Gravity

In ancient Greek and medieval European cosmology, solid crystalline spheres provided a physical structure for the universe and carried the planets in their motions around the Earth. Then Copernicus moved the Earth out of the center of the universe, Tycho Brahe shattered the crystalline spheres with his observations of the comet of 1577, and Kepler replaced circular orbits with ellipses. How, now, could the planets continue to retrace their same paths around the Sun for thousands of years? The answer, encompassing both physical cause and mathematical description, came forth from Isaac Newton.

Newton's remarkable intellectual accomplishments include creation of the calculus, invention of the reflecting telescope, development of the corpuscular theory of light (its colored rays separated by a prism), and development of the principles of gravity and terrestrial and celestial motion. Newtonian thinking came to permeate not only the physical world but also intellectual fields, including politics and economics, where others were encouraged to seek universal natural laws of the sort Newton had found in physics and astronomy. Nor was eighteenth-century literature oblivious to the Newtonian revolution in thought. He changed history, perhaps more so than any other single person.

Newton's father, although a wealthy farmer and lord of his own manor, could not sign his name. Probably he would not have educated his son; his brother did not. Newton was born in 1642, the year Galileo died, three months after his father's death, premature, and so small that he was not expected to survive. When he was three, his mother remarried, and Newton was left in his grandmother's care. Difficult early years probably contributed to his difficult mature personality. He rejoined his mother seven years later, after his stepfather died, leaving his mother wealthy. Her family was educated, and she soon sent Newton off to school. He returned home at age 17 to learn to manage the family manor. But he was a disaster at rural pursuits, his mind lost in other thoughts. His mother's brother, a clergyman, urged her to send young Newton back to school to prepare for university. Newton entered Trinity College, Cambridge, in 1661. He would be appointed a fellow in 1667 and named Lucasian professor in 1669, at age 26. In later years, he would serve as warden of the mint and president of the Royal Society. He died in 1727.

At Cambridge, Newton initially studied Aristotelian physics. But around 1664, his notebooks reveal, he learned of the French scientist René Descartes. In his 1637 Le discours de la méthode pour bien conduire sa raison et chercher la vérité dans les sciences (Discourse on the Method of Rightly Conducting Reason and Seeking Truth in the Sciences), Descartes had begun with the famous phrase "Cogito ergo sum" (I think, therefore I am). From this certainty, he expanded knowledge, one step at a time, to include the existence of God, the reality of the physical world, and its mechanistic nature. Descartes' universe consisted of huge whirlpools, or vortices, of cosmic matter. Our solar system was one of many whirlpools, its planets all moving in the same direction in the same plane around a luminous central body. Planets' moons were swept along by the planets' vortices. All change in motion was the result of percussion of bodies; one object could act on another only by contact. Gravity was the result of celestial matter circulating about the Earth and pushing all terrestrial matter toward the Earth.

Figure 17.1: Whirlpools of Matter.

Descartes, Principia philosophiae,

1644. Descartes speculated that God had divided matter into particles of the same size and imparted to them rotation around their centers and a propensity to rotate as a group around centers equidistant from each other. Distortion caused by unequal pressures of adjoining vortices reshaped circular orbits. Gravity's cause was pressure from celestial matter circulating in all directions about the Earth and tending to push objects toward the Earth. In the diagram, the Sun (S) is in the middle of the vortex AYBM and surrounded by a whirlpool of matter in which its planets circulate. Neighboring vortices each have a star at their centers. From René Descartes, Principia philosophiae (Leiden: Elzevier, 1644).

In contrast to Descartes' explanation of the phenomena of nature in terms of particles bouncing off each other, Newton would explain planetary motions as the result of a mysterious attraction at a distance. The path to his great achievement is almost as mysterious.

In 1665 the plague arrived in Cambridge, the colleges closed, and teachers and students fled to the countryside. There, a flash of insight supposedly gave Newton the idea of universal gravitation. So the legend goes, while musing in a garden, it occurred to him that the power of gravity causing an apple to fall to the Earth might also retain the Moon in its orbit. He did a quick calculation to check his idea, but because he had the wrong number for the radius of the Earth, his answer did not agree with his theory, and he put the theory aside.

Skeptical historians question the story of Newton and the apple, even though it was told by the great man himself. Newton's papers reveal that around 1666 he was thinking more in terms of the Moon fleeing from the Earth than being bound by the force of gravity to the Earth.

In 1679 the English scientist Robert Hooke wrote to Newton asking for his opinion about Hooke's hypothesis that the planetary motions might be compounded of a tangential motion and an attractive motion towards the central body. Years later, Hooke would accuse Newton of stealing and using this idea. Newton then remembered that he had also discussed the problem of determining the Heavenly motions upon philosophical principles with the architect Christopher Wren, in 1677. Wren confirmed this and stated that for many years he had had thoughts upon making out the planets motions by a composition of a descent towards the

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