85
edits
Changes
Jump to navigation
Jump to search
Line 1:
Line 1: − + − Everything in the universe that has mass attracts every other thing that has mass. The mechanism which transports this force has never been observed. Scientists have postulated the existence of a hypothetical particle, the [[graviton]], in order to uphold the naturalistic explanation absent of a supernatural force. The attraction depends on the mass of the objects and the distance between them. For normal objects, this pull is minute, but you can measure the pull between a very large object like the [[Earth]] and another object like you by standing on a scale. Your [[weight]] is the measure of the pull of gravity between you and the planet you are standing on. This [[force]] depends on your [[mass]] and the mass of that planet, but it also depends on your [[distance]] from the center of the planet. The further you are from the planet's center, the weaker the pull between it and your body. If you double your distance, the force is one quarter. At ten times the distance, the force is one hundredth. It drops off with the square of the distance. This is called the [[inverse square law]].<ref>http://hyperphysics.phy-astr.gsu.edu/hbase/forces/isq.html</ref>The force never becomes zero, no matter how far you travel.+ − The law itself states that each particle in the [[universe]] attracts another particle with a force directly [[proportion]]al to the product of their masses and inversely proportional to the [[exponentiation|square]] of the distance between them<ref>Serway and Beichner, Physics for Scientists and Engineers, Fifth Edition</ref>, or: + − + + + + + + + + + + + + + − + − + − + − + − Gravitation is responsible for making objects accelerate towards each other as well as for the formation of the [[Earth]] and [[Sun]], the [[stars]] and the [[planets]].+ − An objects escape speed is the velocity required to escape the gravitational pull of a another object, usually a planet. It can be expressed as:+ + − :<math>v = \sqrt \frac{2 G M}{R}</math>+ − Where:+ + + − ''v'' is necessary velocity+ − ''M'' is the mass of the planet+ − − ''R'' is the radius of the planet − ''G'' is the [[gravitational constant]]: 6.67428x10<sup>-11</sup> N m<sup>2</sup> Kg <sup>-2</sup>+ − ==Gravity and Modern Physics==+ − Newton's [[Theory of Gravity]] was one of the supposed earliest triumphs of modern [[physics]]. It now stands as both one of the most successful and most mysterious areas of that field. It is one of the few scientific theories that does not require a supernatural force to sustain self consistency. On one hand, the [[Theory_of_Relativity|General theory of Relativity]] is one of the most successful [[Scientific_Theory|scientific theories]] to date. On the other hand, how General Relativity might be reconciled with [[Quantum_mechanics|quantum physics]] during the first few milliseconds of the [[Big Bang]] remains an open question, and is one of the hotly contested areas among modern theoretical physicists. In the 1980s [[string theory]] was seen by many physicists as a more likely path towards a particular unification of gravity with the other fundamental forces (electromagnetism, the strong and weak nuclear forces), but the theory has been a failure.<ref>Not Even Wrong, Peter Woit [http://www.math.columbia.edu/~woit/wordpress/]</ref> − ==See also==+ − *[[gravitational gradient]] + + − == References ==+ − <div class="references-small"> − <references /> −
Complete rewrite, needs additional references
'''The theory of Gravity''' is a [[Naturalism|naturalistic]] explanation for the mutual attraction of particles that contain mass. Before general relativity, gravitation was described by [[Sir Isaac Newton|Isaac Newton's]] naturalistic [[Law of Universal Gravitation|law of universal gravitation]] in his [[Philosophiæ Naturalis Principia Mathematica|''Principia Mathematica'']]. Issac Newton's current theory of gravity cannot explain the permutations of space and time as velocity of an object approaches the [[speed of light]]. [[Aristotle]], and other previous thinkers and scientists before Newton never proposed a mathematic explanation for attraction of masses.
'''Gravitation''' is the tendency for matter to attract other matter. Gravitation is what gives everyday objects [[Weight|weight]], and what causes objects that are dropped from a height to fall to the ground. Gravitation also keeps the [[Moon|moon]] in orbit around the [[Earth]] and the Earth in orbit around the [[Sun|sun]].
== Scientific Theories of Gravitation ==
Over the past four centuries, many scientific [[Theory|theories]] of gravitation have been proposed. Three of these theories have gained sufficient widespread acceptance to be considered, for a time, "correct" theories.
:<math>F = G \frac{m_1 m_2}{r^2}</math>
=== Galilean Gravitation ===
Gravitation has been the subject of modern scientific study for about 400 years, beginning with the work of [[Galileo Galilei]] in the late 16th century. According to a biography written by his pupil, Vincenzo Viviani, Galileo dropped objects from the Leaning Tower of Pisa to demonstrate that objects fall at the same rate regardless of mass.<ref>Drake, Stillman. <i>Galileo At Work.</i> (1978)</ref> (Historians now disagree as to whether this experiment actually took place in the way Viviani described.) These results were in contradiction to the ancient Aristotelian conception of gravity, which stated that the speed of an object's fall was proportional to its weight.
Galileo also described a mathematical formula for the distance an object will fall in a given time, though his description was geometric rather than algebraic.<ref>Sharratt, Michael. <i>Galileo: Decisive Innovator.</i> (1994)</ref> His formula, that the distance fallen is proportional to the square of the elapsed time, or <i>d ∝ t<sup>2</sup>,</i> remains valid for many approximations today. However, Galileo's theory that the acceleration of gravity is independent on an object's height above the ground could not stand the test of time.
=== Newtonian Gravitation ===
In 1687, [[Sir Isaac Newton]] published his [[Philosophiæ Naturalis Principia Mathematica|''Principia Mathematica'']], an exhaustive work that laid the foundation for what we know today as classical mechanics. In this work, Newton advanced his [[Law of Universal Gravitation|law of universal gravitation]]. Whereas Galileo described gravitation in terms of motion, Newton described it in terms of a [[Force|force]]. Gravitation itself went from the natural tendency of objects to fall to a vector [[Field theory|field theory]] in which the force between two objects was inversely proportional to the square of the distance between them.
Newton's vector equation for gravitation states that each particle in the [[universe]] attracts another particle with a force directly [[proportion]]al to the product of their masses and inversely proportional to the [[exponentiation|square]] of the distance between them<ref>Serway and Beichner, Physics for Scientists and Engineers, Fifth Edition</ref>, or:
:<math>\mathbf{F} = G \frac{m_1 m_2}{\mathbf{r}^2}</math>
Where:
Where:
''F'' is the magnitude of the gravitational force.
*''<b>F</b>'' is the magnitude of the gravitational force.
''m''<sub>1</sub> and ''m''<sub>2</sub> are the object's masses.
*''m''<sub>1</sub> and ''m''<sub>2</sub> are the object's masses.
''r'' is the distance that separates the objects' centers of mass.
*''<b>r</b>'' is the vector that separates the objects' centers of mass.
''G'' is the [[gravitational constant]]: 6.67428x10<sup>-11</sup> N m<sup>2</sup> Kg <sup>-2</sup>
*''G'' is the [[gravitational constant]]: 6.67428x10<sup>-11</sup> N m<sup>2</sup> kg <sup>-2</sup>
*''G'' is the [[gravitational constant]]: <math>\frac{6.67428 \times 10^{-11} N m^2}{kg^2}</math>
Near the surface of the Earth, where the difference in gravitational acceleration at different heights is very small, Newtonian gravitation "reduces" to, or is mathematically equivalent to within a known margin of error, Galilean gravitation. That is, both Galilean gravitation and Newtonian gravitation can reasonably describe the motion of falling apples and artillery shells. But further from the surface of the Earth, the errors in Galileo's approximation become too great, and Newton's model is required.
Newton's theory of gravitation stood, refined and reformulated, but essentially unchanged, for more than two centuries.
=== Einsteinian Gravitation ===
In 1915, [[Albert Einstein]] published his [[general theory of relativity]], a radical reinterpretation of the phenomenon of gravitation. Whereas Galileo described gravity in terms of motion, and Newton described gravity in terms of force, Einstein described gravity in terms of inertial motion in curved spacetime, such curvature being a function of the matter and fields in a given region. In the famous words of physicist John Wheeler, "Matter tells space how to curve, and space tells matter how to move."<ref>Misner, Thorne & Wheeler. <i>Gravitation.</i> (1973)</ref>
Just as the Newtonian theory reduced to Galilean gravitation near the surface of the Earth, Einstein's theory of gravitation reduces to Newtonian gravity where gravitational fields are relatively weak. Only when gravitational fields are strong, such as the field around the sun within the orbit of [[Mercury]], do Newton's equations fail to produce a good approximation of reality.
=== The Possibility of Quantum Gravitation ===
During the era when Newtonian gravity was preeminent, gravitation was considered to be one of the "fundamental forces of nature," along with electricity, magnetism and the strong and weak nuclear forces. Over the years, this interpretation has fallen out of favor, for a variety of reasons. Research into [[electrodynamics]] revealed the electric and magnetic forces to be two aspects of the same thing, and [[quantum mechanics]] described the strong and weak forces in terms of interactions mediated by [[Boson|bosons]]. Further research into [[quantum chromodynamics]] and the [[Standard Model]] increased our understanding of how electromagnetism and the nuclear forces work, but gravitation remained a thing apart.
Over the past few decades, attention has been turned toward formulating a [[Quantum gravity|quantum field theory of gravity]] which would describe the gravitational interaction in terms of a mechanism similar to the electromagnetic, strong and weak nuclear interactions.
Physicists do not universally agree that a quantum theory of gravity is inevitable, or even possible. Einstein's theory of gravitation described gravity in a way that is so fundamentally different from the other interactions in nature that many physicists no longer gravity to be a force at all. Rather, under the Einsteinian interpretation, gravitation is an inherent property of spacetime and has no analogue at the quantum level at all. Other physicists believe that the Einsteinian interpretation is merely a useful large-scale approximation of an interaction which we have not yet detected on the subatomic scale.
To date, no satisfactory theory of quantum gravity has been advanced, but research continues.
==External Links==
==External Links==
*[http://www.relativitycalculator.com/Newton_Universal_Gravity_Law.shtml Newton's Universal Law of Gravitational Force]
*[http://www.relativitycalculator.com/Newton_Universal_Gravity_Law.shtml Newton's Universal Law of Gravitational Force]
== References ==
{{reflist}}
[[Category:physics]]
[[category:physics]]