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[[Image:Gravity_waves_02_2.jpg |right|frame|<center>Gravity Waves</center>]]

 

'''Gravitation''' is a natural [[phenomenon]] by which all objects with [[mass]] attract each other,[http://math.ucr.edu/home/baez/physics/Relativity/GR/grav_speed.html Does Gravity Travel at the Speed of Light?], and is one of the [[fundamental forces]] of physics. In everyday life, gravitation is most commonly thought of as the agency that gives objects [[weight]]. It is responsible for keeping the Earth and the other planets in their [[orbit]]s 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.  

'''Gravitation''' is a natural [[phenomenon]] by which all objects with [[mass]] attract each other,[https://math.ucr.edu/home/baez/physics/Relativity/GR/grav_speed.html Does Gravity Travel at the Speed of Light?], and is one of the [[fundamental forces]] of physics. In everyday life, gravitation is most commonly thought of as the agency that gives objects [[weight]]. It is responsible for keeping the Earth and the other planets in their [[orbit]]s 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. The terms '''gravitation''' and '''gravity''' are mostly interchangeable in everyday use, but in scientific usage a distinction may be made. "Gravitation" is a general term describing the attractive influence that all objects with mass exert on each other, while "gravity" specifically refers to a [[force]] that is supposed in some theories (such as Newton's) to be the cause of this attraction. By contrast, in [[general relativity]] gravitation is due to spacetime curvatures that cause [[inertia]]lly moving objects to accelerate towards each other.  

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. The terms '''gravitation''' and '''gravity''' are mostly interchangeable in everyday use, but in scientific usage a distinction may be made. "Gravitation" is a general term describing the attractive influence that all objects with mass exert on each other, while "gravity" specifically refers to a [[force]] that is supposed in some theories (such as Newton's) to be the cause of this attraction. By contrast, in [[general relativity]] gravitation is due to spacetime curvatures that cause [[inertia]]lly moving objects to accelerate towards each other.  

== History of gravitational theory ==

== History of gravitational theory ==

=== Early history ===

=== Early history ===

Efforts to understand gravity began in ancient times. [[Indian philosophy|Philosophers]] in [[History of science in early cultures#India|ancient India]] explained the phenomenon from the 8th century BC.<ref>Dick Teresi (2002), ''Lost Discoveries: The Ancient Roots of Modern Science - from the Babylonians to the Maya'', Simon & Schuster, New York, ISBN 0-684-83718-8:

Efforts to understand gravity began in ancient times. [[Indian philosophy|Philosophers]] in [[History of science in early cultures#India|ancient India]] explained the phenomenon from the 8th century BC.

<br>{{quote|"Two hundred years before [[Pythagoras]], 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> According to [[Kanada]], founder of the [[Vaisheshika]] school, "[[Mass|Weight]] causes falling; it is [[Wiktionary:imperceptible|imperceptible]] and known by [[inference]]."<ref>[[Subhash Kak|S. Kak]] (2003). [http://arxiv.org/abs/physics/0310001 Indian Physics: Outline of Early History''], p. 22. ''[[arXiv]]''. [[Louisiana State University]].

<blockquote>"Two hundred years before [[Pythagoras]], 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."</blockquote> According to [[Kanada]], founder of the [[Vaisheshika]] school, "[[Mass|Weight]] causes falling; it is [[Wiktionary:imperceptible|imperceptible]] and known by [[inference]]."[https://arxiv.org/abs/physics/0310001 Indian Physics: Outline of Early History''], p. 22. [[Louisiana State University]].

In the 4th century BC, the [[Greek philosophy|Greek philosopher]] [[Aristotle]] believed that there was no [[result|effect]] without a [[cause]], and therefore no [[Motion (physics)|motion]] without a [[force]]. He hypothesized that everything tried to move towards its proper place in the [[Celestial spheres|crystalline sphere]]s of the heavens, and that physical bodies fell toward the center of the [[Earth]] in proportion to their [[weight]].

In the 4th century BC, the [[Greek philosophy|Greek philosopher]] [[Aristotle]] believed that there was no [[result|effect]] without a [[cause]], and therefore no [[Motion (physics)|motion]] without a [[force]]. He hypothesized that everything tried to move towards its proper place in the [[Celestial spheres|crystalline sphere]]s of the heavens, and that physical bodies fell toward the center of the [[Earth]] in proportion to their [[weight]].

[[Brahmagupta]], in the ''[[Brahmasphutasiddhanta|Brahmasphuta Siddhanta]]'' (AD 628), responded to critics of the [[heliocentrism|heliocentric]] system of [[Aryabhata]] (AD 476–550) 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>[[Brahmagupta]] (628 AD). ''[[Brahmasphutasiddhanta|Brahmasphuta Siddhanta]]'' ("''The Opening of the Universe''").</ref><ref>[[Al-Biruni]] (1030). ''Ta'rikh al-Hind'' (''Indica'').</ref>

[[Brahmagupta]], in the ''[[Brahmasphutasiddhanta|Brahmasphuta Siddhanta]]'' (AD 628), responded to critics of the [[heliocentrism|heliocentric]] system of [[Aryabhata]] (AD 476–550) 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."[[Brahmagupta]] (628 AD). ''[[Brahmasphutasiddhanta|Brahmasphuta Siddhanta]]'' ("''The Opening of the Universe''"). [[Al-Biruni]] (1030). ''Ta'rikh al-Hind'' (''Indica'').

 

In the 9th century, the eldest [[Banū Mūsā]] brother, [[Ja'far Muhammad ibn Mūsā ibn Shākir|Muhammad ibn Musa]], in his ''Astral Motion'' and ''The Force of Attraction'', hypothesized that there was a force of attraction between heavenly bodies,<ref>K. A. Waheed (1978). ''Islam and The Origins of Modern Science'', p. 27. Islamic Publication Ltd., Lahore.</ref> foreshadowing [[Newton's law of universal gravitation]].

 

In 1121, [[Al-Khazini]], in ''The Book of the Balance of Wisdom'', differentiated between [[force]], [[mass]], and [[weight]], and discovered that gravity varies with the distance from the centre of the Earth,<ref name=Abattouy>Professor Mohammed Abattouy (2002). "The Arabic Science of weights: A Report on an Ongoing Research Project", ''The Bulletin of the Royal Institute for Inter-Faith Studies'' '''4''', p. 109-130.

In 1121, [[Al-Khazini]], in ''The Book of the Balance of Wisdom'', differentiated between [[force]], [[mass]], and [[weight]],<ref>[[Donald Routledge Hill]] (1993), ''Islamic Science and Engineering'', p. 61, [[Edinburgh University Press]]. ([[cf.]] Salah Zaimeche PhD (2005), [http://www.muslimheritage.com/uploads/Merv.pdf Merv], p. 5, Foundation for Science Technology and Civilization.)</ref> and discovered that gravity varies with the distance from the centre of the Earth,<ref name=Abattouy>Professor Mohammed Abattouy (2002). "The Arabic Science of weights: A Report on an Ongoing Research Project", ''The Bulletin of the Royal Institute for Inter-Faith Studies'' '''4''', p. 109-130.</ref>

though he believed that the weight of heavy bodies increase as they are farther from the centre of the Earth.N. Khanikoff, ed. and trans. (1858-1860), "Analysis and Extracts of ... Book of the Balance of Wisdom, An Arabic Work on the Water-Balance, Written by 'Al-Khâzinî in the Twelfth Century", chap. 5, sect. 3.1, ''Journal of the American Oriental Society'' '''6''', p. 36.  All these early attempts at trying to explain the force of gravity were philosophical in nature.

though he believed that the weight of heavy bodies increase as they are farther from the centre of the Earth.<ref>N. Khanikoff, ed. and trans. (1858-1860), "Analysis and Extracts of ... Book of the Balance of Wisdom, An Arabic Work on the Water-Balance, Written by 'Al-Khâzinî in the Twelfth Century", chap. 5, sect. 3.1, ''Journal of the American Oriental Society'' '''6''', p. 36.</ref> All these early attempts at trying to explain the force of gravity were philosophical in nature.

=== Scientific revolution ===

=== Scientific revolution ===

Modern work on gravitational theory began with the work of [[Galileo Galilei]] in the late 16th century and early 17th century.  In his famous (though probably apocryphal) experiment dropping balls from the [[Leaning Tower of Pisa|Tower of Pisa]], and later with careful measurements of balls rolling down [[Inclined plane|inclines]], 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. <ref>[[Galileo]] (1638), ''[[Two New Sciences]]'', [http://oll.libertyfund.org/?option=com_staticxt&staticfile=show.php%3Ftitle=753&chapter=109891&layout=html&Itemid=27 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."</ref> (Galileo correctly postulated air resistance as the reason that lighter objects may fall more slowly in an atmosphere.)  Galileo's work set the stage for the formulation of Newton's theory of gravity.

Modern work on gravitational theory began with the work of [[Galileo Galilei]] in the late 16th century and early 17th century.  In his famous (though probably apocryphal) experiment dropping balls from the [[Leaning Tower of Pisa|Tower of Pisa]], and later with careful measurements of balls rolling down [[Inclined plane|inclines]], 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. <ref>[[Galileo]] (1638), ''[[Two New Sciences]]'', [https://oll.libertyfund.org/?option=com_staticxt&staticfile=show.php%3Ftitle=753&chapter=109891&layout=html&Itemid=27 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."</ref> (Galileo correctly postulated air resistance as the reason that lighter objects may fall more slowly in an atmosphere.)  Galileo's work set the stage for the formulation of Newton's theory of gravity.

=== Newton's theory of gravitation ===

=== Newton's theory of gravitation ===

===Gravity and quantum mechanics===

===Gravity and quantum mechanics===

Several decades after the discovery of general relativity it was realized that general relativity is incompatible with [[quantum mechanics]].<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> It is possible to describe gravity in the framework of [[quantum field theory]] like the other [[fundamental forces]], with the attractive force of gravity arises due to exchange of [[virtual particle|virtual]] [[gravitons]], in the same way as the electromagnetic force arises from exchange of virtual [[photons]].<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 [[classical limit]]. However, this approach fails at short distances of the order of the [[Planck length]],<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 [[quantum gravity]] (or a new approach to quantum mechanics) is required. Many believe the complete theory to be [[string theory]],<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> or more currently [[M Theory]].

Several decades after the discovery of general relativity it was realized that general relativity is incompatible with [[quantum mechanics]]. It is possible to describe gravity in the framework of [[quantum field theory]] like the other [[fundamental forces]], with the attractive force of gravity arises due to exchange of [[virtual particle|virtual]] [[gravitons]], in the same way as the electromagnetic force arises from exchange of virtual [[photons]]. This reproduces general relativity in the [[classical limit]]. However, this approach fails at short distances of the order of the [[Planck length]], where a more complete theory of [[quantum gravity]] (or a new approach to quantum mechanics) is required. Many believe the complete theory to be [[string theory]], or more currently [[M Theory]].

== Specifics ==

== Specifics ==

* Stars on the outskirts of galaxies are moving faster than they should. Also galaxies within galaxy clusters are moving faster than they should. [[Dark Matter]] and [[MOND]] have both been proposed as explanations.

* Stars on the outskirts of galaxies are moving faster than they should. Also galaxies within galaxy clusters are moving faster than they should. [[Dark Matter]] and [[MOND]] have both been proposed as explanations.

* The [[expansion of the universe]] 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 is reinterpreted to take this into account, the expansion is not speeding up after all.<ref>[http://space.newscientist.com/channel/astronomy/cosmology/mg19726461.600-dark-energy-may-just-be-a-cosmic-illusion.html]  

* The [[expansion of the universe]] 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 is reinterpreted to take this into account, the expansion is not speeding up after all.<ref>[https://space.newscientist.com/channel/astronomy/cosmology/mg19726461.600-dark-energy-may-just-be-a-cosmic-illusion.html]  

Dark energy may just be a cosmic illusion

Dark energy may just be a cosmic illusion

''New Scientist''</ref>

''New Scientist''</ref>

* The [[Pioneer anomaly|Pioneer spacecraft]] seem to be slowing down in a way which has yet to be explained. <ref>[http://www.economist.com/science/displaystory.cfm?story_id=10804075]  

* The [[Pioneer anomaly|Pioneer spacecraft]] seem to be slowing down in a way which has yet to be explained. <ref>[https://www.economist.com/science/displaystory.cfm?story_id=10804075]  

Wanted: Einstein Jr, Mar 6th 2008

Wanted: Einstein Jr, Mar 6th 2008

''The Economist''</ref>

''The Economist''</ref>

===Recent alternative theories===

===Recent alternative theories===

* [[Brans-Dicke theory]] of gravity (1961)

* [[Brans-Dicke theory]] of gravity (1961)

* [[Induced gravity]] (1967), a proposal by [[Andrei Sakharov]] according to which [[general relativity]] might arise from [[quantum field theory|quantum field theories]] of matter.

* [[Induced gravity]] (1967), a proposal by [[Andrei Sakharov]] according to which [[general relativity]] might arise from [[quantum field theory|quantum field theories]] of matter.

==See also==  

==See also==  

* [[Anti-gravity]], the idea of neutralizing or repelling gravity

* [[Anti-gravity]], the idea of neutralizing or repelling gravity

==Notes==

==Notes==

== External links ==

== External links ==

* [http://www.lightandmatter.com/html_books/1np/ch10/ch10.html Chapter 10. Gravity], from Light and Matter: educational materials for physics and astronomy

* [https://www.lightandmatter.com/html_books/1np/ch10/ch10.html Chapter 10. Gravity], from Light and Matter: educational materials for physics and astronomy

* [http://einstein.stanford.edu/ Gravity Probe B Experiment]  The Official Einstein website from Stanford University

* [https://einstein.stanford.edu/ Gravity Probe B Experiment]  The Official Einstein website from Stanford University

* [http://geophysics.mines.edu/cgem Center for Gravity, Electrical, and Magnetic Studies]

* [https://geophysics.mines.edu/cgem Center for Gravity, Electrical, and Magnetic Studies]

* [http://static.scribd.com/docs/8deo8fwbo2y96.swf Gravity for kids] (flash)

* [https://static.scribd.com/docs/8deo8fwbo2y96.swf Gravity for kids] (flash)

* [http://www.newton.dep.anl.gov/askasci/phy99/phy99x10.htm Ask a scientist], Physics Archive

* [https://www.newton.dep.anl.gov/askasci/phy99/phy99x10.htm Ask a scientist], Physics Archive

* How stuff works:

* How stuff works:

** [http://www.howstuffworks.com/question232.htm How does gravity work?]

** [https://www.howstuffworks.com/question232.htm How does gravity work?]

** [http://www.howstuffworks.com/what-if-zero-gravity.htm What if there were no gravity on Earth?]

** [https://www.howstuffworks.com/what-if-zero-gravity.htm What if there were no gravity on Earth?]

* [http://www.physorg.com/news85310822.html Alternative theory of gravity explains large structure formation -- without dark matter] [[PhysOrg.com]]

* [https://www.physorg.com/news85310822.html Alternative theory of gravity explains large structure formation -- without dark matter] [[PhysOrg.com]]

* [http://www.fourmilab.ch/gravitation/foobar/ Do it yourself, gravitation experiment]

* [https://www.fourmilab.ch/gravitation/foobar/ Do it yourself, gravitation experiment]

[[Category: General Reference]]

[[Category: General Reference]]

[[Category: Physics]]

[[Category: Physics]]