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'''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.  

=== 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.

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.

<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]]."[http://arxiv.org/abs/physics/0310001 Indian Physics: Outline of Early History''], p. 22. [[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]].

=== 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 ===

* 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>

== 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]]