An apple fell: he watched, decided that the earth had pulled it. Hence the force of gravity.

Sir Isaac Newton

Were this story the truth, the whole truth, no one would have heard of Isaac Newton. Countless thinkers had watched apples drop, many had concluded there was some force inside the earth which made them fall. None had ever considered, as Newton did, that even the moon might fall to the earth, that it was only prevented from doing so by the centrifugal force of its movement round the earth. Nor was anyone likely, as Newton did, to calculate the magnitude of this force, then deduce the inverse square law of gravitation from it.

He was one of the greatest all-rounders the world has ever seen. His contributions in pure mathematics, theoretical physics, experi­mental physics, had never, until the time of Einstein, been surpassed. Einstein surpassed him in theoretical physics, Gauss equalled him in pure mathematics, Rutherford, perhaps, in experimental physics. No one has equalled him in mastery of all three together. For sheer intellect, Archimedes may have come near him, but the achieve­ments of the two are impossible to compare. In Greek times, science was far from the stage where it could present the exciting problems it did to Newton.

He devoted most of his intellectual energy, which was enormous, to history, theology, chemistry, alchemy and politics. He gave up science at the age of fifty-four, though at the age of eighty he was still able to discuss improvements to his famous Principia and Opticks with his young disciples. He had made his three greatest discoveries, the Calculus, the Theory of Gravitation, the Spectrum of Light, by the time he was twenty-four.

Newton the man has been completely overshadowed by Newton the thinker, and though we are more concerned here with what he did than what he was, it is worth while having a look at the man himself. He was obsessed with the desire to be considered a gentleman. He had inherited land with a rent of only £30 a year, but it carried with it the legal rights and duties of “Lord of the Manor”. These he exercised throughout his life with the most painstaking attention to detail, even solving small problems of tenancy when he was in the middle of writing the Principia.

He went to endless trouble to prove (to his own satisfaction, if not to others’) that he was a relation of Sir John Newton, Bart.; having satisfied himself on this point he troubled Sir John for years with chatty, cousinly, letters.

Thomas Tenison Archbishop of Canterbury

He left academic life without a qualm and became Warden of the Mint because it provided a good social position and a good income. And yet he might have had the social position and the income if he had agreed to become either Master of Trinity College or Provost of King’s. He refused to become either, as that would mean taking holy orders, and he had religious doubts, even though the Archbishop of Canterbury, Dr Tenison, was prepared to ignore these, raise him to holy orders immediately and give him any preferment in the Church he required.

But it was long before he left Cambridge for the Mint that he composed his Principia, believed to be the most concentrated intellectual effort made by man. For two of his academic years, 1665-1667, the university was closed because of the plague, and he returned to the house of his birth at Woolsthorpe in Lincolnshire. Here, living with his mother in quiet, country surroundings, he found himself in a perfect situation for thought. As a hobby, he worked out, for the first time in history, the area subtended by a hyperbola, with the help of his newly invented Calculus, and for fun he carried the answer to fifty-two places of decimals. He was fascinated by optics; he ground lenses with great dexterity, and from them he fashioned telescopes. He found that the images in his telescopes were blurred, had coloured edges, and when he began to investigate this he made the enormously important discovery that white light is in fact a mixture of rays of all colours, a “spectrum”.

He considered this and decided, not entirely correctly, that the ordinary sort of “refracting” telescope in which light passes from the object viewed, whether it be the moon or a distant star, through a series of lenses to the eyes, could never be made satisfactory, because each colour in the spectrum was bent, or refracted, a little more or a little less than all the other colours, so that they could never be perfectly focused together. He decided to devote some of his energies to a “reflecting” telescope, in which light is reflected from a large concave mirror via a small flat one into the eye, never passing through glass. (It has since been shown that lenses can be made of a combination of different glasses which will reduce this “chromatic aberration” almost to zero.) Newton’s reflecting telescope is the ancestor of the giant 200-inch one at Mount Palomar.

A replica of Newton’s second reflecting telescope that he presented to the Royal Society in 1672 described as the better of the two instruments Newton built.

Throughout the time he was rejecting the refracting telescope, making a reflecting one, his mind was engaged in mathematics. He was fascinated by the theory of Copernicus who had proved that the earth went round the sun and not vice versa, and now New-ton went on to speculate as to why earth and planets moved at all.

Reputed descendants of Newton’s apple tree, at the Botanic Gardens in Cambridge and the Instituto Balseiro library garden

It was at this point, in the garden at Woolsthorpe, that the apple fell.

He returned to Cambridge when the plague had subsided and in that year, 1667, he was elected a Fellow of Trinity College. He had developed his Theory of Gravitation at Woolsthorpe but now, characteristically, he refused to write it down. He accepted a professorship, which was a help to him as it carried a salary of £100 a year with the obligation only to give twenty-four lectures. He had framed his Laws of Motion, the first of which stated that a body at rest will continue at rest unless a force acts on it, and a body moving steadily in a straight line will continue to do so unless a force acts on it. This was in direct opposition to established thought on the subject, why, anyone could see that a ball rolling along the ground stopped ultimately of its own accord. Newton proved mathematically that this stopping was due to friction. As for a ball thrown in the air, that was affected not only by the friction of the air, but by the earth’s gravity.

His second Law of Motion states that force is measured by rate of change of motion, the simplest case being that of gravity, in which the force and therefore the rate of change of velocity are fixed. This he showed to be 32.2 feet per second, in every second. In other words, a falling object is travelling at 32.2 feet per second at the end of the first second, 64.4 feet per second at the end of the second second, 96.6 feet per second at the end of the third, and so on.

Newton’s third law. The skaters’ forces on each other are equal in magnitude, but act in opposite directions. Source

The third Law of Motion is that action and reaction are equal and opposite. For example, the moon pulls the earth with the same force with which the sun pulls the moon, and this can be shown by studying the tides. These laws of motion, and far more besides, were embodied in the book he was eventually persuaded to write, The Mathematical Principles of Natural Philosophy, known, because it was written in Latin, the language of educated men all over Europe, as the Principia. The work, held by many as the greatest scientific book ever written, is in three “books”, and these Laws of Motion are found in the first. In the second book Newton deals with movement against resistance, as, say, an object moving through water. He considers, in a thoroughly practical way, after he has explained the complicated mathematics behind it, the virtues of what we now call “streamlining”, the shaping of a body to minimize this resistance.

But the third book of the Principia is Newton’s triumph. In it he demonstrates the structure of the Universe, the movement of the planets and of their satellites, shows how to find the masses of the sun and of the planets from the mass of the earth. Then he performs an astonishing feat. It was known that the axis of the earth was tilted at about 66 1/2 degrees to the plane in which its orbit lay, but at the same time, though it was known this figure of 66 1/2 degrees varied slightly, no one could think why. Newton proved (i) that the world was not a sphere, after all, but an “oblate spheroid” with flattened ends, and (2) that the sun’s pull on this body’s bulging middle, varying slightly, by an amount which he could show, as the earth moved round the sun, would result in exactly the amount of “equinoctial precession” which we have discovered takes place.

Throughout his life Newton was intensely interested in optics, and his book Opticks, its title spelt in the seventeenth-century way —is a classic on the subject. As we have seen, he discovered that white light was in fact a mixture of coloured lights. He made a prism, demonstrated this spectrum, proved that violet light always came at one end, red at the other. He showed exactly how much bending or refraction there was to each colour; showed that, say, a blue light separated from white through a prism and passed, by itself, through a second prism, could not be made into anything but a blue light. On the other hand, if it be passed through, with all the other colours of the spectrum, it became white.

All Newton’s work—and three million words of it survive, even though he refused to write down much of his thinking, shows a fantastic attention to detail and an ability to sift out one important fact from a mass of trivial ones. Strangely enough, his reluctance to write it down stemmed not from modesty but from an almost pathological horror of controversy. He was horrified by the thought of becoming involved in a dispute: he knew his theories were correct, he was satisfied, he had no desire to get into an argument with men who might not agree with them.

Halley’s Comet

It was only the untiring efforts of the great astronomer Edward I Halley (whose name is remembered with “Halley’s Comet”), constantly reminding Newton that his discoveries must be published, which resulted in the Principia.

Eventually Newton agreed, and dedicated the finished work, which took him little more than a year—to the Royal Society. The famous diarist Samuel Pepys was President of the Society at the time, a shrewd and influential man, but almost without scientific knowledge, and his name, rather surprisingly, is on the title page, just as prominently displayed as that of Newton, in token of the fact that the Society sanctioned the work.

It appeared in 1687, with all three “books” bound as one, and sold at six shillings a copy. It was not an easy book and Newton, who, as we have seen, hated controversy, claimed to have made it deliberately abstruse, “to avoid being bated by little smatterers in mathematics”. The mathematical methods he used were those of classical geometry, which was difficult enough in those days and with which few are familiar now. The same results can be obtained more easily by using Calculus, which he invented, and many believe that he did so first and then provided a geometrical proof because he admired the methods of the ancients.

He was knighted at the age of sixty-two and lived a further twenty-three years, dying at eighty-five in Kensington and being buried in Westminster Abbey. It was not till after his death, until half a century had elapsed from the publishing of the Principia, that his “Newtonian Physics” began to be taught in the Universities. For years, even then, it was only the two Scottish Universities of St Andrews and Edinburgh that did so. Because the demonstrations in the book were hard to follow, many of the foremost intellects of the time had difficulty with them, and the distinguished mathematician Demoivre bought his six-shilling copy and then tore the pages apart, so that he could carry a few with him at a time, study them when he had the leisure.

All scientists, when they had mastered his sweeping theories, were united in their praise. Even Albert Einstein, who was to overthrow a large part of Newton’s teaching, maintained that he himself and all the others were pygmies in comparison with the great seventeenth-century scientist.