Gravitation
Gravitation, the force of attraction
between all objects that tends to pull them toward one another. It is a
universal force, affecting the largest and smallest objects, all forms of
matter, and energy. Gravitation governs the motion of astronomical bodies. It
keeps the moon in orbit around the earth and keeps the earth and the other
planets of the solar system in orbit around the sun. On a larger scale, it
governs the motion of stars and slows the outward expansion of the entire
universe because of the inward attraction of galaxies to other galaxies.
Typically the term gravitation refers to the force in general, and the
term gravity refers to the earth's gravitational pull.
Gravitation is one of the
four fundamental forces of nature, along with electromagnetism and the weak and
strong nuclear forces, which hold together the particles that make up atoms.
Gravitation is by far the weakest of these forces and, as a result, is not
important in the interactions of atoms and nuclear particles or even of
moderatesized objects, such as people or cars. Gravitation is important only
when very large objects, such as planets, are involved. This is true for
several reasons. First, the force of gravitation reaches great distances, while
nuclear forces operate only over extremely short distances and decrease in
strength very rapidly as distance increases. Second, gravitation is always
attractive. In contrast, electromagnetic forces between particles can be
repulsive or attractive depending on whether the particles both have a positive
or negative electrical charge, or they have opposite electrical charges (see
Electricity). These attractive and repulsive forces tend to cancel each
other out, leaving only a weak net force. Gravitation has no repulsive force
and, therefore, no such cancellation or weakening.
The gravitational attraction of
objects for one another is the easiest fundamental force to observe and was the
first fundamental force to be described with a complete mathematical theory by
the English physicist and mathematician Sir Isaac Newton. A more accurate
theory called general relativity was formulated early in the 20th century by
the Germanborn American physicist Albert Einstein. Scientists recognize that
even this theory is not correct for describing how gravitation works in certain
circumstances, and they continue to search for an improved theory.
Gravitation plays a crucial
role in most processes on the earth. The ocean tides are caused by the
gravitational attraction of the moon and the sun on the earth and its oceans.
Gravitation drives weather patterns by making cold air sink and displace less
dense warm air, forcing the warm air to rise. The gravitational pull of the
earth on all objects holds the objects to the surface of the earth. Without it,
the spin of the earth would send them floating off into space.
The gravitational attraction of
every bit of matter in the earth for every other bit of matter amounts to an
inward pull that holds the earth together against the pressure forces tending
to push it outward. Similarly, the inward pull of gravitation holds stars
together. When a star's fuel nears depletion, the processes producing the
outward pressure weaken and the inward pull of gravitation eventually compresses
the star to a very compact size (see Star, Black Hole).
Freefall
Falling objects accelerate in response
to the force exerted on them by Earth’s gravity. Different objects accelerate
at the same rate, regardless of their mass. This illustration shows the speed
at which a ball and a cat would be moving and the distance each would have
fallen at intervals of a tenth of a second during a short fall.
If an object held near
the surface of the earth is released, it will fall and accelerate, or pick up
speed, as it descends. This acceleration is caused by gravity, the force of
attraction between the object and the earth. The force of gravity on an object
is also called the object's weight. This force depends on the object's mass, or
the amount of matter in the object. The weight of an object is equal to the
mass of the object multiplied by the acceleration due to gravity.
A bowling ball that weighs
16 lb is actually being pulled toward the earth with a force of 16 lb. In the
metric system, the bowling ball is pulled toward the earth with a force of 71
newtons (a metric unit of force abbreviated N). The bowling ball also pulls on
the earth with a force of 16 lb (71 N), but the earth is so massive that it
does not move appreciably. In order to hold the bowling ball up and keep it
from falling, a person must exert an upward force of 16 lb (71 N) on the ball.
This upward force acts to oppose the 16 lb (71 N) downward weight force,
leaving a total force of zero. The total force on an object determines the
object's acceleration.
If the pull of gravity
is the only force acting on an object, then all objects, regardless of their
weight, size, or shape, will accelerate in the same manner. At the same place
on the earth, the 16 lb (71 N) bowling ball and a 500 lb (2200 N) boulder will
fall with the same rate of acceleration. As each second passes, each object
will increase its downward speed by about 9.8 m/sec (32 ft/sec), resulting in
an acceleration of 9.8 m/sec/sec (32 ft/sec/sec). In principle, a rock and a
feather both would fall with this acceleration if there were no other forces
acting. In practice, however, air friction exerts a greater upward force on the
falling feather than on the rock and makes the feather fall more slowly than
the rock.
The mass of an object
does not change as it is moved from place to place, but the acceleration due to
gravity, and therefore the object's weight, will change because the strength of
the earth's gravitational pull is not the same everywhere. The earth's pull and
the acceleration due to gravity decrease as an object moves farther away from
the center of the earth. At an altitude of 4000 miles (6400 km) above the
earth's surface, for instance, the bowling ball that weighed 16 lb (71 N) at
the surface would weigh only about 4 lb (18 N). Because of the reduced weight
force, the rate of acceleration of the bowling ball at that altitude would be
only one quarter of the acceleration rate at the surface of the earth. The pull
of gravity on an object also changes slightly with latitude. Because the earth
is not perfectly spherical, but bulges at the equator, the pull of gravity is
about 0.5 percent stronger at the earth's poles than at the equator.
III


EARLY IDEAS ABOUT GRAVITATION

The ancient Greek philosophers
developed several theories about the force that caused objects to fall toward
the earth. In the 4th century bc,
the Greek philosopher Aristotle proposed that all things were made from some
combination of the four elements, earth, air, fire, and water. Objects that
were similar in nature attracted one another, and as a result, objects with
more earth in them were attracted to the earth. Fire, by contrast, was
dissimilar and therefore tended to rise from the earth. Aristotle also
developed a cosmology, that is, a theory describing the universe, that was
geocentric, or earthcentered, with the moon, sun, planets, and stars moving
around the earth on spheres. The Greek philosophers, however, did not propose a
connection between the force behind planetary motion and the force that made
objects fall toward the earth.
At the beginning of the
17th century, the Italian physicist and astronomer Galileo discovered that all
objects fall toward the earth with the same acceleration, regardless of their
weight, size, or shape, when gravity is the only force acting on them. Galileo
also had a theory about the universe, which he based on the ideas of the Polish
astronomer Nicolaus Copernicus. In the mid16th century, Copernicus had proposed
a heliocentric, or suncentered system, in which the planets moved in circles
around the sun, and Galileo agreed with this cosmology. However, Galileo
believed that the planets moved in circles because this motion was the natural
path of a body with no forces acting on it. Like the Greek philosophers, he saw
no connection between the force behind planetary motion and gravitation on
earth.
In the late 16th and early
17th centuries the heliocentric model of the universe gained support from
observations by the Danish astronomer Tycho Brahe, and his student, the German
astronomer Johannes Kepler. These observations, made without telescopes, were
accurate enough to determine that the planets did not move in circles, as
Copernicus had suggested. Kepler calculated that the orbits had to be ellipses
(slightly elongated circles). The invention of the telescope made even more
precise observations possible, and Galileo was one of the first to use a
telescope to study astronomy. In 1609 Galileo observed that moons orbited the
planet Jupiter, a fact that could not reasonably fit into an earthcentered
model of the heavens.
The new heliocentric theory
changed scientists' views about the earth's place in the universe and opened
the way for new ideas about the forces behind planetary motion. However, it was
not until the late 17th century that Isaac Newton developed a theory of
gravitation that encompassed both the attraction of objects on the earth and
planetary motion.
IV


NEWTON'S THEORY OF GRAVITATION

Gravitational Forces
Because the Moon has significantly less
mass than Earth, the weight of an object on the Moon’s surface is only
onesixth the object’s weight on Earth’s surface. This graph shows how much an
object that weighs w on Earth would weigh at different points between the Earth
and Moon. Since the Earth and Moon pull in opposite directions, there is a
point, about 346,000 km (215,000 mi) from Earth, where the opposite
gravitational forces would cancel, and the object's weight would be zero.
To develop his theory of
gravitation, Newton first had to develop the science of forces and motion
called mechanics. Newton proposed that the natural motion of an object is
motion at a constant speed on a straight line, and that it takes a force to
slow down, speed up, or change the path of an object. Newton also invented
calculus, a new branch of mathematics that became an important tool in the
calculations of his theory of gravitation.
Newton proposed his law of
gravitation in 1687 and stated that every particle in the universe attracts
every other particle in the universe with a force that depends on the product
of the two particles' masses divided by the square of the distance between
them. The gravitational force between two objects can be expressed by the
following equation: F= GMm/d^{2} where F is the gravitational
force, G is a constant known as the universal constant of gravitation, M
and m are the masses of each object, and d is the distance
between them. Newton considered a particle to be an object with a mass that was
concentrated in a small point. If the mass of one or both particles increases,
then the attraction between the two particles increases. For instance, if the mass
of one particle is doubled, the force of attraction between the two particles
is doubled. If the distance between the particles increases, then the
attraction decreases as the square of the distance between them. Doubling the
distance between two particles, for instance, will make the force of attraction
one quarter as great as it was.
According to Newton, the
force acts along a line between the two particles. In the case of two spheres,
it acts along the line between their centers. The attraction between objects
with irregular shapes is more complicated. Every bit of matter in the irregular
object attracts every bit of matter in the other object. A simpler description
is possible near the surface of the earth where the pull of gravity is
approximately uniform in strength and direction. In this case there is a point
in an object (even an irregular object) called the center of gravity, at which
all the force of gravity can be considered to be acting.
Newton's law affects all
objects in the universe, from raindrops in the sky to the planets in the solar
system. It is therefore known as the universal law of gravitation. In order to
know the strength of gravitational forces in general, however, it became
necessary to find the value of G, the universal constant of gravitation.
Scientists needed to perform an experiment, but gravitational forces are very
weak between objects found in a common laboratory and thus hard to observe. In
1798 the English chemist and physicist Henry Cavendish finally measured G with
a very sensitive experiment in which he nearly eliminated the effects of
friction and other forces. The value he found was 6.754 x 10^{11} Nm^{2}/kg^{2}—close
to the currently accepted value of 6.670 x 10^{11} Nm^{2}/kg^{2}
(a decimal point followed by 10 zeros and then the number 6670). This value is
so small that the force of gravitation between two objects with a mass of 1
metric ton each, 1 meter from each other, is about 67 millionths of a newton,
or about 15 millionths of a pound.
Gravitation may also be
described in a completely different way. A massive object, such as the earth,
may be thought of as producing a condition in space around it called a
gravitational field. This field causes objects in space to experience a force.
The gravitational field around the earth, for instance, produces a downward
force on objects near the earth surface. The field viewpoint is an alternative
to the viewpoint that objects can affect each other across distance. This way
of thinking about interactions has proved to be very important in the
development of modern physics.
Newton's law of gravitation
was the first theory to accurately describe the motion of objects on the earth
as well as the planetary motion that astronomers had long observed. According
to Newton's theory, the gravitational attraction between the planets and the
sun holds the planets in elliptical orbits around the sun. The earth's moon and
moons of other planets are held in orbit by the attraction between the moons
and the planets. Newton's law led to many new discoveries, the most important
of which was the discovery of the planet Neptune. Scientists had noted
unexplainable variations in the motion of the planet Uranus for many years.
Using Newton's law of gravitation, the French astronomer Urbain Leverrier and
the British astronomer John Couch each independently predicted the existence of
a more distant planet that was perturbing the orbit of Uranus. Neptune was
discovered in 1864, in an orbit close to its predicted position.
B


Problems with Newton's Theory

Frames of Reference
A situation can appear different when
viewed from different frames of reference. Try to imagine how an observer's
perceptions could change from frame to frame in this illustration.
Scientists used Newton's theory
of gravitation successfully for many years. Several problems began to arise,
however, involving motion that did not follow the law of gravitation or
Newtonian mechanics. One problem was the observed and unexplainable deviations
in the orbit of Mercury (which could not be caused by the gravitational pull of
another orbiting body).
Another problem with Newton's
theory involved reference frames, that is, the conditions under which an
observer measures the motion of an object. According to Newtonian mechanics,
two observers making measurements of the speed of an object will measure
different speeds if the observers are moving relative to each other. A person
on the ground observing a ball that is on a train passing by will measure the
speed of the ball as the same as the speed of the train. A person on the train
observing the ball, however, will measure the ball's speed as zero. According
to the traditional ideas about space and time, then, there could not be a
constant, fundamental speed in the physical world because all speed is
relative. However, near the end of the 19th century the Scottish physicist
James Clerk Maxwell proposed a complete theory of electric and magnetic forces
that contained just such a constant, which he called c. This constant speed was
300,000 km/sec (186,000 mi/sec) and was the speed of electromagnetic waves,
including light waves. This feature of Maxwell's theory caused a crisis in
physics because it indicated that speed was not always relative.
Albert Einstein resolved this
crisis in 1905 with his special theory of relativity. An important feature of
Einstein's new theory was that no particle, and even no information, could
travel faster than the fundamental speed c. In Newton's gravitation
theory, however, information about gravitation moved at infinite speed. If a
star exploded into two parts, for example, the change in gravitational pull
would be felt immediately by a planet in a distant orbit around the exploded
star. According to Einstein's theory, such forces were not possible.
Though Newton's theory contained
several flaws, it is still very practical for use in everyday life. Even today,
it is sufficiently accurate for dealing with earthbased gravitational effects
such as in geology (the study of the formation of the earth and the
processes acting on it), and for most scientific work in astronomy. Only when
examining exotic phenomena such as black holes (points in space with a
gravitational force so strong that not even light can escape them) or in
explaining the big bang (the origin of the universe) is Newton's theory
inaccurate or inapplicable.
V


EINSTEIN'S THEORY OF RELATIVITY

In 1915 Einstein formulated
a new theory of gravitation that reconciled the force of gravitation with the
requirements of his theory of special relativity. He proposed that
gravitational effects move at the speed of c. He called this theory
general relativity to distinguish it from special relativity, which only holds
when there is no force of gravitation. General relativity produces predictions
very close to those of Newton's theory in most familiar situations, such as the
moon orbiting the earth. Einstein's theory differed from Newton's theory, however,
in that it described gravitation as a curvature of space and time.
In Einstein's general theory of
relativity, he proposed that space and time may be united into a single,
fourdimensional geometry consisting of 3 space dimensions and 1 time dimension.
In this geometry, called spacetime, the motions of particles from point to
point as time progresses are represented by curves called world lines. If there
is no gravity acting, the most natural lines in this geometry are straight
lines, and they represent particles that are moving always in the same
direction with the same speed—that is, particles that have no force acting on
them. If a particle is acted on by a force, then its world line will not be
straight. Einstein also proposed that the effect of gravitation should not be
represented as the deviation of a world line from straightness, as it would be
for an electrical force. If gravitation is present, it should not be considered
a force. Rather, gravitation changes the most natural world lines and thereby
curves the geometry of spacetime. In a curved geometry, such as the
twodimensional surface of the earth, there are no straight lines. Instead,
there are special curves called geodesics, an example of which are great
circles around the earth. These special curves are at each point as straight as
possible, and they are the most natural lines in a curved geometry. The effect
of gravitation would be to influence the geodesics in spacetime. Near sources
of gravitation the space is strongly curved and the geodesics behave less and
less like those in flat, uncurved spacetime. In the solar system, for example,
the effect of the sun and the earth is to cause the moon to move on a geodesic
that winds around the geodesic of the earth 12 times a year.
A


Testing Einstein's Theory

Einstein's theory required
verification, but the level of precision in measurement needed to distinguish
between Einstein's theory and Newton's theory was difficult to achieve in the
early 20th century and remains so today. There were two predictions, however,
that could be tested. One involved deviations in the orbit of Mercury.
Astronomers had observed that the ellipse of Mercury's orbit itself
rotated—that is, the point nearest the sun, called the perihelion, slowly
advanced around the sun. The rate of advancement predicted by Newton's theory
differed slightly from what astronomers had measured, but Einstein's theory
predicted the correct rate.
The second test involved
measuring the bending of light as it passed around the sun. Both Newton's and
Einstein's theories predicted that light would be deflected by gravitation. But
the amount of deflection predicted by the two theories differed. The light to
be measured in such a test originates in distant stars. However, under ordinary
conditions the sun's brightness prevents scientists from observing the light
from these stars. This problem disappears during an eclipse, when the moon
blocks the sun's light. In 1919 a special British expedition took photographs
during an eclipse. Scientists measured the deflection of starlight as it passed
by the sun and arrived at numbers that agreed with Einstein's prediction.
Subsequent eclipse observations also have confirmed Einstein's theory.
Other physicists have proposed
relativistic theories of gravitation to compete with Einstein's. In these
competing theories, almost all of which are geometrical like Einstein's,
gravitational effects move at the speed c. They differ mostly in the
mathematical details. Even using the technology of the late 20th century,
scientists still find it very difficult to test these theories with experiments
and observations. But Einstein's theory has passed all tests that have been
made so far.
B


Applications of Einstein's Theory

Einstein's general relativity
theory predicts special gravitational conditions. The Big Bang theory, which
describes the origin and early expansion of the universe, is one conclusion
based on Einstein's theory that has been verified in several independent ways.
Another conclusion suggested by
general relativity, as well as other relativistic theories of gravitation, is
that gravitational effects move in waves. Astronomers have observed a loss of
energy in a pair of neutron stars (stars composed of densely packed neutrons)
that are orbiting each other. The astronomers theorize that energycarrying
gravitational waves are radiating from the pair, depleting the stars of their
energy. Very violent astrophysical events, such as the explosion of stars or
the collision of neutron stars, can produce gravitational waves strong enough
that they may eventually be directly detectable with extremely precise
instruments. Astrophysicists are designing such instruments with the hope that
they will be able to detect gravitational waves by the beginning of the 21st
century.
Another gravitational effect
predicted by general relativity is the existence of black holes. The idea of a
star with a gravitational force so strong that light cannot escape from its
surface can be traced to Newtonian theory. Einstein modified this idea in his
general theory of relativity. Because light cannot escape from a black hole,
for any object—a particle, spacecraft, or wave—to escape, it would have to move
past light. But light moves outward at the speed c. According to relativity, c
is the highest attainable speed, so nothing can pass it. The black holes that
Einstein envisioned, then, allow no escape whatsoever. An extension of this
argument shows that when gravitation is this strong, nothing can even stay in
the same place, but must move inward. Even the surface of a star must move
inward, and must continue the collapse that created the strong gravitational
force. What remains then is not a star, but a region of space from which
emerges a tremendous gravitational force.
Einstein's theory of gravitation
revolutionized 20thcentury physics. Another important revolution that took
place was quantum theory. Quantum theory states that physical interactions, or the
exchange of energy, cannot be made arbitrarily small. There is a minimal
interaction that comes in a packet called the quantum of an interaction. For
electromagnetism the quantum is called the photon. Like the other interactions,
gravitation also must be quantized. Physicists call a quantum of gravitational
energy a graviton. In principle, gravitational waves arriving at the earth
would consist of gravitons. In practice, gravitational waves would consist of
apparently continuous streams of gravitons, and individual gravitons could not
be detected.
Einstein's theory did not
include quantum effects. For most of the 20th century, theoretical physicists
have been unsuccessful in their attempts to formulate a theory that resembles
Einstein's theory but also includes gravitons. Despite the lack of a complete
quantum theory, it is possible to make some partial predictions about quantized
gravitation. In the 1970s, British physicist Stephen Hawking showed that
quantum mechanical processes in the strong gravitational pull just outside of
black holes would create particles and quanta that move away from the black
hole, thereby robbing it of energy.
An important trend in
modern theoretical physics is to find a theory of everything (TOE), in which
all four of the fundamental forces are seen to be really different aspects of
the same single universal force. Physicists already have unified
electromagnetism and the weak nuclear force and have made progress in joining
these two forces with the strong nuclear force (see Grand Unification
Theories). However, relativistic gravitation, with its geometric and
mathematically complex character, poses the most difficult challenge. Einstein
spent most of his later years searching for an allencompassing theory by
trying to make electromagnetism geometrical like gravitation. The modern
approach is to try to make gravitation fit the pattern of the other fundamental
forces. Much of this work involves looking for mathematical patterns. A TOE is
difficult to test using experiments because the effects of a TOE would be
important only in the rarest circumstances.