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  12/11/13Gravitation - Wikipedia, the free encyclopediaen.wikipedia.org/wiki/Gravity1/11 Hammer and feather drop: Apollo 15astronaut David Scott on the Moonrecreating Galileo's famous gravityexperiment. (1.38 MB, ogg/Theoraformat). Gravitation From Wikipedia, the free encyclopedia  (Redirected from Gravity) Gravitation , or   gravity , is a natural phenomenon by which all physical bodies attract each other. It is most commonly experienced as the agentthat gives weight to objects with mass and causes them to fall to theground when dr opped.Gravitation is one of the four fundamental interactions of nature, alongwith electromagnetism, and the nuclear strong force and weak force. Inmodern physics, the phenomenon of  gravitation is most accuratelydescribed by the general theory of relativity by Einstein, in which the phenomenon itself is a consequence of the curvature of spacetimegoverning the motion of inertial objects. The simpler Newton's law of universal gravitation postulates the gravity force proportional to masses of interacting bodies and inversely proportional to the square of the distance between them. It provides an accurate approximation for most physicalsituations including calculations as critical as spacecraft trajectory.From a cosmological perspective, gravitation causes dispersed matter to coalesce, and coalesced matter to remain intact, thus accounting for the existence of planets, stars, galaxies and most of the macroscopic objects in the universe. It is responsible for keeping the Earth and the other planets in their orbits around the Sun; for keeping theMoon in its orbit around the Earth; for the formation of tides; for natural convection, by which fluid flow occursunder the influence of a density gradient and gravity; for heating the interiors of forming stars and planets to veryhigh temperatures; and for various other  phenomena observed on Earth and throughout the universe. Contents 1 History of gravitational theory1.1 Scientific revolution1.2  Newton's theory of gravitation 1.3 Equivalence principle1.4 General relativity1.5 Gravity and quantum mechanics2 Specifics2.1 Earth's gravity2.2 Equations for a falling body near the surface of the Earth2.3 Gravity and astronomy2.4 Gravitational radiation2.5 Speed of gravity3 Anomalies and discrepancies4 Alternative theories4.1 Historical alternative theories4.2 Recent alternative theories    12/11/13Gravitation - Wikipedia, the free encyclopediaen.wikipedia.org/wiki/Gravity2/11 Sir Isaac Newton, an English physicistwho lived from 1642 to 1727 5 See also6 Footnotes7 References8 Further reading9 External links History of gravitational theory  Main article: History of gravitational theory Scientific revolution Modern work on gravitational theory began with the work of Galileo Galilei in the late 16th and early 17thcenturies. In his famous (though possibly apocryphal [1] ) experiment dropping balls from the Tower of Pisa, andlater with careful measurements of balls rolling down inclines, Galileo showed that gravitation accelerates all objectsat the same rate. This was a major departure from Aristotle's belief that heavier objects accelerate faster. [2]  Galileo postulated air resistance as the reason that lighter objects may fall slower in an atmosphere. Galileo's work set thestage for the formulation of Newton's theory of gravity. Newton's theory of gravitation  Main article: Newton's law of universal gravitation In 1687, English mathematician Sir Isaac Newton published  Principia , which hypothesizes the inverse-square law of universalgravitation. In his own words, “I deduced that the forces whichkeep the planets in their orbs must [be] reciprocally as the squaresof their distances from the centers about which they revolve: andthereby compared the force requisite to keep the Moon in her Orbwith the force of gravity at the surface of the Earth; and found themanswer pretty nearly.” [3]  Newton's theory enjoyed its greatest success when it was used to predict the existence of Neptune based on motions of Uranus thatcould not be accounted for by the actions of the other planets.Calculations by both John Couch Adams and Urbain Le Verrier  predicted the general position of the planet, and Le Verrier'scalculations are what led Johann Gottfried Galle to the discovery of  Neptune.A discrepancy in Mercury's orbit pointed out flaws in Newton'stheory. By the end of the 19th century, it was known that its orbitshowed slight perturbations that could not be accounted for entirelyunder Newton's theory, but all searches for another perturbing body (such as a planet orbiting the Sun even closer than Mercury) had been fruitless. The issue was resolved in 1915 by Albert Einstein's new theory of generalrelativity, which accounted for the small discrepancy in Mercury's orbit.  12/11/13Gravitation - Wikipedia, the free encyclopediaen.wikipedia.org/wiki/Gravity3/11 Two-dimensional analogy of spacetime distortiongenerated by the mass of an object. Matter changesthe geometry of spacetime, this (curved) geometry being interpreted as gravity. White lines do notrepresent the curvature of space but insteadrepresent the coordinate system imposed on thecurved spacetime, which would be rectilinear in aflat spacetime. Although Newton's theory has been superseded, most modern non-relativistic gravitational calculations are stillmade using Newton's theory because it is a much simpler theory to work with than general relativity, and givessufficiently accurate results for most applications involving sufficiently small masses, speeds and energies. Equivalence principle The equivalence principle, explored by a succession of researchers including Galileo, Loránd Eötvös, and Einstein,expresses the idea that all objects fall in the same way. The simplest way to test the weak equivalence principle is todrop two objects of different masses or compositions in a vacuum, and see if they hit the ground at the same time.These experiments demonstrate that all objects fall at the same rate when friction (including air resistance) isnegligible. More sophisticated tests use a torsion balance of a type invented by Eötvös. Satellite experiments, for example STEP, are planned for more accurate experiments in space. [4] Formulations of the equivalence principle include:The weak equivalence principle: The trajectory of a point mass in a gravitational field depends only onits initial position and velocity, and is independent of its composition. [5] The Einsteinian equivalence principle: The outcome of any local non-gravitational experiment in a freely falling laboratory is independent of the velocity of the laboratory and its location in spacetime. [6] The strong equivalence principle requiring both of the above. General relativity See also: Introduction to general relativity In general relativity, the effects of gravitation are ascribed tospacetime curvature instead of a force. The starting pointfor general relativity is the equivalence principle, whichequates free fall with inertial motion, and describes free-falling inertial objects as being accelerated relative to non-inertial observers on the ground. [7][8]  In Newtonian physics,however, no such acceleration can occur unless at least oneof the objects is being operated on by a force.Einstein proposed that spacetime is curved by matter, andthat free-falling objects are moving along locally straight paths in curved spacetime. These straight paths are calledgeodesics. Like Newton's first law of motion, Einstein'stheory states that if a force is applied on an object, it woulddeviate from a geodesic. For instance, we are no longer following geodesics while standing because the mechanicalresistance of the Earth exerts an upward force on us, andwe are non-inertial on the ground as a result. This explainswhy moving along the geodesics in spacetime is consideredinertial.  12/11/13Gravitation - Wikipedia, the free encyclopediaen.wikipedia.org/wiki/Gravity4/11 Einstein discovered the field equations 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, non-linear, differentialequations. The solutions of the field equations are the components of the metric tensor of spacetime. A metrictensor describes a geometry of spacetime. The geodesic paths for a spacetime are calculated from the metrictensor. Notable solutions of the Einstein field equations include:The Schwarzschild solution, which describes spacetime surrounding a spherically symmetric non-rotatinguncharged massive object. For compact enough objects, this solution generated a black hole with a centralsingularity. For radial distances from the center which are much greater than the Schwarzschild radius, theaccelerations predicted by the Schwarzschild solution are practically identical to those predicted by Newton's theory of gravity.The Reissner-Nordström solution, in which the central object has an electrical charge. For charges with ageometrized 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 eventhorizons.The Kerr-Newman solution for charged, rotating massive objects. This solution also produces black holeswith multiple event horizons.The cosmological Friedmann-Lemaître-Robertson-Walker solution, which predicts the expansion of theuniverse.The tests of general relativity included the following: [9] General relativity accounts for the anomalous perihelion precession of Mercury. [10] The prediction that time runs slower at lower potentials has been confirmed by the Pound–Rebkaexperiment, the Hafele–Keating experiment, and the GPS.The prediction of the deflection of light was first confirmed by Arthur Stanley Eddington from hisobservations during the Solar eclipse of May 29, 1919. [11][12]  Eddington measured starlight deflections twicethose predicted by Newtonian corpuscular theory, in accordance with the predictions of general relativity.However, his interpretation of the results was later disputed. [13]  More recent tests using radio interferometricmeasurements of quasars passing behind the Sun have more accurately and consistently confirmed thedeflection of light to the degree predicted by general relativity. [14]  See also gravitational lens.The time delay of light passing close to a massive object was first identified by Irwin I. Shapiro in 1964 ininterplanetary spacecraft signals.Gravitational radiation has been indirectly confirmed through studies of binary pulsars.Alexander Friedmann in 1922 found that Einstein equations have non-stationary solutions (even in the presence of the cosmological constant). In 1927 Georges Lemaître showed that static solutions of theEinstein equations, which are possible in the presence of the cosmological constant, are unstable, andtherefore the static universe envisioned by Einstein could not exist. Later, in 1931, Einstein himself agreedwith the results of Friedmann and Lemaître. Thus general relativity predicted that the Universe had to be non-static—it had to either expand or contract. The expansion of the universe discovered by Edwin Hubble in1929 confirmed this prediction. [15] The theory's prediction of frame dragging was consistent with the recent Gravity Probe B results. [16] General relativity predicts that light should lose its energy when travelling away from the massive bodies. The
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