gravitation, the attractive force force, commonly, a "push" or "pull," more properly defined in physics as a quantity that changes the motion, size, or shape of a body. Force is a vector quantity, having both magnitude and direction.
..... Click the link for more information. existing between any two particles of matter matter, anything that has mass and occupies space. Matter is sometimes called koinomatter (Gr. koinos=common) to distinguish it from antimatter, or matter composed of antiparticles .
..... Click the link for more information. .

The Law of Universal Gravitation

Since the gravitational force is experienced by all matter in the universe, from the largest galaxies down to the smallest particles, it is often called universal gravitation. (Based upon observations of distant supernovas around the turn of the 21st cent., a repulsive force, termed dark energy dark energy, repulsive force that opposes the self-attraction of matter (see gravitation ) and causes the expansion of the universe to accelerate. The search for dark energy was triggered by the discovery (1998) in images from the Hubble Space Telescope of a distant
..... Click the link for more information. , that opposes the self-attraction of matter has been proposed to explain the accelerated expansion of the universe.) Sir Isaac Newton Newton, Sir Isaac, 1642–1727, English mathematician and natural philosopher (physicist), who is considered by many the greatest scientist that ever lived.
..... Click the link for more information. was the first to fully recognize that the force holding any object to the earth is the same as the force holding the moon, the planets, and other heavenly bodies in their orbits. According to Newton's law of universal gravitation, the force between any two bodies is directly proportional to the product of their masses (see mass mass, in physics, the quantity of matter in a body regardless of its volume or of any forces acting on it. The term should not be confused with weight , which is the measure of the force of gravity (see gravitation ) acting on a body.
..... Click the link for more information. ) and inversely proportional to the square of the distance between them. The constant of proportionality in this law is known as the gravitational constant; it is usually represented by the symbol G and has the value 6.670 × 10⁻¹¹ N-m²/kg² in the meter-kilogram-second (mks) system of units. Very accurate early measurements of the value of G were made by Henry Cavendish.

The Relativistic Explanation of Gravitation

Newton's theory of gravitation was long able to explain all observable gravitational phenomena, from the falling of objects on the earth to the motions of the planets. However, as centuries passed, very slight discrepancies were observed between the predictions of Newtonian theory and actual events, most notably in the motions of the planet Mercury. The general theory of relativity relativity, physical theory, introduced by Albert Einstein, that discards the concept of absolute motion and instead treats only relative motion between two systems or frames of reference.
..... Click the link for more information. proposed in 1916 by Albert Einstein Einstein, Albert (īn`stīn), 1879–1955, American theoretical physicist, known for the formulation of the relativity theory, b.
..... Click the link for more information. explained these differences and provided a geometric explanation for gravitational phenomena, holding that matter causes a curvature of the space-time framework in its immediate neighborhood.

The Search for Gravity Waves

Tantalizing evidence for the existence of gravity waves, which are predicted by Einstein's general theory of relativity and would be analogous to electromagnetic waves, comes from astronomical observations of a binary pulsar pulsar, in astronomy, a neutron star that emits brief, sharp pulses of energy instead of the steady radiation associated with other natural sources. The study of pulsars began when Antony Hewish and his students at Cambridge Univ.
..... Click the link for more information. designated 1913 + 16. The rate at which the two neutron stars in the binary rotate around each other is changing in a manner that is consistent with the emission of gravity waves. A hypothetical particle, given the name graviton, has been suggested as the mediator of the gravitational force; it is analogous to the photon photon (fō`tŏn), the particle composing light and other forms of electromagnetic radiation , sometimes called light quantum.
..... Click the link for more information. , the particle embodying the quantum properties of electromagnetic waves (see quantum theory quantum theory, modern physical theory concerned with the emission and absorption of energy by matter and with the motion of material particles; the quantum theory and the theory of relativity together form the theoretical basis of modern physics.
..... Click the link for more information. ). The search for gravity waves continues with the building of large interferometers that would be sensitive enough to detect the faint waves directly (see interference interferometer. When the wavelength of the light is known, the interferometer indicates the thickness of the film by the interference patterns it forms. The reverse process, i.e., the measurement of the length of an unknown light wave, can also be carried out by the interferometer.
..... Click the link for more information. ). Millions of dollars have already been spent on the Laser Interferometer Gravitational Wave Observatory (LIGO), supported by the National Science Foundation, and work is beginning on the even more ambitious Laser Interferometer Space Antenna (LISA).

The Force of Gravity

The term gravity is commonly used synonymously with gravitation, but in correct usage a definite distinction is made. Whereas gravitation is the attractive force acting to draw any bodies together, gravity indicates that force in operation between the earth and other bodies, i.e., the force acting to draw bodies toward the earth. The force tending to hold objects to the earth's surface depends not only on the earth's gravitational field field, in physics, region throughout which a force may be exerted; examples are the gravitational, electric, and magnetic fields that surround, respectively, masses, electric charges, and magnets. The field concept was developed by M.
..... Click the link for more information. but also on other factors, such as the earth's rotation. The measure of the force of gravity on a given body is the weight weight, measure of the force of gravity on a body (see gravitation ). Since the weights of different bodies at the same location are proportional to their masses, weight is often used as a measure of mass .
..... Click the link for more information. of that body; although the mass of a body does not vary with location, its weight does vary. It is found that at any given location, all objects are accelerated equally by the force of gravity, observed differences being due to differences in air resistance, etc. Thus, the acceleration acceleration, change in the velocity of a body with respect to time. Since velocity is a vector quantity, involving both magnitude and direction, acceleration is also a vector. In order to produce an acceleration, a force must be applied to the body.
..... Click the link for more information. due to gravity, symbolized as g, provides a convenient measure of the strength of the earth's gravitational field at different locations. The value of g varies from about 9.832 meters per second per second (m/sec²) at the poles to about 9.780 m/sec² at the equator. Its value generally decreases with increasing altitude. Because variations in the value of g are not large, for ordinary calculations a value of 9.8 m/sec², or 32 ft/sec², is commonly used.

Bibliography

See A. S. Eddington, Space, Time and Gravitation (1920); J. A. Wheeler, A Journey into Gravity and Spacetime (1990); M. Bartusiak, Einstein's Unfinished Symphony: Listening to the Sounds of Space-Time (2000).

gravitation

Universal force of attraction that acts between all bodies that have mass. Though it is the weakest of the four known forces, it shapes the structure and evolution of stars, galaxies, and the entire universe. The laws of gravity describe the trajectories of bodies in the solar system and the motion of objects on Earth, where all bodies experience a downward gravitational force exerted by Earth's mass, the force experienced as weight. Isaac Newton was the first to develop a quantitative theory of gravitation, holding that the force of attraction between two bodies is proportional to the product of their masses and inversely proportional to the square of the distance between them. Albert Einstein proposed a whole new concept of gravitation, involving the four-dimensional continuum of space-time which is curved by the presence of matter. In his general theory of relativity, he showed that a body undergoing uniform acceleration is indistinguishable from one that is stationary in a gravitational field.