$$\require{cancel}$$

# Birth of Physics in the Rennaisance

[ "article:topic" ]

By the middle of the 15th century, the wealth of Europe allowed the resurgence of interest in science, which was seen as an aid to trade and commerce. Much of the early ideas had been preserved by Islamic scholars; with the decline of Arab power in Europe, the torch of learning was passed on.

### Francis Bacon (1561-1626).

As Lord Chancellor of England under James I, Francis Bacon was a strong proponent of science as a fount of support for human well-being. He believed that true knowledge of the natural world had to be based on unbiased observations by objective observers. He saw medieval crafts as important untapped sources of knowledge, the uncovering of which could lead to the conquest of Nature for the advantage of mankind and the greater glory of God. A philosopher rather than a scientist - he underestimated the power of mathematics, and failed to understand the importance of the heliocentric model of the solar system - Bacons visionary ideas were nevertheless enormously influential in the burgeoning development and perceived importance of the scientific method.

### Nicolaus Copernicus (1473-1543)

The initiator of the revolution which occurred in cosmology was born in a small town near the border between Prussia and Poland. His father died when he was young and he was adopted by his uncle, the bishop of the small principality of Ermland, recently won by Poland from the crusading order of Teutonic Knights. Copernicus, on his return from his studies in Italy, was kept busy aiding his uncle in resisting the attempts of the Teutonic Knights to re-occupy Ermland, until, in 1512, his uncle died, possibly by poisoning by the Knights. Copernicus moved to Frauenberg on the Baltic Sea to take up the post of a canon of the Cathedral. He spent the next thirty years there, as an ecclesiastic administrator, politician, physician, and scholar.

Copernicus's aim was to remove some of the many inconsistencies of the Ptolemaic system of the universe. He first suggested that the sun was at the center of the universe in a short publication entitled Commentariolus. After several years, this theory reached the attention of Georg Rheticus, Professor of Mathematics and Astronomy of Wittenberg. Rheticus came to study for two years in Frauenberg, and in 1540, in a work entitled Narratio Primo, gave the first published account of the Copernican theory. Three years later Copernicus finally published a full account of his life's work in De Revolutionibus Orbicum Coelestium (On the Revolutions of the Heavenly Spheres).

The history of the publication of this work is interesting. Apparently embittered by Copernicus's lack of acknowledgement of his efforts, Rheticus left the supervision of the printing to Andreas Osiander, a Lutheran pastor. When the book was printed, Osiander had inserted an unauthorized preface which stated that the theory it contained was to be considered to be no more than a convenient mathematical method for calculating the motions of the heavenly bodies. According to legend, Copernicus collapsed and died on reading this preface that certainly misrepresented his own belief in the truth of his theory.

Copernicus believed that a scientific theory of the heavens should both account for the observations and also preserve the Pythagorean belief that the motions of the heavenly bodies should be circular and uniform. The absurdly complicated system of eighty or so epicycles which were needed in the Ptolemaic system seemed to break at least this latter requirement. Copernicus realized that this system could be simplified by assigning a daily rotation to the earth, and allowing it and the other planets to orbit a stationary sun (a suggestion made many years earlier by Aristarchus.) Although he was able to break away from the ancients' mystical belief that the earth was somehow different from the planets in its motion, he continued to maintain a belief in the uniform and circular nature of the orbits of the heavenly bodies. It was left up to Kepler to remove this final prejudice.

With our advantage of hindsight, it is may seem surprising that Copernicus's theory did not sweep European science by storm. Yet it was not until 76 years after the publication of De Revolutionibus that church declared its theories to be heretical and banned them. There seem to be several reasons for this; Copernicus was of a shy and retiring nature, very different in personality from the exuberant and combative Galileo; his magnum opus, published only at the end of his life was obscurely written, filled with long computational tables, and opened with a preface that seemed to disavow its main thesis. Although it did make computation easier, it still required forty eight epicycles to arrive at results which attained an accuracy of one percent or so, no better than that of the Ptolemaic system. With little support from scientists or scholars it did not become a threat to the established order decreed by the church until it was reinforced by the work of Kepler and Galileo.

One of the objections to the Copernican view of the earth and the solar system was that, if the earth truly rotated, a stone thrown vertically upwards should be left behind by the rotation of the earth, and would fall some distance away from the point of projection. With no true theory of mechanics or gravitation, Copernicus was unable to provide a convincing explanation, which had to wait the developments of Galileo and Newton.

### Tycho Brahe (1546-1601)

Three years after the death of Copernicus, one of the greatest observational astronomers was born in Copenhagen. His name was Tycho Brahe. His interest in Astronomy was decided at the age of fourteen when he observed a partial eclipse of the sun which had been predicted by contemporary astronomers. In 1572, as a student at the University of Copenhagen, he observed the sudden appearance of a very bright star, probably a supernova. Brahe was able to prove, using his own observations, that the supernova had to lie a long way outside the solar system. Such a phenomenon was unthinkable in the ancients' understanding of the heavens, which were supposed to be fixed for ever in a system of perfect harmony, and Brahe's fame began to spread. In 1575 the King of Denmark gave him the island of Hven, which is within sight of Hamlet's Elsinore, with sufficient funds to establish the most advance astronomical observatory of its day. Without a telescope (which was to be invented only after his death) he accumulated the most comprehensive and wonderfully accurate measurements that the world had ever seen.

Brahe seems to have been quarrelsome, opinionated and conceited. During his student days, he lost his nose in a duel and wore a gold and silver replacement for the rest of his life. During his very productive time in Hven, he was a tyrannical dictator to his tenants, and the subject of a petition to the King who had recently succeeded to the throne. This king was less inclined to put up with Brahe's offensive ways than had been his father, and thelatter left Hven for good in . He moved to Prague, under the protection of Rudolph II, the king of Bohemia. Soon after the young Johannes Kepler joined him and the course of science was changed forever. Initially he was not keen on sharing his observations with this young competitor, and it was only after Brahe's death, and a threatened lawsuit that Kepler, was able to obtain access to the full range of Brahe's wonderful measurements. Brahe finally succumbed to his over-indulgence in food and drink by contracting a urinary infection, from which he expired.

Interestingly, Brahe did not believe in the Copernican system. The phenomenon of "parallax" occurs when you line up your finger with an object across the room; then when you move your head to right or left, your finger appears to move relative to the object. In a similar way, if the earth orbited the sun, Brahe argued that the apparent position of nearer stars should change relative to that of the distant ones. His observations showed no such change. Accordingly he believed in a system in which the planets moved around the sun; however, both sun and planets were considered to orbit the stationary earth. This system had the advantage that it avoided the problems associated with a spinning earth. This system is mathematically very close to the Copernican but avoids the problems associated with a spinning earth.

### Johannes Kepler (1571-1630)

The man whose work led to the correct model of the solar system was born in Weil, Germany. After a somewhat unhappy childhood he was educated at a theological seminary and the University of Tubingen. During much of his life he was on the move to avoid the religious persecution and the wars which were ravaging Europe. He believed that the planets influence earthly events, and all his life he attempted to find a divine order in the Universe. One of his theories suggested that the planetary orbitals fitted in to Pythagoras's five "perfect" solids; yet another, published in The Harmony of the World was that the ratios of the speeds of the planets were dictated by the notes of the musical scale. In 1600 he moved to Prague to join Tycho Brahe, twenty five years his senior. The meeting of these two geniuses must rank as one of the most perfect collaborations in the history of physics, although it was fraught with jealousy from Brahe and bad temper and childishness from Kepler. Although Brahe had the temperament and experimental ability to make the most comprehensive and precise astronomical measurements the world had ever seen, he lacked the organizational skills and the mathematical ability which allowed Kepler to unravel the three laws of planetary motion which bear his name.

Contrary to the heavenly perfection he had hoped to prove, his detailed and astonishingly careful analysis of Brahe's observations led him to realize that:

1. planets revolve around the sun, in ellipses with the sun at one focus;
2. the area swept out by a line from the sun to the planet is the same in equal time intervals;
3. the square of the length of each planet's year is proportional to the cube of the major axis of the orbit.

### Galileo Galilei (1564-1642)

Sometimes called the "Father of Modern Physics", Galileo Galilei was born in Padua, Italy. His influence on the development of experimental physics continues to this day, although he cannot be credited with all that is sometimes claimed for him. Indeed one writer has pointed out that he is famous for having dropped weights off the Leaning Tower of Pisa, which he almost certainly did not, for inventing the telescope, which he certainly did not, and for developing the Law of Inertia, which, in the way he stated it, is wrong ! Nonetheless his insistence on the experimental method laid the foundations of modern mechanics, and his willingness to fly in the face of the Pope's displeasure ensured the final acceptance of a heliocentric view of the solar system.

Galileo was an excellent student and began his university studies as a medical student at the University of Pisa. However his interests lay more in the direction of mathematics and science, in which he was sufficiently successful that he was appointed to the professorship of mathematics first at the University of Pisa, and subsequently at the University of Padua. After he became famous, he was appointed as the Philosopher-in-residence at the court of the Grand Duke of Tuscany in Florence.

Galileo seems to have been a bellicose and outspoken man, who made little attempt to conceal his contempt for those of his contemporaries who did not agree with him. He was also an unabashed showman, who adored the limelight and the fame that his discoveries brought him. Although the Church was both narrow minded and oppressive, Galileo almost seems to have gone out of his way to provoke a confrontation which came to a head with the publication of his book Dialogues on the Ptolemaic and Copernican Systems. The Pope took offence at what he believed was Galileo's portayal of him as a simpleton, and Galileo was finally forced by the Inquisition to recant the "heresies" of the heliocentric view. His book was placed on the Index of banned books, to be removed from the list only in 1835. He ended his life under house arrest, where he completed his Dialogue on Two New Sciences which summarised his work on mechanics.

According to legend his first discovery came as he watched the huge chandeliers in the Pisa Cathedral oscillate back and forth. Using his pulse as a timer, he realised that the time for one oscillation of the chandelier was independent of the amplitude of the swing - a fact which explains why pendula are still used as markers of time. His other work in mechanics was even more far reaching. He disproved Aristotle's assertion that a heavier body would fall faster than a lighter one by asking what would happen if the two bodies were lightly joined together. In this case Aristotle's theory could not decide whether the joined body would fall faster than either or that it would fall with a speed intermediate between the speeds of the two. This "thought experiment" was confirmed by a variety of ingenious experiments using inclined planes to slow down the fall of the bodies so that his crude water clocks could make measurements of the times of fall. (Simon Stevin of Bruges was probably the first to actually drop two weights from a high place to disprove the Aristotelian view).

He first gained fame through his exploitation of the idea of the Dutchman Lippershey who had invented the telescope. Galileo built his own, and trained it on the heavens. To his delight, he saw the craters on the moon, the phases of Venus, and the moons of Jupiter. This was far from the perfect heavens that the ancients had required, and immediately converted Galileo to the Copernican view of the solar system. Legend has it that some of the scholars declined his invitation to use his telescope on the grounds that it might shake their faith!

However it was his earthbound experiments that were his most important contribution to the development of physics. By letting balls roll down inclined planes, the effect of gravity could be sufficiently moderated to allow Galileo to time their passage using the primitive water clocks at his disposal. He showed that the speed of the balls as they accelerated down a smooth graduated inclined plane increased uniformly with time (or, what is the same thing, the distance they travelled was proportional to the square of the time) but independent of the weight of the balls. By allowing the balls to roll up an opposed inclined plane, he found that they reached almost the same height as the one they had started from. By reducing the angle of the second plane, and hypothesising a situation with no friction, he deduced that in the limiting case in which the second inclined plane was horizontal, the ball would continue to move on the horizontal surface for ever. Thus he propounded his Law of Inertia. Galileo's great genius in this development was to realize that the local effects of friction or air resistance masked the universal nature of the motion, and that a body which had no net force on it would continue to move without further intervention. However, he imagined that, in the absence of friction, the inertia of the body would make it move in a curved path on the surface of the earth; his law was finally corrected by Newton.

In addition to this Law, Galileo understood that that each influence on a body acted independently of any others; this was an early statement of what we would now call the Principle of Superposition. Using these results, Galileo was finally able to give a correct description of the path of a projectile. In the absence of air resistance, two independent motions combine; the constant speed in the horizontal direction, and the vertical motion in which the projectile rises vertically until the deceleration caused by gravity causes it to stop and to accelerate back to earth. The resultant path is a parabola.

Even in its slightly incorrect form, Galileo's Law of Inertia and the Principle of Superposition did allow for the resolution of two problems associated with the Copernican view of the solar system. One of the objections was that, if the earth truly rotated, a stone thrown vertically upwards would be left behind by the rotation of the earth, and would fall to the west of the the point of projection. Galileo's Law explained that the stone would retain its initial inertia in the direction of the earth's rotation, irrespective of its vertical direction. In fact he suggested that, for example, an object dropped from the top of the mast of a ship would still fall to the bottom of the mast; this experiment, performed in 1640, triumphantly confirmed Galileo's prediction. The other objection to the Copernican view was that some divine intervention was necessary to keep the planets moving around their orbits - in fact one theory had posited that this was the work of God's angels. Galileo had only to assume that God started things off; thereafter the planets would continue to move under their own inertia. Since Galileo did not have a theory of gravity this was as far as he could go in cosmology.

You can much more information and pictures pertaining to Galileo by clicking here.

### René Descartes (1596-1650).

Born in Bretagne, Descartes spent much of his life in Holland. Although the Cartesian view of the split between the mind and the body has come under criticism in an age in which holistic thought is more fashionable, it nonetheless allowed philosophers to disassociate the material world from religious belief. Such a departure from Aristotles view of nature offended the church and Descartes books were prohibited from 1663 till 1740, when the French reclaimed them as an alternative to the work of Newton.

Descartes attempted to develop a system of philosophy starting from first principles. He decided that the only thing he could be sure of was that he existed; cogito ergo sum (I think therefore I am), and from this certainty he attempted to develop an understanding of nature, using this as a basis for all further deductions and his intuition. In the spirit of this enquiry he made outstanding developments in mathematics in an attempt to give a complete mechanical understanding of the physical world: give me motion and extension, and I will construct the world; the rules of nature are the rules of mechanics were two of his oft-quoted sayings. His work in mathematics was brilliant - he was the originator of co-ordinate geometry, in which geometrical principles can be expressed purely in algebraic terms - and the x-y axes that we use today for most graphs still bear his name.

He was more of a mathematician than an experimental scientist and, as a result, some of his physics is incorrect. However he gave us two of the most fundamental principles of physics. He was the first scientist to enunciate what we now call the Law of Inertia: namely that the natural motion of bodies is in a straight line. In addition he was the first to realize that the most important measure of a bodys motion is what we now call its momentum - the product of its mass by its speed, and that this momentum is conserved in collisions between bodies.

Descartes is seldom given his fair place in most English-language science education: Newton fares almost as badly in the French language!

### Isaac Newton (1642-1727)

Isaac Newton was arguably the most influential physicist that ever lived. His contributions to physics were both wide-ranging and revolutionary. The Mechanics he developed is still the foundation of many fields of Physics and his three Laws are even more important today than they were when they were first written down. Only at the end of the 19th century was it superseded, and then only for speeds close to that of light. His work on Optics showed for the first time that light is composed of many colors. His theory of Gravitation cleared away the mists that had surrounded all previous versions of cosmology, completely explained Kepler's laws of planetary motion, and was the only accepted version until Einstein's development of General Relativity three hundred years later; even now the simplicity of the Newtonian approach still makes it the basis of choice for any calculations which do not require the sophistication of Einstein's theory. His invention of calculus (which he called fluxions), was developed almost as a sideline to his work on gravity. The fact that we now use the notation of Leibniz who developed calculus independently does not take away from the monumental nature of Newton's achievement. His Principia Mathematica published when he was forty-four years of age, has been called the greatest achievement of the human intellect. A statement of his Laws of Motion and their implication is given in the section on Mechanics.

Newton's earliest work was on optics. He was interested in reducing chromatic aberrations in glass lenses. After many experiments he discovered that white light is composed of a spectrum of colors from red to violet. His work on optics was published only later in his life (1704) and led, finally, to his knighting by Queen Anne.

The story goes that one of Newton's friends, Edmund Halley, asked him what would be the shape of planetary orbits under the action of a force that was inversely proportional to the square of the distance over which it acts. Newton answered that he had solved this problem and that the answer would be an ellipse. Apparently Newton had realised that the gravitational force was universal - i.e. that it was a force which existed between any two bodies - and that as a result it could describe the motion of an apple falling towards earth just as well as it could explain the orbit of the moon around the sun. He had also calculated that the force had to be an inverse square which produced the required agreement with Kepler's three laws of planetary motion. it was in order to do these calculations, he developed calculus, one of the greatest advances in mathematics since the time of the Ancient Greeks. At Halley's urging he prepared to publish his discovery, and in the process had to lay out his entire scheme of mechanics.

In spite of being the initiator of a system of mechanics which ushered in Modern Physics, Newton had still one foot in medieval times. In later life, he devoted most of his time to alchemy and a chronological study of the bible, most of which would now be considered to be worthless.

### Thomas Young (1773-1829).

Thomas Young was an English physician and physicist, with a brilliant mind and eclectic interests. By the age of fourteen it is said that he was acquainted with Latin, Greek, French, Italian, Hebrew, Arabic and Persian. So great was his knowledge that he was called called Phenomena Young by his fellow students at Cambridge. He studied medicine in London, Edinburgh, and Göttingen and set up medical practice in London. His initial interest was in sense perception, and he was the first to realize that the eye focuses by changing the shape of the lens. He discovered the cause of astigmatism, and was the initiator, with Helmoltz, of the three color theory of perception, believing that the eye constructed its sense of color using only three receptors, for red, green and blue. In 1801 he was appointed Professor of Physics at Cambridge university. His famous double-slit experiment established that light was a wave motion, although this conclusion was strongly opposed by contemporary scientists who believed that Newton, who had proposed that light was corpuscular in nature, could not possibly be wrong. However Young's work was soon confirmed by the French scientists Fresnel and Arago. He proposed that light was a transverse wave motion (as opposed to longitudinal) whose wavelength determined the color. Since it was thought that all wave motions had to be supported in a material medium, light waves were presumed to travel through a so-called aether, which was supposed to fill the entire universe. He became very interested in Egyptology, and his studies of the Rosetta stone, discovered on one of Napoleon's expeditions in 1814, contributed greatly to the subsequent deciphering of the ancient Egyptian hieroglyphic writing. He did work in surface tension, elasticity (Young's modulus, a measure of the rigidity of materials, is named after him), and gave one of the earliest scientific definitions of energy.