To Explain the World: The Discovery of Modern Science (31 page)

I should add that in his
Principles of Philosophy
Descartes offered a significant qualitative improvement to Buridan’s notion of impetus.
⁷
He argued that “all movement is, of itself, along straight lines,” so that (contrary to both Aristotle and Galileo) a force is required to keep planetary bodies in their curved orbits. But Descartes made no attempt at a calculation of this force. As we will see in
Chapter 14
, it remained for Huygens to calculate the force required to keep a body moving at a given speed on a circle of given radius, and for Newton to explain this force, as the force of gravitation.

In 1649 Descartes traveled to Stockholm to serve as a teacher of the reigning Queen Christina. Perhaps as a result of the cold Swedish weather, and having to get up to meet Christina at an unwontedly early hour, Descartes in the next year, like Bacon,
died of pneumonia. Fourteen years later his works joined those of Copernicus and Galileo on the Index of books forbidden to Roman Catholics.

The writings of Descartes on scientific method have attracted much attention among philosophers, but I don’t think they have had much positive influence on the practice of scientific research (or even, as argued above, on Descartes’ own most successful scientific work). His writings did have one negative effect: they delayed the reception of Newtonian physics in France. The program set out in the
Discourse on Method,
of deriving scientific principles by pure reason, never worked, and never could have worked. Huygens when young considered himself a follower of Descartes, but he came to understand that scientific principles were only hypotheses, to be tested by comparing their consequences with observation.
⁸

On the other hand, Descartes’ work on optics shows that he too understood that this sort of scientific hypothesis is sometimes necessary. Laurens Laudan has found evidence for the same understanding in Descartes’ discussion of chemistry in the
Principles of Philosophy.
⁹
This raises the question whether any scientists actually learned from Descartes the practice of making hypotheses to be tested experimentally, as Laudan thought was true of Boyle. My own view is that this hypothetical practice was widely understood before Descartes. How else would one describe what Galileo did, in using the hypothesis that falling bodies are uniformly accelerated to derive the consequence that projectiles follow parabolic paths, and then testing it experimentally?

According to the biography of Descartes by Richard Watson,
¹⁰
“Without the Cartesian method of analyzing material things into their primary elements, we would never have developed the atom bomb. The seventeenth-century rise of Modern Science, the eighteenth-century Enlightenment, the nineteenth-century Industrial Revolution, your twentieth-century personal computer, and the twentieth-century deciphering of the brain—all Cartesian.” Descartes did make a great contribution to mathematics,
but it is absurd to suppose that it is Descartes’ writing on scientific method that has brought about any of these happy advances.

Descartes and Bacon are only two of the philosophers who over the centuries have tried to prescribe rules for scientific research. It never works. We learn how to do science, not by making rules about how to do science, but from the experience of doing science, driven by desire for the pleasure we get when our methods succeed in explaining something.

14

The Newtonian Synthesis

With Newton we come to the climax of the scientific revolution. But what an odd bird to be cast in such a historic role! Newton never traveled outside a narrow strip of England, linking London, Cambridge, and his birthplace in Lincolnshire, not even to see the sea, whose tides so much interested him. Until middle age he was never close to any woman, not even to his mother.
*
He was deeply concerned with matters having little to do with science, such as the chronology of the Book of Daniel. A catalog of Newton manuscripts put on sale at Sotheby’s in 1936 shows 650,000 words on alchemy, and 1.3 million words on religion. With those who might be competitors Newton could be devious and nasty. Yet he tied up strands of physics, astronomy, and mathematics whose relations had perplexed philosophers since Plato.

Writers about Newton sometimes stress that he was not a modern scientist. The best-known statement along these lines
is that of John Maynard Keynes (who had bought some of the Newton papers in the 1936 auction at Sotheby’s): “Newton was not the first of the age of reason. He was the last of the magicians, the last of the Babylonians and Sumerians, the last great mind which looked out on the visible and intellectual world with the same eyes as those who began to build our intellectual inheritance rather less than 10,000 years ago.”
*
But Newton was not a talented holdover from a magical past. Neither a magician nor an entirely modern scientist, he crossed the frontier between the natural philosophy of the past and what became modern science. Newton’s achievements, if not his outlook or personal behavior, provided the paradigm that all subsequent science has followed, as it became modern.

Isaac Newton was born on Christmas Day 1642 at a family farm, Woolsthorpe Manor, in Lincolnshire. His father, an illiterate yeoman farmer, had died shortly before Newton’s birth. His mother was higher in social rank, a member of the gentry, with a brother who had graduated from the University of Cambridge and become a clergyman. When Newton was three his mother remarried and left Woolsthorpe, leaving him behind with his grandmother. When he was 10 years old Newton went to the one-room King’s School at Grantham, eight miles from Woolsthorpe, and lived there in the house of an apothecary. At Grantham he learned Latin and theology, arithmetic and geometry, and a little Greek and Hebrew.

At the age of 17 Newton was called home to take up his duties as a farmer, but for these he was found to be not well suited. Two years later he was sent up to Trinity College, Cambridge, as a sizar, meaning that he would pay for his tuition and room and board by waiting on fellows of the college and on those students who had been able to pay their fees. Like Galileo at Pisa, he began his education with Aristotle, but he soon turned away to
his own concerns. In his second year he started a series of notes,
Questiones quandam philosophicae
, in a notebook that had previously been used for notes on Aristotle, and which fortunately is still extant.

In December 1663 the University of Cambridge received a donation from a member of Parliament, Henry Lucas, establishing a professorship in mathematics, the Lucasian chair, with a stipend of £100 a year. Beginning in 1664 the chair was occupied by Isaac Barrow, the first professor of mathematics at Cambridge, 12 years older than Newton. Around then Newton began his study of mathematics, partly with Barrow and partly alone, and received his bachelor of arts degree. In 1665 the plague struck Cambridge, the university largely shut down, and Newton went home to Woolsthorpe. In those years, from 1664 on, Newton began his scientific research, to be described below.

Back in Cambridge, in 1667 Newton was elected a fellow of Trinity College; the fellowship brought him £2 a year and free access to the college library. He worked closely with Barrow, helping to prepare written versions of Barrow’s lectures. Then in 1669 Barrow resigned the Lucasian chair in order to devote himself entirely to theology. At Barrow’s suggestion, the chair went to Newton. With financial help from his mother, Newton began to spread himself, buying new clothes and furnishings and doing a bit of gambling.
¹

A little earlier, immediately after the restoration of the Stuart monarchy in 1660, a society had been formed by a few Londoners including Boyle, Hooke, and the astronomer and architect Christopher Wren, who would meet to discuss natural philosophy and observe experiments. At the beginning it had just one foreign member, Christiaan Huygens. The society received a royal charter in 1662 as the Royal Society of London, and has remained Britain’s national academy of science. In 1672 Newton was elected to membership in the Royal Society, which he later served as president.

In 1675 Newton faced a crisis. Eight years after beginning his fellowship, he had reached the point at which fellows of a Cam
bridge college were supposed to take holy orders in the Church of England. This would require swearing to belief in the doctrine of the Trinity, but that was impossible for Newton, who rejected the decision of the Council of Nicaea that the Father and the Son are of one substance. Fortunately, the deed that had established the Lucasian chair included a stipulation that its holder should not be active in the church, and on that basis King Charles II was induced to issue a decree that the holder of the Lucasian chair would thenceforth never be required to take holy orders. So Newton was able to continue at Cambridge.

Let’s now take up the great work that Newton began at Cambridge in 1664. This research centered on optics, mathematics, and what later came to be called dynamics. His work in any one of these three areas would qualify him as one of the great scientists of history.

Newton’s chief experimental achievements were concerned with optics.
*
His undergraduate notes, the
Questiones quandam philosophicae
, show him already concerned with the nature of light. Newton concluded, contrary to Descartes, that light is not a pressure on the eyes, for if it were then the sky would seem brighter to us when we are running. At Woolsthorpe in 1665 he developed his greatest contribution to optics, his theory of color. It had been known since antiquity that colors appear when light passes through a curved piece of glass, but it had generally been thought that these colors were somehow produced by the glass. Newton conjectured instead that white light consists of all the colors, and that the angle of refraction in glass or water depends slightly on the color, red light being bent somewhat less than blue light, so that the colors are separated when light passes through
a prism or a raindrop.
*
This would explain what Descartes had not understood, the appearance of colors in the rainbow. To test this idea, Newton carried out two decisive experiments. First, after using a prism to create separate rays of blue and red light, he directed these rays separately into other prisms, and found no further dispersion into different colors. Next, with a clever arrangement of prisms, he managed to recombine all the different colors produced by refraction of white light, and found that when these colors are combined they produce white light.

The dependence of the angle of refraction on color has the unfortunate consequence that the glass lenses in telescopes like those of Galileo, Kepler, and Huygens focus the different colors in white light differently, blurring the images of distant objects. To avoid this chromatic aberration Newton in 1669 invented a telescope in which light is initially focused by a curved mirror rather than by a glass lens. (The light rays are then deflected by a plane mirror out of the telescope to a glass eyepiece, so not all chromatic aberration was eliminated.) With a reflecting telescope only six inches long, he was able to achieve a magnification by 40 times. All major astronomical light-gathering telescopes are now reflecting telescopes, descendants of Newton’s invention. On my first visit to the present quarters of the Royal Society in Carlton House Terrace, as a treat I was taken down to the basement to look at Newton’s little telescope, the second one he made.

In 1671 Henry Oldenburg, the secretary and guiding spirit of the Royal Society, invited Newton to publish a description of his telescope. Newton submitted a letter describing it and his work on color to
Philosophical Transactions of the Royal Society
early in 1672. This began a controversy over the originality and significance of Newton’s work, especially with Hooke, who had been curator of experiments at the Royal Society since 1662, and holder of a lectureship endowed by Sir John Cutler since 1664. No feeble opponent, Hooke had made significant contributions to astronomy, microscopy, watchmaking, mechanics, and city planning. He claimed that he had performed the same experiments on light as Newton, and that they proved nothing—colors were simply added to white light by the prism.

Newton lectured on his theory of light in London in 1675. He conjectured that light, like matter, is composed of many small particles—contrary to the view, proposed at about the same time by Hooke and Huygens, that light is a wave. This was one place where Newton’s scientific judgment failed him. There are many observations, some even in Newton’s time, that show the wave nature of light. It is true that in modern quantum mechanics light is described as an ensemble of massless particles, called photons, but in the light encountered in ordinary experience the number of photons is enormous, and in consequence light does behave as a wave.

In his 1678
Treatise on Light
, Huygens described light as a wave of disturbance in a medium, the ether, which consists of a vast number of tiny material particles in close proximity. Just as in an ocean wave in deep water it is not the water that moves along the surface of the ocean but the disturbance of the water, so likewise in Huygens’ theory it is the wave of disturbance in the particles of the ether that moves in a ray of light, not the particles themselves. Each disturbed particle acts as a new source of disturbance, which contributes to the total amplitude of the wave. Of course, since the work of James Clerk Maxwell in the nineteenth century we have known that (even apart from quantum effects) Huygens was only half right—light
is
a wave, but a wave of disturbances in electric and magnetic fields, not a wave of disturbance of material particles.