It was Christmas Eve 1642 when Hannah Newton first felt the birth pains. Her child was not expected until January or February but the contractions became regular and consistent. The child was a boy, born on Christmas Day. Hannah was a widow. She had experienced marriage, death and birth all within the space of nine months. She chose to call the child Isaac, after his deceased father.
The Newtons lived at Woolsthorpe Manor in the village of Colsterworth in Lincolnshire. It lay just off the Roman road of Ermine Street, which was at that time a part of the Great North Road from London to Scotland. The young Isaac Newton (1642–1727) was precocious; he enjoyed reading and he enjoyed constructing things. He received the rudiments of an education, but at the age of 14, when he was old enough to help with the farm, his mother took him away from school. It was a mistake. Young Isaac was discovered reading under the hedge when he should have been tending to the sheep. Sometimes he wandered around in a dream. On one occasion he walked all the way home from Grantham holding a horse’s bridle. He was so wrapped up in his own thoughts he had not noticed that the horse—which should have been attached to the bridle—had gone its own way. Isaac’s mother despaired of her son. He would never make a farmer she decided, so after discussing the matter with her relatives and with the schoolmaster at Grantham she agreed to let him try for entry to Cambridge where he could train to become a country parson.
Isaac Newton was sent back to school to prepare for a university education. He worked hard and easily qualified for entrance to Trinity College at Cambridge. In the autumn of 1661 he arrived to claim his place at the university. At first he had some problems integrating into undergraduate life, but he was fortunate to meet up with another student called John Wickens. The two decided to share a room together, and Wickens was happy to put up with the absentminded Newton and to assist with the many experiments conducted by his room-mate.
Newton obtained his Bachelor of Arts degree, but his interests ranged far outside the curriculum and he did not pass with distinction. He was able to remain at university, however, to study for a Master’s degree. In 1665 a great plague broke out in London and hundreds of people were dying by the week. Cambridge University took no chances; the authorities closed the colleges down for fear of the plague and the students were sent home to fend for themselves. Newton ended up spending most of his time at his mother’s rural home. The enforced seclusion in the countryside seemed to have a beneficial effect on him, however, and without the distractions of the university he became totally absorbed in his own ideas and able to continue with his many experiments.
In one experiment, conducted in 1666, he acquired a glass prism and passed a ray of sunlight through it. The prism split the sunlight into the colors of the spectrum. These colors had been witnessed by many before him, but Newton went on to make great discoveries about the nature of light. He described his feelings in his own words:
I procured me a Triangular glass-Prisme, to try therewith the celebrated Phenomena of Colorus. And in order thereto having darkened my chamber, and made a small hole in my window-shuts, to let in a convenient quantity of the Sun’s light, I placed my Prisme at his entrance, that it might thereby be refracted to the opposite wall. It was at first a very pleasing divertisment, to view the vivid and intense colours produced thereby; but after a while applying myself to consider them more circumspectly, I became surprised to see them in an oblong form; which according to the received laws of Refraction, I expected should be circular.
As a result of his experiments with light, Newton discovered the reason why the telescopes of his time always seemed to produce colored fringes around the image. He realized that the problem could be avoided by making a telescope that used a mirror, rather than a lens, to collect and focus the light. He actually constructed a telescope on these principles, and he was so pleased with his instrument that he sent it to the Royal Society in London for their perusal. This, and his treatise on light, both represented a step forward in the science of astronomy and the Royal Society were sufficiently impressed by these contributions to welcome Isaac Newton as one of their members.
But the nature of light was only one of Newton’s amazing findings during the years 1665 and 1666. The story that Newton’s theory of gravitation was inspired by the fall of an apple seems to be apocryphal, yet it must be founded in truth, for in old age he told the story to his first biographer William Stukeley (1687–1765) who related it as follows:
After dinner, the weather being warm, we went into the garden and drank tea, under the shade of some apple trees, only he and myself. Amidst other discourse, he told me, he was just in the same situation, as when formerly, the notion of gravitation came into his mind. It was occasion’d by the fall of an apple, as he sat in a contemplative mood. Why should that apple always descend perpendicularly to the ground, thought he to himself. Why should it not go sideways or upwards, but constantly to the Earth’s centre? Assuredly the reason is, that the Earth draws it. There must be a drawing power in matter: and the sum of the drawing power must be in the Earth’s centre, not in any side of the Earth. Therefore does the apple fall perpendicularly, or towards the centre. If matter thus draws matter, it must be in proportion of its quantity. Therefore the apple draws the Earth, as well as the Earth draws the apple. That there is a power, like that we here call gravity, which extends itself thro the universe.
Newton contemplated the force of gravity. He was convinced that every object in the universe had a gravitational attraction for every other object, and he felt sure that this force was governed by an inverse square law. The apple on the tree was attracted by the gravity of the Earth. Was the Moon in her passage across the sky governed by the same rule? Was the Moon drawn to the Earth by the same gravity as the apple? Newton did some calculations. He was not sure about the size of the Earth, but he found that the calculations fitted the theory “pretty nearly.” He was also unsure about which measurement he should take with regard to the distance from the apple to the Earth. Was it the few feet from the ground that he could easily measure? Was it the distance to the center of the Earth? Was it the distance to an unknown point somewhere under the Earth? He did not know the answer but he kept his calculations and his thoughts for future reference.
He also began to think about mechanics. He thought about the nature of force and acceleration. He wondered about how they were related and whether the laws of mechanics were the same on Earth as in the heavens. Why did the heavens enjoy perpetual motion when everything on Earth ran down because of friction and air resistance? The mathematics to answer his questions did not exist. The equations involved quantities in a state of flux, and he developed a method called “fluxions” to handle the problems. He had invented what we now call calculus. Newton himself described his years at Woolsthorpe:
In the beginning of the year 1665 I found the Method of approximating series and the Rule for Reducing any dignity of any Binomial into such a series. The same year in May I found the method of Tangents of Gregory and Slusius, and in November had the direct method of fluxions and the next year in January had the Theory of Colours and in May following I had entrance into ye inverse method of fluxions. And the same year I began to think of gravity extending to the orb of the Moon, and having found out how to estimate the force with which a globe revolving within a sphere presses the surface of the sphere, from Kepler’s rule of the periodical times of the planets being in a sesquialterate proportion of their distances from the centre of their orbs I deduced that the forces which keep the planets in their orbs must [be] reciprocally as the squares of their distances from the centres about which they revolve: and thereby compared the force requisite to keep the Moon in her orb with the force of gravity at the surface of the Earth and found them to answer pretty nearly. All this was in the two plague years of 1665 and 1666, for in those days I was in the prime of my age for invention, and minded mathematics and philosophy more than at any time since.
His mathematical genius was soon appreciated at Cambridge, and at the recommendation of his tutor Isaac Barrow (1630–77), Newton was offered the seat of the Lucasian Professor of Mathematics. For a time his income was secure. He retired into his ivory tower and worked on his many other interests, in particular theology and alchemy. By 1666 he had solved the main mathematical problems relating to gravitation, but he never prepared his work for publication, and for nearly 20 years these momentous ideas about the universe existed only in his head.
Gravity is the force that draws everything to the ground—a fact that was well known to people long before Isaac Newton was even born. However, Newton succeeded in explaining how the force of gravity works, not just on Earth but throughout the whole universe. In the 1660s, in his garden at Woolsthorpe, Newton observed an apple falling to the ground. He began to wonder if the force on the apple was the same as the force that held the Moon in orbit around the Earth. His calculations led him to believe that it probably was, although it was nearly 20 years before he published his ideas. The force acting on the Moon and an object such as an apple are the same, and Newton was able to assert that every particle of matter in the universe exerts a gravitational attraction on every other particle of matter in the universe.
The great physicist Albert Einstein (1879–1955) developed the theory of mechanics and gravitation beyond the ideas of Newton, but for practical purposes there is no difference between the two concepts.
It was December 1683 and in London there was a minor epidemic of the smallpox. It was a bleak and cold Christmas and the smoke from the coal fires left a great smog over the shivering city. The sluggish waters of the River Thames cooled toward freezing point as the sharp frosts of an exceptionally hard winter set in. The narrow arches of Old London Bridge restricted the river water on its passage to the sea, and as the winter approached the ice formed on the slow-moving waters of the Thames above the bridge. It soon became thick enough to support the weight of a man.
On New Year’s Day the river had become completely frozen from one bank to the other, and the foolhardy were boasting about crossing the ice to the other side. On January 9 the diarist John Evelyn (1620–1706) crossed over the ice himself. By this time the Londoners felt so safe on the frozen surface of their river that even a coach and horses could make the crossing in safety and whole new streets were appearing on the river. John Evelyn described the scene:
The frost still continuing more and more severe, the Thames before London was planted with bothes in formal streetes, as in a Citty, or Continual faire, all sorts of Trades and shops furnished, and full of Commodities, even to a Printing presse, where the People and Lady’s tooke a fansy to have their names Printed and the day and yeare set downe, when printed on the Thames … Coaches now plied from Westminster to the Temple, and from several other stairs too and fro, as in the streetes; also on sleds, sliding with skates; There was likewise Bull-baiting, Horse and Coach races, Pupet plays and interludes, Cookes and Tipling, and lewder places; so as it seem’d to be a bacchanalia, Triumph or Carnoval on the Water, whilst it was a severe Judgement upon the land: the trees not onely splitting as if lightning-strock, but Men and Cattel perishing in divers places, and the very seas so locked up with ice, that no vessels could stirr out, or come in: The fowl Fish and birds, and all our exotique Plants and Greenes universaly perishing; many Parks of deere destroied, and all sorts of fuell so deare that there were great Contributions to preserve the poore alive; nor was this severe weather much lesse intense in other parts of Europe even as far as Spaine, and the most southern tracts …
In spite of the severe weather the Royal Society still managed to hold its regular meetings, albeit poorly attended by reason of the extreme cold. The problem of gravitation was a topic very much under discussion, and the astronomers were convinced that the Sun somehow exerted an attractive force on the planets. In Holland Christiaan Huygens (1629–95) had recently published a formula for what he called the centrifugal force—this was the outward thrust experienced by a body such as a planet moving uniformly around a circle. (It was the same outward force that is experienced by a child riding on a roundabout.) This formula for centrifugal force, the astronomers knew, was one of the key factors required to solve the problem of the planetary orbits.
After one of the meetings of the Royal Society three of the members, namely Edmond Halley (1656–1742), Christopher Wren (1632–1723) and Robert Hooke (1635–1703) retired to a coffee house to discuss between themselves the problem of gravitation and how it related to what they called “the system of the world.” At this time Wren was turned 50 and Hooke was in his late 40s. They knew each other from their days at Oxford after the Civil War. Halley was the junior member; he was only 26. All three of them had come to the conclusion that the law of gravity was an inverse square law. All three knew that this hypothesis could not be raised to the status of a law unless Kepler’s laws of planetary motion, which by now were accepted as established fact, could be proved from it.
But the required proof was a very intricate and taxing mathematical problem. Christopher Wren had tried to solve the problem and he had to admit failure. Edmond Halley was a very able mathematician but he also admitted that his attempt had failed. Perhaps it was a problem that could never be solved. Robert Hooke was not a man to admit failure, however; he coyly announced that he had solved the problem but that he would keep his solution to himself so that others trying to solve it would know the value of his work. Christopher Wren did not believe his friend Hooke had a solution, however, and he offered a book to the value of 40 shillings to anybody who could solve the problem.
A few weeks later it became clear to both Wren and Halley that Hooke’s demonstration was not forthcoming. If the problem were to be solved then there was but one person in the whole of England with the mathematical skills to do it, and they all knew that he lived at Trinity College in Cambridge. Somebody must travel to Cambridge and confront Isaac Newton with the problem.
Robert Hooke was definitely the wrong man to make the visit. Newton had taken offense when Hooke had exposed an error in his work to the Royal Society, and Hooke’s credibility with Newton was at rock bottom. Christopher Wren was the obvious man to go to Cambridge. First, Newton thought very highly of Wren’s skill as a mathematician. Second, while at Cambridge, Wren could witness the progress on the library at Trinity College which he had designed. But Wren was too busy rebuilding London after the Great Fire to spare any time uncovering the secrets of the universe. It was the younger man, Edmond Halley, who had to visit Cambridge and gain an audience with the Lucasian Professor of Mathematics.
Edmond Halley had not met Newton before, but he knew of his awesome reputation. He also knew of Newton’s desire for privacy and that he did not suffer fools gladly. Halley was in no hurry to go to Cambridge, and by the time he made the journey in August 1684 the hard winter was history. In the English countryside it was harvest time, and the corn was being reaped with sickle and scythe by every available farmhand in the rural villages. When Halley arrived at Cambridge he had no problem in finding the Lucasian Professor of Mathematics. He was greeted by a figure with a sharp nose, a prematurely gray but full head of hair and slightly protruding and deeply penetrating eyes. It was Isaac Newton himself. Halley had little notion about how Newton would respond to his request, and he must have been pleased when Newton turned out to be friendly and cooperative. After a little small talk and a few formalities Halley came to the point of his visit by asking the all-important question regarding the system of the world. What did Isaac Newton think would be the orbit of a planet around the Sun, supposing the attraction of the Sun to be “reciprocal to the square of its distance from it”? Newton replied immediately that it would be an ellipse. Halley asked him how he knew the answer. “Why,” said Newton, “I have calculated it.”
Edmond Halley was amazed. He asked Newton for his calculation. The absentminded professor obligingly began to rummage through his papers as Halley watched with trepidation. Newton was unable to find his paper, but Halley did not doubt for a moment that the man before him was telling the truth. Isaac Newton had already solved the key problem at the heart of the theory of gravitation! Torn between joy and dismay Halley watched in awe as he realized that the man before him carried one of the great secrets of the universe in his head! But try as he might Newton was unable to find the paper with the written solution. It was small wonder that Halley’s next act was to ask Newton to rework his calculation.
It says much for Halley’s tact and diplomacy that he returned from Cambridge with a promise from Newton to renew the necessary calculation and to supply Halley with a copy. It leaves a few questions unanswered, however. Was Newton’s study a disorganized chaos of papers and half-completed experiments, or was it as neatly organized as his mind? Perhaps Newton knew exactly where his calculation was but wanted time to think about it before showing it to Halley. Why was Newton, who was always complaining about the demands on his time, so cooperative with Edmond Halley? Newton was well aware of Halley’s astronomical work in the southern hemisphere, and at their meeting the two must have discussed many topics in which they had a common interest. Halley told Newton about his findings in St. Helena, where his pendulum clock ran more slowly at the top of a mountain. Newton had read the account already in the Philosophical Transactions, but there was no substitute for talking to the author face-to-face. Newton probably expounded his ideas on the comet of 1680–81, and he set Halley thinking about the motion of comets and the possibility that they could return after a number of years. The outcome was that when Halley left Cambridge he became progressively more convinced that Newton’s ideas must be written up and published for the world to see and read. He knew that the Royal Society was by far the best body to handle the publication.
Halley was very fortunate. He arrived at Cambridge at a time when Newton’s researches on alchemy were getting nowhere and his academic life was progressing toward a dead end. The latest discoveries on gravitation had given Newton some new ideas to work on, and he realized that his golden opportunity had arrived and that the time was right to make his researches known to the world. He decided it was his duty to write up and publish these ideas for posterity. He knew they would generate controversy in the world of natural philosophy, but for once he was prepared to publish them and face the consequences.
It took Isaac Newton about a year to put his great work together, which he called Philosophiae Naturali Principia Mathematica (The Mathematical Principles of Natural Philosophy), usually simply known as Principia. Much of it was a synopsis of the problems he had solved over the previous 20 years. He started by formulating his three laws of motion, which today form the basis of mechanics and dynamics. Newton’s three laws of motion describe the way bodies move when acted on by a force. The first law states that a body remains at rest or in uniform motion in a straight line unless acted on by a force. We know that moving bodies on Earth slow down and stop, but this is because they are acted on by the forces of friction and wind pressure. If these forces were removed the bodies would continue to move in a straight line forever unless an external force acted upon them.
The second law states that a force changes the motion of a body in the direction of the force. The acceleration of the body depends on its mass and the value of the force. A good example is a train moving in a straight line with a constant force. In such conditions it generates uniform acceleration. For a planet moving around the Sun and drawn to it by gravity, the law is complex but still holds true.
The third law states that for every action there is an equal and opposite reaction. If you sit on a chair, your weight becomes a downward force on the chair. The chair exerts an equal upward force, known as the reaction, on you. When two billiard balls strike each other the result is more complex, but Newton’s law shows that the balls experience equal and opposite reactions. These laws appear to apply only to earthbound objects, but Newton went on to show how the same laws could be applied to the motion of the planets around the Sun. He showed how the whole system of the Sun and the planets could be explained from a single law—the law of universal gravitation. Newton used his method of fluxions to arrive at his results, but he reworked them into a classical form so that mathematicians could understand them more readily. The result was a work explaining the whole system of the world in detail, but encompassed in classical mathematics. Future generations would use calculus to solve the problems, leaving Newton’s classical proofs in an isolated time warp of their own.
It was 1687 when Newton’s great work Principia was published and his astronomical works became known to the world. He is remembered as a great scientist and philosopher as well as an astronomer. He retained an interest in astronomy until his death in 1727, and during his lifetime he saw many advances in both the practical and theoretical sides of astronomy.
Finally, we must thank Humphrey Newton (fl. 1629–95), a distant relative and assistant to Isaac Newton, for leaving us with this simple but fascinating picture of a master scientist and his often distracted thoughts:
When he has sometimes taken a turn or two [about the garden], has made a sudden stand up, turn’d himself about, [he would] run up the stairs like another Archimedes, with an “eureka” fall to write on his desk standing without giving himself the leisure to draw a chair to sit down on. At some seldom times when he designed to dine in the hall, would turn to the left hand and go out into the street, when making a stop when he found his mistake, would hastily turn back, and then sometimes instead of going into the hall, would return to his chamber again.