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== History of gravitational theory ==
{{main|History of gravitational theory}}
=== Early history ===
There have been many attempts to understand and explain gravity since ancient times.

] in ] made attempts at explaining gravity from the ].<ref>Dick Teresi (2002), ''Lost Discoveries: The Ancient Roots of Modern Science - from the Babylonians to the Maya'', ], New York, ISBN 0-684-83718-8:
<br>{{quote|"Two hundred years before ], philosophers in northern India had understood that gravitation held the solar system together, and that therefore the sun, the most massive object, had to be at its centre."}}</ref> ], founder of the ] school, attempted to explain gravity: "] causes falling; it is ] and known by ]."<ref>] (2003). , p. 22. '']''. ].</ref>

The ] philosopher ] in the ] believed that there was no ] without a ], and therefore no ] without a ]. He hypothesized that everything tried to move towards its proper place in the crystalline ]s of the heavens, and that physical bodies fell toward the center of the ] in proportion to their ].

], in the '']'' (] CE), responded to critics of the ] system of ] (476-550 CE) stating that "all heavy things are attracted towards the center of the earth" and that "all heavy things fall down to the earth by a law of nature, for it is the nature of the earth to attract and to keep things, as it is the nature of water to flow, that of fire to burn, and that of wind to set in motion... The earth is the only low thing, and seeds always return to it, in whatever direction you may throw them away, and never rise upwards from the earth."<ref>] (628 CE). '']'' ("''The Opening of the Universe''").</ref><ref>] (1030). ''Ta'rikh al-Hind'' (''Indica'').</ref>

In the 9th century, ] (later known as Alkindus in Latin) stated his law of ] gravity: "All terrestrial objects are attracted towards the center of the Earth."<ref name=Qadir>Asghar Qadir (1989). ''Relativity: An Introduction to the Special Theory'', p. 6-11. World Scientific, ]. ISBN 9971506122.</ref>

=== The scientific revolution ===
Modern work on gravitational theory began with the work of ] in the late ] and early ]. In his famous experiment dropping balls at the ] and later with careful measurements of balls rolling down ]s, Galileo showed that gravitation accelerates all objects at the same rate. This was a major departure from Aristotle's belief that heavier objects are accelerated faster. (Galileo correctly postulated air resistance as the reason that lighter objects are perceived to fall more slowly.) Galileo's work set the stage for the formulation of Newton's theory of gravity.

In the 1660s, influenced by the ideas of ], ] explained his law of ] gravity: "All objects are pulled towards the ] with a force proportional to their mass and inversely proportional to the square of their distance to the Sun."<ref name=Qadir/> In the late ], as a result of Robert Hooke's suggestion that there is a gravitational force which depends on the ] of the ], ] was able to ] derive ] three ] ], including the ] ] for the seven known ]s.<ref name=Qadir/>

=== Newton's theory of gravitation ===
{{main|Law of universal gravitation}}
In 1687, English mathematician ] published '']'', which hypothesizes the ] 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.”

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 by the actions of the other planets. Calculations by ] and ] both predicted the general position of the planet, and Le Verrier's calculations are what led ] to the discovery of Neptune.

Ironically, it was another discrepancy in a planet's orbit that helped to doom Newton's theory. By the end of the 19th century, it was known that the orbit of ] 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 ], which accounted for the discrepancy in Mercury's orbit.

Although Newton's theory has been superseded, most modern non-relativistic gravitational calculations are based on Newton's work because it is a much easier theory to work with and gives fairly accurate results for most applications.

===General relativity===

{{main|Introduction to general relativity}}
In ''']''', the effects of gravitation are ascribed to ] ] instead of to a force. The starting point for general relativity is the ], which equates free fall with inertial motion. The issue that this creates is that free-falling objects can accelerate with respect to each other. In ], no such acceleration can occur unless at least one of the objects is being operated on by a force (and therefore is not moving inertially).

To deal with this difficulty, Einstein proposed that spacetime is curved by matter, and that free-falling objects are moving along locally straight paths in curved spacetime. (This type of path is called a ]). More specifically, 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.

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 two ].
* The ] for rotating massive objects. This solution also produces black holes with multiple event horizons.
* The ] ], which predicts the expansion of the ].

General relativity has enjoyed much success because of how its predictions of phenomena which are not called for by the theory of gravity have been regularly confirmed. For example:
* General relativity accounts for the anomalous ] ] of the planet ].
* The prediction that time runs slower at lower potentials has been confirmed by the ], the ], and the ].
* The prediction of the deflection of light was first confirmed by ] in ], and has more recently been strongly confirmed through the use of a ] which passes behind the ] as seen from the ]. See also ].
* The ] passing close to a massive object was first identified by ] in ] in interplanetary spacecraft signals.
* ] has been indirectly confirmed through studies of binary ]s.
* The expansion of the universe (predicted by the ]) was confirmed by ] in ].

===Gravity and quantum mechanics===

{{main|Graviton|Quantum gravity}}
Several decades after the discovery of general relativity it was realized that it cannot be the complete theory of gravity because it is incompatible with ].<ref>{{cite book | author=Randall, Lisa | title=Warped Passages: Unraveling the Universe's Hidden Dimensions | publisher=Ecco | year=2005 | id=ISBN 0-06-053108-8}}</ref> Later it was understood that it is possible to describe gravity in the framework of ] like the other ]. In this framework the attractive force of gravity arises due to exchange of ] ], in the same way as the electromagnetic force arises from exchange of virtual ].<ref>{{cite book |last= Feynman |first= R. P. |coauthors= Morinigo, F. B., Wagner, W. G., & Hatfield, B. |title= Feynman lectures on gravitation |publisher= Addison-Wesley |year= 1995 |isbn=0201627345 }}</ref><ref>{{cite book | author=Zee, A. |title=Quantum Field Theory in a Nutshell | publisher = Princeton University Press | year=2003 | id=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>{{cite book | author=Randall, Lisa | title=Warped Passages: Unraveling the Universe's Hidden Dimensions | publisher=Ecco | year=2005 | id=ISBN 0-06-053108-8}}</ref> where a more complete theory of ] is required. Many believe the complete theory to be ].<ref>{{cite book | author=Greene, Brian | title=The elegant universe: superstrings, hidden dimensions, and the quest for the ultimate theory | publisher=Vintage Books |location = New York| year=2000 | id=ISBN 0375708111}}</ref>

It is notable that in general relativity, gravitational radiation, which under the rules of quantum mechanics must be composed of gravitons, is created only in situations where the curvature of spacetime is oscillating, such as is the case with co-orbiting objects. The amount of 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 ] have been created to study the problem. No confirmed detections have been made of this hypothetical radiation, but as the science behind LIGO is refined and as the instruments themselves are endowed with greater sensitivity over the next decade, this may change.


== Specifics == == Specifics ==

Revision as of 10:44, 7 June 2007

For other uses, see Gravity (disambiguation). "Gravity" redirects here. For other uses, see Gravity (disambiguation).

Gravitation is a natural phenomenon by which all objects attract each other. In everyday life, gravitation is most familiar as the agency that endows objects with weight. Gravitation is responsible for keeping the Earth and the other planets in their orbits around the Sun; for keeping the Moon in its orbit around the Earth; for the formation of tides; for convection (by which hot fluids rise); for heating the interiors of forming stars and planets to very high temperatures; and for various other phenomena that we observe. Gravitation is also the reason for the very existence of the Earth, the Sun, and most macroscopic objects in the universe; without it, matter would not have coalesced into these large masses, and life, as we know it, would not exist.

Modern physics describes gravitation using the general theory of relativity, but the much simpler Newton's law of universal gravitation provides an excellent approximation in most cases.

In scientific terminology gravitation and gravity are distinct. "Gravitation" is the attractive influence that all objects exert on each other, while "gravity" specifically refers to a force which all massive (objects with mass) objects are theorized to exert on each other to cause gravitation. Although these terms are interchangeable in everyday use, in theories other than Newton's, gravitation is caused by factors other than gravity. For example in general relativity, gravitation is due to spacetime curvatures which causes inertially moving objects to tend to accelerate towards each other. Another (discredited) example is Le Sage's theory of gravitation, in which massive objects are effectively pushed towards each other.

The gravitational force keeps the planets in orbit about the Sun.

CAts!!!!!

Specifics

Earth's gravity

Main article: Earth's gravity

Every planetary body, including the Earth, is surrounded by its own gravitational field, which exerts an attractive force on any object. This field is proportional to the body's mass and varies inversely with the square of distance from the body. The gravitational field is numerically equal to the acceleration of objects under its influence, and its value at the Earth's surface, denoted g, is approximately 9.8 m/s². This means that, ignoring air resistance, an object falling freely near the earth's surface increases in speed by 9.807 m/s (32.174 ft/s or 22 mi/h) for each second of its descent. Thus, an object starting from rest will attain a speed of 9.807 m/s (32.17 ft/s) after one second, 19.614 m/s (64.34 ft/s) after two seconds, and so on. According to Newton's 3rd Law, the Earth itself experiences an equal and opposite force to that acting on the falling object, meaning that the Earth also accelerates towards the object. However, because the mass of the Earth is huge, the measurable acceleration of the Earth by this same force is negligible, when measured relative to the system's center of mass.

Equations for a falling body

Main article: Equations for a falling body

Under normal Earth-bound conditions, when objects move owing to a constant gravitational force a set of kinematical and dynamical equations describe the resultant trajectories. For example, Newton’s law of gravitation simplifies to F = ma, where m is the mass of the body and a is the acceleration. This assumption is reasonable for objects falling to Earth over the relatively short vertical distances of our everyday experience, but does not necessarily hold over larger distances, such as spacecraft trajectories, because the acceleration far from the surface of the Earth will not in general be g which is acceleration due to gravity (9.8 m/s). A further example is the expression that we use for the calculation of potential energy Ep of a body at height h ( Ep = mgh or as Ep = Wh, with W meaning weight). This expression can be used only over small distances h from the Earth. Similarly the expression for the maximum height reached by a vertically projected body, h = u 2 / 2 g {\displaystyle h=u^{2}/2g} is useful for small heights and small initial velocities only. In case of large initial velocities we have to use the principle of conservation of energy to find the maximum height reached.

Gravity and astronomy

Main article: Gravity (astronomy)

The discovery and application of Newton's law of gravity accounts for the detailed information we have about the planets in our solar system, the mass of the Sun, the distance to stars, quasars and even the theory of dark matter. Although we have not traveled to all the planets nor to the Sun, we know their mass. The mass is 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.

Alternative theories

Main article: Alternatives to general relativity

Historical alternative theories

Recent alternative theories

See also

Notes

  • Template:Fnb Proposition 75, Theorem 35: p.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
  • Template:Fnb 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.)

References

  • Halliday, David (2001). Physics v. 1. New York: John Wiley & Sons. ISBN 0-471-32057-9. {{cite book}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  • Serway, Raymond A. (2004). Physics for Scientists and Engineers (6th ed. ed.). Brooks/Cole. ISBN 0-534-40842-7. {{cite book}}: |edition= has extra text (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)
  • Tipler, Paul (2004). Physics for Scientists and Engineers: Mechanics, Oscillations and Waves, Thermodynamics (5th ed. ed.). W. H. Freeman. ISBN 0-7167-0809-4. {{cite book}}: |edition= has extra text (help)

External links

Fundamental interactions of physics
Physical forces
Hypothetical forces
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