution

The Friedmann–Lemaître–Robertson–Walker solution, discovered in 1922 by Alexander Friedmann and then confirmed in 1927 by Georges Lemaître. This solution was revolutionary for predicting the expansion of the Universe, which was confirmed seven years later after a series of measurements by Edwin Hubble.[52] It even showed that general relativity was incompatible with a static universe, and Einstein later conceded that he had been wrong to design his field equations to account for a Universe that was not expanding.[53]

cosmological

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 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.

distribution of velocities

: Various spacecraft have experienced greater acceleration than expected during gravity assist maneuvers.

Flyby anomaly

: The expansion of the universe seems to be speeding up.[88] Dark energy has been proposed to explain this.[89]

Accelerated expansion

Anomalous increase of the : Recent measurements indicate that planetary orbits are widening faster than if this were solely through the Sun losing mass by radiating energy.

astronomical unit

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.[90]

cosmic microwave background radiation

Extra massive hydrogen clouds: The spectral lines of the 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.[90]

Lyman-alpha forest

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.

Aristotelian theory of gravity

(1784) also called LeSage gravity but originally proposed by Fatio and further elaborated by Georges-Louis Le Sage, based on a fluid-based explanation where a light gas fills the entire Universe.

Le Sage's theory of gravitation

Ann. Chem. Phys. 13, 145, (1908) pp. 267–271, Weber–Gauss electrodynamics applied to gravitation. Classical advancement of perihelia.

Ritz's theory of gravitation

(1912, 1913), an early competitor of general relativity.

Nordström's theory of gravitation

(1921)

Kaluza–Klein theory

(1922), another early competitor of general relativity.

Whitehead's theory of gravitation

I. Bernard Cohen (1999) [1687]. "A Guide to Newton's Principia". The Principia : mathematical principles of natural philosophy. By Newton, Isaac. Translated by I. Bernard Cohen. University of California Press.  9780520088160. OCLC 313895715.

ISBN

Halliday, David; Robert Resnick; Kenneth S. Krane (2001). Physics v. 1. New York: John Wiley & Sons.  978-0-471-32057-9.

ISBN

Serway, Raymond A.; Jewett, John W. (2004). (6th ed.). Brooks/Cole. ISBN 978-0-534-40842-8.

Physics for Scientists and Engineers

Tipler, Paul (2004). Physics for Scientists and Engineers: Mechanics, Oscillations and Waves, Thermodynamics (5th ed.). W.H. Freeman.  978-0-7167-0809-4.

ISBN

; Misner, Charles W.; Wheeler, John Archibald (1973). Gravitation. W.H. Freeman. ISBN 978-0-7167-0344-0.

Thorne, Kip S.

Panek, Richard (2 August 2019). . The Washington Post.

"Everything you thought you knew about gravity is wrong"

The Feynman Lectures on Physics Vol. I Ch. 7: The Theory of Gravitation

, Encyclopedia of Mathematics, EMS Press, 2001 [1994]

"Gravitation"

, Encyclopedia of Mathematics, EMS Press, 2001 [1994]

"Gravitation, theory of"