Born: December 25, 1642 [Jan. 4, 1643, New Style], Woolsthorpe, Lincolnshire, England
Died: March 20 [March 31], 1727, London
English physicist and mathematician Sir Isaac Newton was the culminating figure of the scientific revolution of the 17th century. In optics, his discovery of the composition of white light integrated the phenomena of colours into the science of light and laid the foundation for modern physical optics.
In mechanics, his three laws of motion, the basic principles of modern physics, resulted in the formulation of the law of universal gravitation. In mathematics, he was the original discoverer of the infinitesimal calculus. Newton’s Philosophiae Naturalis Principia Mathematica (Mathematical Principles of Natural Philosophy), 1687, was one of the most important single works in the history of modern science.
Newton was elected to a fellowship in Trinity College in 1667, and from 1670 to 1672 he delivered a series of lectures and developed them into the essay “Of Colours,” which was later revised to become Book One of his Opticks. Newton held that light consists of material corpuscles in motion. The corpuscular conception of light was always a speculative theory on the periphery of his optics, however. The core of Newton’s contribution had to do with colours. He realized that light is not simple and homogeneous-
The ultimate source of Newton’s conviction that light is corpuscular was his recognition that individual rays of light have immutable properties. He held that individual rays (that is, particles of given size) excite sensations of individual colours when they strike the retina of the eye. He also concluded that rays refract at distinct angles-
In 1675 Newton brought forth a second paper, an examination of the colour phenomena in thin films, which was identical to most of Book Two as it later appeared in the Opticks. The purpose of the paper was to explain the colours of solid bodies by showing how light can be analyzed into its components by reflection as well as refraction. The paper was significant in demonstrating for the first time the existence of periodic optical phenomena. He discovered the concentric coloured rings in the thin film of air between a lens and a flat sheet of glass; the distance between these concentric rings (Newton’s rings) depends on the increasing thickness of the film of air.
A second piece which Newton had sent with the paper of 1675 provoked new controversy. Entitled “An Hypothesis Explaining the Properties of Light,” it was in fact a general system of nature. Robert Hooke, who had earlier established himself as an opponent of Newton’s ideas, apparently claimed that Newton had stolen its content from him. The issue was quickly controlled, however, by an exchange of formal, excessively polite letters that fail to conceal the complete lack of warmth between the men.
Newton was also engaged in another exchange on his theory of colours with a circle of English Jesuits in Liège, perhaps the most revealing exchange of all. Although their objections were shallow, their contention that his experiments were mistaken lashed him into a fury. The correspondence dragged on until 1678, when a final shriek of rage from Newton, apparently accompanied by a complete nervous breakdown, was followed by silence. For six years he withdrew from intellectual commerce except when others initiated a correspondence, which he always broke off as quickly as possible.
During his time of isolation, Newton, who was always somewhat interested in alchemy, now immersed himself in it. His conception of nature underwent a decisive change. Newton’s “Hypothesis of Light” of 1675, with its universal ether, was a standard mechanical system of nature. However, about 1679, Newton abandoned the ether and its invisible mechanisms and began to ascribe the puzzling phenomena-
More than 35 years later, in the second English edition of the Opticks, Newton accepted an ether again, although it was an ether that embodied the concept of action at a distance by positing a repulsion between its particles. As he conceived of them, attractions were quantitatively defined, and they offered a bridge to unite the two basic themes of 17th-
In 1684 Newton was at work on the problem oforbital dynamics, and two and a half years later, a short tract he had written, entitled De Motu (“On Motion”), had grown into Philosophiae Naturalis Principia Mathematica. This work is not only Newton’s masterpiece but also the fundamental work for the whole of modern science. Significantly, De Motu did not state the law of universal gravitation. For that matter, even though it was a treatise on planetary dynamics, it did not contain any of the three Newtonian laws of motion. Only when revising De Motu did Newton embrace the principle of inertia (the first law) and arrive at the second law of motion.
The mechanics of the Principia was an exact quantitative description of the motions of visible bodies. It rested on Newton’s three laws of motion:
1. that a body remains in its state of rest unless it is compelled to change that state by a force impressed on it;
2. that the change of motion (the change of velocity times the mass of the body) is proportional to the force impressed;
3. that to every action there is an equal and opposite reaction. Using these laws, Newton found that the centripetal force holding the planets in their given orbits about the Sun must decrease with the square of the planets’ distances from the Sun.
Newton also compared the distance by which the Moon, in its orbit of known size, is diverted from a tangential path in one second with the distance that a body at the surface of the Earth falls from rest in one second. When the latter distance proved to be 3,600 (60 × 60) times as great as the former, he concluded that one and the same force, governed by a single quantitative law, is operative in all three cases, and from the correlation of the Moon’s orbit with the measured acceleration of gravity on the surface of the Earth, he applied the ancient Latin word gravitas (literally, “heaviness” or “weight”) to it. The law of universal gravitation, which he also confirmed from such further phenomena as the tides and the orbits of comets, states that every particle of matter in the universe attracts every other particle with a force that is proportional to the product of their masses and inversely proportional to the square of the distance between their centres. The Principia immediately raised Newton to international prominence.