Difference between revisions of "Gravity"

Revision as of 03:27, March 8, 2007

Gravity is considered by scientists and evolutionists to be one of the fundamental forces of the universe. It is a theory which suggests that all masses are attracted to each other because of invisible particles called gravitons or invisible curves in space. The idea was first developed by Isaac Newton, and has been worked on by prominent scientists like Johannes Kepler and Albert Einstein.

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-{{otheruses}}
+Gravity is considered by scientists and [[evolutionists]] to be one of the [[fundamental forces]] of the [[universe]]. It is a theory which suggests that all masses are attracted to each other because of invisible particles called gravitons or invisible curves in [[space]]. The idea was first developed by [[Isaac Newton]], and has been worked on by prominent scientists like [[Johannes Kepler]] and [[Albert Einstein]].
-{{redirect|Gravity}}
-'''Gravitation'''  is a phenomenon through which all objects attract 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 many cases.
+== References ==
-Gravitation is 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 large masses (stars and planets) and life would not exist. Gravitation is also responsible for keeping the Earth and the other planets in their [[orbit]]s around the Sun; the [[Moon]] in its orbit around the Earth; for heating interiors of forming stars and planets to very high temperatures, for the formation of [[tides]], for rising hot air or water (convection), and for various other natural phenomena that we observe.
-[[Image:Solar sys8.jpg|right|350px|thumb|The '''gravitational force''' keeps the planets in orbit about the Sun.]]
-== History of gravitational theory ==
-{{main|History of gravitational theory}}
-=== Early history ===
-Since the time of the [[Greek philosophy|Greek]] philosopher [[Aristotle]] in the [[4th century BC]], there have been many attempts to understand and explain gravity. Aristotle believed that there was no [[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 crystalline [[sphere]]s of the heavens, and that physical bodies fell toward the center of the [[Earth]] in proportion to their [[weight]].
-Another early explanation was that of the [[Hindu astronomy|Indian astronomer]] [[Brahmagupta]] who, in his ''[[Brahmasphutasiddhanta|Brahmasphuta Siddhanta]]'' ([[628 CE]]), responded to critics of the [[heliocentrism|heliocentric]] system of [[Aryabhata]] (perhaps 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>[[Brahmagupta]] (628 CE). ''[[Brahmasphutasiddhanta|Brahmasphuta Siddhanta]]'' ("''The Opening of the Universe''").</ref><ref>[[Al-Biruni]] (1030 CE). ''Ta'rikh al-Hind'' ("''Chronicles of India''").</ref>
-Modern work on gravitational theory began with the work of [[Galileo Galilei]] in the late [[16th century]] and early [[17th century]].  In his famous experiment dropping balls at the [[Leaning Tower of Pisa|Tower of Pisa]] and later with careful measurements of balls rolling down [[incline]]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 fall more slowly.)  Galileo's work set the stage for the formulation of Newton's theory of gravity.
-=== Newton's theory of gravitation ===
-{{main|Law of universal gravitation}}
-In 1687, English mathematician [[Sir Isaac Newton]] published ''[[Principia]]'', which hypothesizes the [[inverse-square law]] 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 [[Neptune]] based on motions of [[Uranus]] that could not be accounted by the actions of the other planets.  Calculations by [[John Couch Adams]] and [[Urbain Le Verrier]] both predicted the general position of the planet, and Le Verrier's calculations are what led [[Johann Gottfried Galle]] 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 [[Mercury (planet)|Mercury]] could not be accounted for entirely under Newton's theory, and all searches for another perturbing body (such as a planet orbiting the [[Sun]] even closer than Mercury) have been fruitless.  This issue was resolved in 1915 by [[Albert Einstein]]'s new [[general relativity]] theory. This theory accounted for the discrepancy in Mercury's orbit. However, it is currently known that the theoretical formula for the anomalous perihelion motion of a planet, such as Mercury's perihelion, can be derived solely based on an extremely simple proposition founded in classical physics,<ref>Kidman, J. N. (1977).  Quantum gravitation and the perihelion anomaly.  ''Lettere al Nuovo Cimento, 18,'' pp. 181-182.  {{PDFlink|[http://www.aspden.org/papers/bib/1977e.pdf on-line]|91.1&nbsp;[[Kibibyte|KiB]]<!-- application/pdf, 93373 bytes -->}}''</ref> without the need of any excursion into Einstein's space-time notion.
-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 sufficient for most applications.
-===General relativity===
-{{main|Introduction to general relativity}}
-In this '''theory''', [[inertia|inertial motion]] occurs when objects are in [[free-fall]] instead of when they are at [[rest (physics)|rest]] with respect to a massive object such as the Earth (as is the case in classical mechanics).  This view of inertia creates a problem:  In flat spacetimes such as those of classical mechanics and [[special relativity]], there is no way that inertial observers can accelerate with respect to each other, as free-falling bodies can do as they are each accelerated towards the center of a massive object.
-To deal with this difficulty, Einstein proposed that [[spacetime]] is [[curvature|curved]] by the presence of matter, and that free-falling objects are following the [[geodesic (general relativity)|geodesic]]s of the spacetime.  More specifically, Einstein discovered the [[field equation]]s of general relativity, which relate the presence of matter and the curvature of spacetime and are named after him.  The [[Einstein field equations]] are a set of 10 [[simultaneous equation|simultaneous]], [[Nonlinearity|non-linear]], [[differential equation]]s whose solutions give the components of the [[metric tensor (general relativity)|metric tensor]] of spacetime.  A metric tensor describes a geometry of spacetime. The geodesic paths for objects in inertial motion in a spacetime are calculated from the metric tensor of that spacetime.  Notable solutions of the Einstein field equations include:
-* The [[Schwarzschild solution]], which describes spacetime surrounding a [[spherical symmetry|spherically symmetric]] non-[[rotation|rotating]] uncharged massive object.  For compact enough objects, this solution generated a [[black hole]] with a central [[gravitational singularity|singularity]].  For radial distances from the center which are much greater than the [[Schwarzschild radius]], the accelerations predicted by the Schwarzschild solution are practically identical to those predicted by Newton's theory of gravity.
-* The [[Reissner-Nordström black hole|Reissner-Nordström solution]], in which the central object has an electrical charge.  For charges with a [[gemetrized coordinates|geometrized]] length which are less than the geometrized length of the mass of the object, this solution produces black holes with two [[event horizons]].
-* The [[Kerr solution]] for rotating massive objects.  This solution also produces black holes with multiple event horizons.
-* The [[physical cosmology|cosmological]] [[Robertson-Walker coordinates|Robertson-Walker solution]], which predicts the expansion of the [[universe]].
-General relativity has enjoyed much success because of how its predictions have been regularly confirmed.  For example:
-* General relativity accounts for the anomalous [[precession]] of the planet [[Mercury (planet)|Mercury]].
-* The prediction that time runs slower at lower potentials has been confirmed by the [[Pound-Rebka experiment]], the [[Hafele-Keating experiment]], and the [[GPS]].
-* The prediction of the deflection of light was first confirmed by [[Arthur Eddington]] in [[1919]], and has more recently been strongly confirmed through the use of a [[quasar]] which passes behind the [[Sun]] as seen from the [[Earth]]. See also [[gravitational lensing]].
-* The [[time delay of light]] passing close to a massive object was first identified by [[Irwin Shapiro]] in [[1964]] in interplanetary spacecraft signals.
-* [[Gravitational radiation]] has been indirectly confirmed through studies of binary [[pulsar]]s.
-* The expansion of the universe (predicted by the [[Robertson-Walker metric]]) was confirmed by [[Edwin Hubble]] in [[1929]].
-== Specifics ==
-===Earth's gravity===
-{{main|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&nbsp;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 [[foot (unit of length)|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&nbsp;ft/s) after one second, 19.614&nbsp;m/s (64.34&nbsp;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|Equations for a falling body}}
-Under normal moon-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 = mpg'', where m is the [[mass]] of the body. 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 close from the surface of the Moon will not in general be '''g'''. A further example is the expression that we use for the calculation of potential energy ''P.E.'' of a body at height ''h'' ( ''P.E. = mgh''). 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, <math>h = u^2/2g</math> 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|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, [[quasar]]s 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 center]]s, [[galaxy|galaxies]] orbit a center of mass in clusters, and clusters orbit in [[supercluster]]s.
-===Gravity versus gravitation===
-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 are theorized to exert on each other to cause gravitation.  Although these terms are used interchangeably 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 [[inertia]]lly moving objects to tend to accelerate towards each other.  Another (but discredited) example is [[Le Sage's theory of gravitation]], in which massive objects are effectively pushed towards each other.
-==Alternative theories==
-{{main|Alternatives to general relativity}}
-'''Historical alternative theories'''
-* [[Aristotelian theory of gravity]]
-* [[Le Sage's theory of gravitation]] (1784) also called '''LeSage gravity''', proposed by [[Georges-Louis Le Sage]], based on a fluid-based explanation where a light gas fills the entire universe.
-* [[Nordström's theory of gravitation]] (1912, 1913), an early competitor of general relativity.
-* [[Whitehead's theory of gravitation]] (1922), another early competitor of general relativity.
-'''Recent alternative theories'''
-* [[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.
-* [[Rosen bi-metric theory]] of gravity
-* In the [[modified Newtonian dynamics]] (MOND) (1981), [[Mordehai Milgrom]] proposes a modification of [[Newton's Second Law]] of motion for small accelerations.
-* The new and highly controversial [[Process Physics]] theory attempts to address gravity
-* The [[self-creation cosmology]] theory of gravity (1982) by G.A. Barber in which the [[Brans-Dicke theory]] is modified to allow mass creation.
-* [[Nonsymmetric gravitational theory]] (NGT) (1994) by John Moffat
-* The satirical theory of [[Intelligent falling]] (2002, in its first incarnation as "Intelligent grappling")
-* [[Tensor-vector-scalar gravity]] (TeVeS) (2004), a relativistic modification of MOND by [[Jacob Bekenstein]]
-==See also==
-{{portal | Gravitation}}
-{{col-start}}
-{{col-break}}
-* [[Artificial gravity]]
-* [[Escape velocity]]
-* [[General relativity]]
-* [[g-force]]
-* [[Gravitational wave]]s
-* [[Gravitational binding energy]]
-* [[Gravity Research Foundation]]
-* [[Divergence theorem#Gravity|Gravity and the divergence theorem]]
-* [[Kepler's laws of planetary motion#Kepler's third law|Kepler's third law of planetary motion]]
-{{col-break}}
-* [[Newton's laws of motion]]
-* [[n-body problem|''n''-body problem]]
-* [[Pioneer anomaly|The Pioneer spacecraft anomaly]]
-* [[Scalar Gravity]]
-* [[Speed of gravity]]
-* [[Standard gravitational parameter]]
-* [[standard gravity]]
-* [[Weight]]
-* [[Weightlessness]]
-* [[Lagrange Points]]
-{{col-end}}
-==Notes==
-<div class="references-small">
-* {{fnb|1}} 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
-* {{fnb|3}} [[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.)
-</div>
-==References==
-<div class="references-small">
 <references />
-*{{cite book | last = Halliday | first =  David | coauthors = Robert Resnick; Kenneth S. Krane | title = Physics v. 1 | location = New York | publisher =  John Wiley & Sons | year = 2001 | id = ISBN 0-471-32057-9 }}
-*{{cite book | last = Serway | first =  Raymond A. | coauthors =  Jewett, John W. | title = Physics for Scientists and Engineers | edition = 6th ed. | publisher = Brooks/Cole | year = 2004 | id = ISBN 0-534-40842-7 }}
-*{{cite book | last = Tipler | first =  Paul | title = Physics for Scientists and Engineers: Mechanics, Oscillations and Waves, Thermodynamics | edition = 5th ed. | publisher = W. H. Freeman | year = 2004 | id = ISBN 0-7167-0809-4 }}
-Gravity makes things fall.
-==External links==
-* [http://www.lightandmatter.com/html_books/1np/ch10/ch10.html Gravity] - a chapter from an online textbook
-* {{PDFlink|[http://physnet.org/home/modules/pdf_modules/m101.pdf ''Newton's Law of Universal Gravitation'']|194&nbsp;[[Kibibyte|KiB]]<!-- application/pdf, 199568 bytes -->}} on [http://www.physnet.org Project PHYSNET]
-* [http://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]
-* [http://www.physorg.com/news85310822.html Alternative theory of gravity explains large structure formation -- without dark matter] [[PhysOrg.com]]
-{{FundamentalForces}}
-[[Category:Fundamental physics concepts]]
-[[Category:Gravity]]
-[[Category:Introductory physics]]
-[[Category:Celestial mechanics]]
-<!--interwiki-->
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Difference between revisions of "Gravity"

Revision as of 03:27, March 8, 2007

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