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Gravitation, or gravity, is a force that attracts all objects
in the universe;  it is the most familiar of the four
FUNDAMENTAL INTERACTIONS of matter.  Gravitation has several
basic characteristics that distinguish it from the other
fundamental interactions.  First, it is universal, affecting
all forms of matter and energy in essentially the same way,
whereas all the other interactions directly affect only certain 
types of particles.  The electromagnetic force, for example,
affects only charged particles.  Second, gravity is always
attractive, since it interacts with mass-energy, which is
always positive.  In electromagnetism, on the other hand,
charges can either attract or repel.  Third, gravitation is a
long-range interaction.  Electromagnetism is also long-range,
but the strong and weak nuclear forces generally operate only
within a distance the size of an atomic nucleus.  Fourth,
gravity is the weakest of the four fundamental forces.  It has
a negligible effect on elementary particles.  The
electromagnetic attractive force between a proton and an
electron is nearly 10 to the 40th power times greater than the
gravitational force at the same separation.  Because gravity is a 
long-range attractive force affecting all matter, however, it is 
the dominant force in the universe.
Throughout history there have been many attempts to describe or 
explain gravitation. About 330 BC, Aristotle claimed that the
four elements--earth, water, air, and fire--have their natural
places, toward which they tend to travel. He argued that
objects containing greater amounts of earth than others would
fall toward the Earth faster and that their speed would
increase as they neared their natural place.
GALILEO GALILEI deduced (1604) that gravity imparts a definite
acceleration, rather than a velocity, and that this
acceleration is the same for all objects traveling in a vacuum. 
The universality of gravitational acceleration is known as the
weak equivalence principle.
Sir Isaac NEWTON made the most significant contribution to
gravitational theory when he perceived (1606) that the orbit of 
the Moon depended on the same type of force that causes an
apple to fall to Earth.  This proposition required that the
magnitude of the force decrease in inverse proportion to the
square of the distance from the Earth's center.  Newton
combined the inverse square law with his three LAWS OF MOTION
to formulate a theory of universal gravitation, which stated
that there is a gravitational attraction between every pair of
objects, inversely proportional to the square of the distance
between them.
Rene DESCARTES (1596-1650) had earlier proposed a
nonquantitative theory of gravitation based on the inward
pressure of vortices on planets (see PHYSICS, HISTORY OF), but
Newton did not offer a cause for the attraction.  In fact, he
avoided even calling it an attraction, speaking instead of
"bodies gravitating towards one another." This description was
sufficient to deduce KEPLER'S LAWS of planetary motion, the
oceanic tides (see TIDE), and the PRECESSION OF THE EQUINOXES.
In 1846 it was used to predict and discover a new planet,
Neptune.  Thus Newton's theory of gravitation stands as one of
the greatest advancements of scientific knowledge.
Expressed mathematically, Newton's theory states that there is
an attractive force F, given by F = Gm(1)m(2)/r squared,
between two particles having masses m(1) and m(2) and
separation distance r.  G is the gravitational constant of
proportionality, an unknown quantity that could not be
determined by solar-system observations, which give only ratios 
of masses and thus the product of G and some mass.  The value
of G was first determined in 1798 by Henry Cavendish, who
measured the force of gravitational attraction between two
spheres of known mass.  This experiment has come to be known as 
"weighing the Earth," because once G was determined, the
Earth's mass, m(e), could be determined from the astronomically 
known value for Gm(e).  The experiment has been repeated many
times with increasing precision.  The currently accepted value
of G is 6.67259 times 10 to the minus 8 power cm cubed /g sec

In 1905, Albert EINSTEIN developed his theory of special
RELATIVITY, which modified Newton's theory of gravitation.
Einstein sought to describe gravitation in a way that was
independent of the motion of observers and of the coordinates
chosen to label events.  His work led to a geometrical theory
that described gravity purely by the structure of the
SPACE-TIME CONTINUUM.  According to this geometrical theory,
gravity affects all forms of matter and energy, all of which
move in space-time.  Thus Einstein's theory obeys the weak
equivalence principle and gives the same gravitational
acceleration for all freely falling objects.
In addition to describing the effect of gravity on matter,
Einstein described the effect of matter on gravity.  This
theory, which Einstein completed in 1915, is called general
relativity.  Although Einstein's theory is much different from
Newton's, it predicts nearly the same effects in systems in
which gravitational fields are weak and velocities are slow
compared to the velocity of light.  Planetary motion had
provided a particularly accurate verification of Newton's
theory, but Einstein's accounted for some phenomena in the
solar system not considered by Newton.
One such phenomenon was the perihelion precession of Mercury.
In the 19th century, it was observed that the rate of
precession differed by 43'' per century from what Newton's
theory predicted;  Einstein's theory predicted precisely such a 
difference in the precession rate.  Another such phenomenon is
the bending of light rays by the Sun's gravitational field,
which Newton's theory did not predict at all.  Einstein's
prediction was confirmed by Arthur S. Eddington during a total
eclipse in 1919, and later by others to 1% accuracy.  Einstein
also predicted the gravitational RED SHIFT, a change in the
frequency of electromagnetic waves escaping from a strong
gravitational field, which was confirmed by Robert Pound and
Glen Rebka in 1960.  In 1964, Irving Shapiro used general
relativity to predict a time delay of signals passing near the
Sun, an effect since confirmed.
In addition, Einstein's theory of general relativity predicts
several qualitatively new effects in other systems and is
especially useful in dealing with COSMOLOGY.  Relativity
asserts that the universe must be either expanding or
contracting.  Einstein was not sufficiently bold to believe
this prediction, however, so he modified his equations to allow 
for a static universe.  In 1929, though, Edwin Hubble
discovered that the universe is expanding.  Whether gravity
will eventually cause it to collapse is a subject of current
investigation and debate.  General relativity also predicts
GRAVITATIONAL WAVES from masses in nonuniform motion, but these 
waves are so weak that they have not yet been definitely
detected.  Finally, Einstein's theory predicts GRAVITATIONAL
COLLAPSE of sufficiently massive objects into BLACK HOLES.
Today there is mounting evidence that several astronomical
systems may contain black holes.
Einstein's general relativity is not the only 20th-century
theory of gravity, though it is perhaps the simplest and most
elegant.  All viable theories of gravity must, like Einstein's, 
be complete, self-consistent, and relativistic.  They must also 
have the correct Newtonian limit, uphold the weak equivalence
principle, and predict the same Einstein shift as measured by
all ideal clocks at the same position.  There is strong
experimental evidence for accepting these criteria as
fundamental, and it has been conjectured by L. I. Schiff that
they can be satisfied only by geometrical, or metric, theories. 
The strongest rival theory has been the Brans-Dicke theory.
Like general relativity, it is a geometrical theory that
satisfies the fundamental criteria.  Its field equations are
different, however, and it claims that the geometry of
space-time is affected not only by matter but also by an
additional scalar field.  Unlike Einstein's calculations, the
Brans-Dicke theory cannot predict the perihelion shift of
Some recent theories attempt to explain gravitation
non-geometrically, proposing instead that particles called
gravitons are responsible.  These so-called supersymmetry
theories place gravitational phenomena within the realm of
quantum physics.  They are part of an attempt to show that the
four fundamental interactions of nature are related, and that
they were a single, united force at the birth of the universe
Multidimensional analysis has also raised the issue of the
constancy of the gravitational constant G.  This idea was
proposed earlier by British physicist Paul Dirac in his
so-called "large numbers" hypothesis.  Dirac noted that the
ratio of the strength of the electromagnetic force to that of
the gravitational force (approximately 10 to the 40th power) is 
roughly equivalent to the age of the universe in atomic terms.
He wondered whether there might be a deep physical connection
requiring this similarity and proposed that this would be the
case if G slowly decreased as the age of the universe
increased.  If G were decreasing, however, gravitational time
would change with respect to atomic time;  thus far,
experiments have not shown this to be happening.
Don N. Page
Bibliography:  Bergmann, P. G., The Riddle of Gravitation
(1968);  Hawking, S. W., and Israel, W., Three Hundred Years
of Gravitation (1987;  repr.  1989);  Mathews, P. M., et al.,
eds., Gravitation, Quantum Fields and Superstrings (1988);
Misner, Charles, et al., Gravitation (1973);  National Research 
Council, Gravitation, Cosmology, and Cosmic-Ray Physics (1986); 
Thorne, K. S., Gravitational Radiation (1989);  Zee, Anthony
An Old Man's Toy (1989).

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