(1642-1727) British Mathematical Physicist (Mechanics, Gravitation), Experimentalist (Optics), Astronomer
Sir Isaac Newton launched the modern age of scientific discovery and invention that continues to this day. His was a rare genius, capable of creating the mathematical tools—the calculus—he needed to pursue his physical investigations. His three laws of motion and his law of gravitation, clearly defining the nature of mass, weight, force, and acceleration, remain the foundation of our understanding of the mechanical dynamics of the macroscopic world. His mathematical description of gravitational force gave a physical basis to the Sun-centered universe proposed by Copernicus and defended by galileo galilei and erased the artificial boundary separating terrestrial and celestial events. His breakthroughs in optics, including the discovery that white light is composed of a spectrum of colors and the invention of the reflecting telescope, would suffice to secure the place of a lesser figure in the annals of physics; in Newton’s case, they rank among the secondary achievements of an unparalleled career.
Isaac Newton was born in Woolsthorpe, in Lincolnshire, England, on December 25, 1642, by the old Julian calendar, the year Galileo died. His father, an illiterate but propertied farmer, had died three months earlier and Isaac himself, a premature, sickly baby, surprised everyone by surviving. His mother’s remarriage to a wealthy, elderly clergyman in the next town, when Isaac was three, was a blow to the boy. Left in the care of grandparents, who treated him as an orphan, he found solace in making drawings and diagrams of mechanical things and in executing a few, such as water clocks, kites with fiery lanterns attached, and a model mill powered by a mouse. He was sent to school in nearby Grantham at age 12 but forced to withdraw four years later when his newly widowed mother demanded his services as manager of her estate. When Isaac proved wholly unsuited to the task, an uncle succeeded in reen-rolling him at school, where he studied for his university entrance examinations.
In 1661, Newton entered Trinity College, Cambridge, where, despite his mother’s wealth, he was obliged to work his way through the first three years by waiting tables and cleaning rooms for better subsidized students. He wrote, in a journal of his thoughts, “Plato is my friend, Aristotle is my friend, but my best friend is truth.” He plunged avidly into his studies and in 1664 was elected a scholar, a status that included four years of financial support. But the bubonic plague would interrupt his university career. Spreading across Europe, it reached Cambridge in 1665, forcing the university to close its doors.
Newton would later call the next two years, spent at home in seclusion, “the prime of my age for invention.” In light of what he accomplished, this seems an understatement. During this period he laid the foundations of differential and integral calculus, which he called the “method of fluxions.” Newton was led to invent this new branch of mathematics by his desire to describe motion using analytic geometry. The problem was that plane geometry was capable of describing only linear motion. To solve this problem, he reasoned that, by dividing the straight lines of a polygon into infinitesimally smaller straight-line segments, a circle would result in the limit of an infinitely large number of line segments of van-ishingly small length. From this insight flowed the calculus, the algebra of infinitesimal quantities, which enabled him to describe curvilinear motion mathematically.
Isaac Newton launched the modern age of scientific discovery and invention. Newton’s three laws of motion and his law of gravitation remain the foundation of our understanding of the mechanical dynamics of the macroscopic world.
The calculus was the mathematical language in which he would express his theory of universal dynamics. And, indeed, it was during this extraordinary two-year period that Newton experienced his major physical insights: he formulated the three laws of motion and the essentials of his gravitation theory, in addition to completing important work on optics. Amazingly, however, he would not share his findings with the world until several years later. True to his own complex personal dynamics, the withdrawn young man, fearing criticism and cherishing his “peace of mind,” was roused to publish only with external encouragement, most frequently someone else’s claiming credit for what Newton knew he had already discovered. At such times, he would defend his claim to primacy fiercely and even ruthlessly.
Thus, the publication in 1667 by Nicolas Mercator of a book with some methods for dealing with infinite series spurred Newton to write his own treatise, De Analysi, expounding his own wider-ranging mathematical results. By now he was back in Cambridge and had been elected a fellow of Trinity College. It was his mentor Isaac Barrow who disseminated Newton’s work to the mathematics community, establishing the young man’s reputation. When Barrow resigned as Lucasian Professor of Mathematics in 1669 to pursue divinity studies, Newton, age 26, was given this prestigious chair. Newton would remain at Cambridge for almost 30 years, living modestly and never marrying. An indifferent lecturer, whose classes were sparsely attended, Newton devoted his days and nights to his solitary studies, which, in addition to physics and mathematics, included chemistry, alchemy, theology, and mysticism. Newton would remain a passionate student of theology all his life, asserting the existence of a deity as the “first cause” of all natural phenomena.
Despite his quest for anonymity Newton soon became famous for his invention of the reflecting telescope. In need of a telescope for observing the motion of comets and planets, Newton was dissatisfied with the Galilean-style refractor telescope, then the only one in use, which had a large lens at the front end to gather light. Refractors tended to introduce spurious colors (i.e., chromatic aberration); to eliminate this, Newton used a mirror instead of a lens to collect light. The resulting efficient and inexpensive instrument became the most popular telescope in the world and remains the proto type of today’s huge astronomical reflecting telescopes. When, in 1672, Newton presented one to the Royal Society, the most influential of numerous scientific societies that were formed in the 17 th century, it elected him a fellow and urged him to write a paper on his work in optics. Newton obliged, at last publishing the results of his 1665-1666 experiments with light and color. He reported that sunlight, when passed through a prism, dispersed into a spectrum of colors; when passed through a second prism, the colors in the spectrum combined and once more formed white light. In this way he proved that colors are a property of light and not of the prism. He also investigated other optical phenomena including thin film interference effects such as Newton’s rings. Although this paper was generally well received, criticism by the eminent robert hooke made Newton once more recoil from the ordeal of publishing. Later Hooke would claim that Newton had stolen some of his optical results. Newton’s response was to wait until Hooke was dead before publishing his Opticks in 1704.
Given his aversion to subjecting his work to public scrutiny, Newton might never have published his most important findings without the intervention of the eminent astronomer Edmond Halley, who urged him to write his magnum opus and then saw that it was published. Over a two- to three-year period, between 1684 and 1686, Newton wrote his Mathematical Principles of Natural Philosophy, known today as the Principia, which was published in 1687. Here, in what is considered the greatest scientific treatise ever written, Newton proposed his laws of motion, and, most centrally, the theory of gravity. In developing his system, he built upon a synthesis of Kepler’s laws of planetary motion and Galileo’s laws of motion and gravitation. He would later say
If I have seen further than other men, it is because I have stood on the shoulders of giants.
Newton “saw further” by divining the unifying physical principles underlying his predecessors’ observations. He invented the concept of mass, a physical property of every object in the universe, and said that all objects with mass possess inertia, the tendency to resist any change in its state of motion. In his first law, the law of inertia, he states: “Every body remains at rest or in uniform motion in a straight line unless it is compelled to change that state.” Here Newton affirms Galileo’s contention, contradicting Aristotle, that no force is needed to sustain an object in motion. In Newton’s universe, when an object is set in motion or changes its velocity or direction of motion, a force is responsible. Newton defined force in his second law, the law of acceleration, which states, “A force accelerates a body by an amount proportional to its mass.” Since acceleration is the rate of change of velocity with time, the law can be stated as, Force equals mass times acceleration or F = ma. Newton’s third law of motion states, “To every action there always exists an equal and opposite reaction.” The force of action and the counterforce of reaction are always mutual; that formulation indicated how objects could be made to move and led to the law of conservation of momentum.
Newton then took these concepts and, by wedding them to his theory of the universality of the gravitational force, explained the dynamics of the entire solar system, validating Kepler’s laws of planetary motion. According to legend, Newton saw an apple fall in his orchard around 1665-1666, thought of it in terms of an attractive gravitational force toward the Earth, and realized the same force might extend as far as the Moon. He was familiar with Galileo’s work on projectiles and suggested that the Moon’s motion in orbit could be understood as a natural extension of that theory. Since the law of inertia tells us that the Moon would move in a straight line unless acted upon by an outside force, Newton reasoned that a force—gravity—is acting on it. Calculating the force needed to hold the Moon in its orbit, as compared with the force pulling an object to the ground, he deduced his famous inverse-square law of gravitation: “The force of gravitational attraction between two bodies is proportional to their masses, and decreases with increasing distance between them, as the inverse of the square of that distance, so if the distance is doubled, the force is down by a factor of four.” In order for the law to work, Newton had made the key assumption that gravity emanates from the center of the Earth. Using exact computation, he calculated the relative masses of heavenly bodies from their gravitational forces. Since comets were shown to obey the same laws, he conjectured that they might periodically return. Using his law of action-reaction, he was also able to describe the tides as resulting from the gravitational pull of both the Sun and the Moon: the action force holds the Moon in its orbit; the reaction force of gravity from the Moon moves the tides around the Earth. However, he never pretended to understand what actually caused gravitation, suggesting to those who found the idea of attraction across empty space objectionable that it might be caused by the impact of unseen particles.
The publication of the Principia had an electrifying effect throughout Europe and turned Newton into a figure of awe and reverence. After its appearance he seems to have grown bored with Cambridge. As a firm opponent of the attempt by King James II to make the universities into Catholic institutions, he was elected a member of Parliament for the University of Cambridge to the Convention Parliament of 1689 and sat again in 1701-1702. The excessive strain of his studies and the attendant disputes caused him to suffer severe depression in 1692, when he was described as having “lost his reason.”
Four years later, in 1696, he moved to London as Warden of the Royal Mint. In 1699, he became Master of the Mint, an office he retained until his death. The Royal Society of London first elected him president in 1703 and annually reelected for the rest of his life. He was knighted by Queen Anne in 1705.
His major work, Opticks, in which he summed up his life’s work on light, appeared in 1704. Although he held that light rays were corpuscular in nature, he integrated into his ideas the concept of periodicity, holding that “ether waves” were associated with light corpuscles. The corpuscle concept lent itself to an analysis by forces and established an analogy between the action of material bodies and that of light, reinforcing the universalizing tendency of the Prin-cipia. However, in the 1800s, the investigation of interference effects by thomas young led to the establishment of the wave theory of light.
As Newtonian science gained acceptance in Europe, he became the most highly esteemed natural philosopher in Europe. His last decades were spent revising his major works, polishing his studies of ancient history, and defending himself against critics, such as Leibnitz, with whom he engaged in bitter dispute over who had invented the calculus. He died at age 84 on March 20, 1727, and was buried with great pomp in Westminster Abbey.
Newton seemed to understand that his work heralded the beginning of an era in which the scientific method would continue to unlock the basic laws governing the universe. He wrote:
To myself I seem to have been only like a boy playing on the seashore and diverting myself in now and then finding a smoother pebble or a prettier shell than ordinary, whilst the great ocean of truth lay all unexplored before me.
Despite this apparent modesty, Newton would have been pleased, but not altogether surprised, to hear the reply of Apollo 8 astronaut Bill Anders who was asked who was “driving” the spacecraft to the Moon by his eight-year-old son. “I think,” said the astronaut, “Isaac Newton is doing most of the driving now.”
