gravitation

n.

1. Physics.
   1. The natural phenomenon of attraction between physical objects with mass or energy.
   2. The act or process of moving under the influence of this attraction.
2. A movement toward a source of attraction: the gravitation of the middle classes to the suburbs.

gravitational grav'i·ta'tion·al adj.
gravitationally grav'i·ta'tion·al·ly adv.
gravitative grav'i·ta'tive adj.

The mutual attraction between all masses and particles of matter in the universe. In a sense this is one of the best-known physical phenomena. During the eighteenth and nineteenth centuries gravitational astronomy, based on Newton's laws, attracted many of the leading mathematicians and was brought to such a pitch that it seemed that only extra numerical refinements would be needed in order to account in detail for the motions of all celestial bodies. In the twentieth century, however, A. Einstein shattered this complacency, and the subject is currently in a healthy state of flux.

Newton's law of gravitation

Newton's law of universal gravitation states that every two particles of matter in the universe attract each other with a force that acts in the line joining them, the intensity of which varies as the product of their masses and inversely as the square of the distance between them. Or, the gravitational force F exerted between two particles with masses m₁ and m₂ separated by a distance d is given by the equation below, where G is called the constant of gravitation.

Gravitational constant

In 1774, G was determined by measuring the deflection of the vertical by the attraction of a mountain. This method is much inferior to the laboratory method in which the gravitational force between known masses is measured. In the torsion balance two small spheres, each of mass m, are connected by a light rod, suspended in the middle by a thin wire. The deflection caused by bringing two large spheres each of mass M near the small ones on opposite sides of the rod is measured, and the force is evaluated by observing the period of oscillation of the rod under the influence of the torsion of the wire (see illustration). This is known as the Cavendish experiment, in honor of H. Cavendish, who achieved the first reliable results by this method in 1797–1798. More recent determinations using various refinements yield the results: constant of gravitation G = 6.67 × 10⁻¹¹ SI (mks) units; mass of Earth = 5.98 × 10²⁴ kg. The result of the best available laboratory measurements, announced in 2002, is G = (6.6742 ± 0.0010) × 10⁻¹¹ in SI (mks) units.

Diagram of the torsion balance.

In newtonian gravitation, G is an absolute constant, independent of time, place, and the chemical composition of the masses involved. Partial confirmation of this was provided before Newton's time by the experiment attributed to Galileo in which different weights released simultaneously from the top of the Tower of Pisa reached the ground at the same time. Newton found further confirmation, experimenting with pendulums made out of different materials. Early in this century, R. Eötvös found that different materials fall with the same acceleration to within 1 part in 10⁷. The accuracy of this figure has been extended to 1 part in 10¹¹, using aluminum and gold, and to 0.9 × 10⁻¹² with a confidence of 95%, using aluminum and platinum.

Mass and weight

In the equations of motion of newtonian mechanics, the mass of a body appears as inertial mass, a measure of resistance to acceleration, and as gravitational mass in the expression of the gravitational force. The equality of these masses is confirmed by the Eötvös experiment. It justifies the assumption that the motion of a particle in a gravitational field does not depend on its physical composition. In Newton's theory the equality can be said to be a coincidence, but not in Einstein's theory, where this equivalence becomes a cornerstone of relativistic gravitation.

While mass in newtonian mechanics is an intrinsic property of a body, its weight depends on certain forces acting on it. For example, the weight of a body on the Earth depends on the gravitational attraction of the Earth on the body and also on the centrifugal forces due to the Earth's rotation. The body would have lower weight on the Moon, even though its mass would remain the same. See also Centrifugal force.

Gravity

This should not be confused with the term gravitation. Gravity is the older term, meaning the quality of having weight, and so came to be applied to the tendency of downward motion on the Earth. Gravity or the force of gravity is today used to describe the intensity of gravitational forces, usually on the surface of the Earth or another celestial body. So gravitation refers to a universal phenomenon, while gravity refers to its local manifestation. See also Earth, gravity field of.

Accuracy of newtonian gravitation

A discrepancy in newtonian gravitation was discovered by U.J.J. Leverrier in the orbit of Mercury. Because of the action of the other planets, the perihelion of Mercury's orbit advances. But allowance for all known gravitational effects still left an observed motion of about 43 seconds of arc per century unaccounted for by Newton's theory. Attempts to account for this by adding an unknown planet or by drag with an interplanetary medium were unsatisfactory, and a very small change was suggested in the exponent of the inverse square of force. This particular discordance was accounted for by A. Einstein's general theory of relativity in 1916, but the final word on the subject has yet to be said. See also Relativity.

Gravitational lens

Light is deflected when it passes through a gravitational field, and an analogy can be made to the refraction of light passing through a lens. It has been suggested that a galaxy situated between an observer and a more distant source might have a focusing effect, and that this might account for some of the observed properties of quasi-stellar objects. The multiple images of the quasar (Q0957 + 561 A,B) are almost certainly caused by the light from a single body passing through a gravitational lens. While this is the best-studied gravitational lens, many other examples of this phenomenon have been discovered. See also Gravitational lens; Quasar.

Relativistic theories

In spite of his success and the absence of a reasonable alternative, Newton's theory was heavily criticized, not least with regard to its requirement of “action at a distance” (that is, through a vacuum). Newton himself considered this to be “an absurdity,” and he recognized the weaknesses in postulating in his system of mechanics the existence of preferred reference systems (that is, inertial reference systems) and an absolute time.

The theory of relativity grew from attempts to describe electromagnetic phenomena in moving systems. No physical effect can propagate with a speed exceeding that of light in vacuum; therefore, Newton's theory must be the limiting case of a field theory in which the speed of propagation approaches infinity. Einstein's field theory of gravitation (general relativity) is based on the identification of the gravitational field with the curvature of space-time. The geometry of space-time is affected by the presence of matter and radiation. The relationship between mass-energy and the space-time curvature is therefore a relativistic generalization of the newtonian law of gravitation. The relativistic theory is mathematically far more complicated than Newton's. Instead of the single newtonian potential described above, Einstein worked with 10 quantities that form a tensor.

An important step in Einstein's reasoning is his “principle of equivalence”: A uniformly accelerated reference system imitates completely the behavior of a uniform gravitational field. This principle requires that all bodies fall in a gravitational field with precisely the same acceleration, a result that is confirmed by the Eötvös experiment mentioned earlier. Also, if matter and antimatter were to repel one another, it would be a violation of the principle. See also Free fall.

Gravitational waves

The existence of gravitational waves, or gravitational “radiation,” was predicted by Einstein shortly after he formulated his general theory of relativity. They are now a feature of any relativity theory. Gravitational waves are “ripples in the curvature of space-time.” In other words, they are propagating gravitational fields, or propagating patterns of strain, traveling at the speed of light. They carry energy and can exert forces on matter in their path, producing, for instance, very small vibrations in elastic bodies. The gravitational wave is produced by change in the distribution of some matter. It is not produced by a rotating sphere, but would result from a rotating body not having symmetry about its axis of rotation: a pulsar, perhaps. In spite of the relatively weak interaction between gravitational radiation and matter, the measurement of this radiation is now technically possible.

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For more information on gravitation, visit Britannica.com.

The force, first described mathematically by Isaac Newton, whereby any two objects in the universe are attracted toward each other. Gravitation holds the moon in orbit around the Earth, the planets in orbit around the sun, and the sun in the Milky Way. It also accounts for the fall of objects released near the surface of the Earth. The modern theory of gravitation is the general theory of relativity.

A cynical view of the world by Ambrose Bierce

n.

The tendency of all bodies to approach one another with a strength proportion to the quantity of matter they contain -- the quantity of matter they contain being ascertained by the strength of their tendency to approach one another. This is a lovely and edifying illustration of how science, having made A the proof of B, makes B the proof of A.

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IN BRIEF: The force by which every particle or mass of matter attracts and is attracted by every other particle or mass.

Gravitation cannot be held responsible for people falling in love. — Albert Einstein (1879-1955).

Dansk (Danish)
n. - tyngdekraft, gravitation, massetiltrækning

Nederlands (Dutch)
het graviteren, effect van zwaartekracht

Français (French)
n. - gravitation

Deutsch (German)
n. - Schwerkraft, Gravitation, Streben

Ελληνική (Greek)
n. - (φυσ.) βαρύτητα, καταφορά

Italiano (Italian)
gravitazione

Português (Portuguese)
n. - gravitação (f)

Русский (Russian)
гравитация

Español (Spanish)
n. - gravitación, gravedad

Svenska (Swedish)
n. - gravitation, dragning

中文（简体）(Chinese (Simplified))
引力, 沉下, 重力

中文（繁體）(Chinese (Traditional))
n. - 引力, 沈下, 重力

한국어 (Korean)
n. - 중력, 하강, 경향

日本語 (Japanese)
n. - 引力作用, 引力, 重力, 引き寄せられること

العربيه (Arabic)
‏(الاسم) الجاذبيه الارضيه, انجذاب‏

עברית (Hebrew)
n. - ‮כבידה, כוח הכובד, משיכה, תנועה‬

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