Gravitation
Gravitation, the mutual attraction of all bodies and particles of matter in the universe. Although gravitation is a relatively weak force, it is responsible for much of the large-scale structure and organization of the universe. The motion of the moon around the earth, of the earth around the sun, and of the sun around the core of the Milky Way galaxy are all the result of gravitation.
The term gravitation is closely related to the term gravity. Gravity refers specifically to the gravitational attraction a large body has toward comparatively small masses on or near its surface. It is because of the earth's gravity that objects fall to the ground. The force with which an object is pulled by gravity is the object's weight.
Sir Isaac Newton was the first to propose that the same type of force that causes an object such as an apple to fall to the ground also holds the moon in its orbit around the earth. He presented his theory of gravitation in 1687. The essence of this theory is the law of universal gravitation, which states: Every particle of matter attracts every other particle of matter with a force proportional to the product of their masses and inversely proportional to the square of their distance apart.
This law can be expressed in the form
F = G M1 M2/d2
where F is the gravitational force; M1 and M2 are the masses of the two particles; d is their distance apart; and G is the universal gravitational constant. The value of G must be determined by experiment. When the force, masses, and distance are measured in newtons, kilograms, and meters, respectively, G has a numerical value of 6.67 X 10-11.
To calculate the gravitational force between large bodies, it is generally necessary to consider each body as being composed of small particles and to calculate the gravitational interaction of all the particles. In certain cases, the calculations can be greatly simplified. For example, a spherical body, such as the earth, exerts a gravitational force on other bodies as if all of its mass were concentrated at its center.
The gravitational forces with which two bodies attract each other are equal in strength and opposite in direction. An apple at the earth's surface, for example, attracts the earth with a force equal in strength to the force with which the earth attracts the apple. Because of the earth's much greater mass, however, the effect of the apple's gravitational attraction of the earth is not noticeable.
Newton's theory of gravitation has proved to be very accurate and useful. A notable success of the theory was the discovery of the planet Neptune in 1846; Neptune was found where calculations based on Newton's theory predicted a planet should be in order to account for irregularities in the orbit of Uranus.
In the 20th century, several new theories of gravitation have been developed. The most thoroughly investigated and best established of these theories is that of Albert Einstein. It is a cornerstone of his general theory of relativity, which was published in 1916.
In the general theory of relativity, gravitation is presented as an effect resulting from a geometrical property of space called curvature. The curvature is affected by the distribution of matter in space. Einstein's concept of gravitation is largely based on the principle of equivalence. According to this principle, the effects caused by the acceleration of a system cannot be distinguished from those caused by gravitational attraction.
For conditions in the vicinity of the sun and planets, the differences between the predictions of Newton's theory and Einstein's theory of gravitation are minute and difficult to observe. The predictions of the two theories differ appreciably only for bodies moving at speeds approaching the speed of light and for regions containing bodies of extremely large mass.
Several phenomena predicted by Einstein's theory have been used to test it. These phenomena include the slowing and deflection of light by the sun's gravity, and the gradual shifting of Mercury's orbit around the sun. Although some observations have been inconclusive, others strongly support the theory.
Using Einstein's theory, physicists have predicted the existence of a variety of astronomical phenomena. These include gravitational waves—weak disturbances in space that are generated by accelerated matter and travel at the speed of light; gravitational lenses—massive bodies with a gravitational field that focuses light from a more distant body, forming a multiple image of the body; and black holes—objects whose gravity is so great that neither matter nor light can escape from it. Indirect evidence has been found for both black holes and gravitational waves. Some experiments conducted in the late 1960's seemed to provide direct evidence for gravitational waves, but the results of the experiments have not been confirmed. Since the late 1970's, astronomers have discovered several quasars that appear in photographs as double images near the image of an intervening galaxy. Many astronomers believe that the intervening galaxies in these photographs are gravitational lenses.