Introduction Francis Bacon Nicolaus Copernicus Tycho Brahe Johannes Kepler Galileo Galilei René Descartes Isaac Newton Thomas Young
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 centre 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 unauthorised 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 realised 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.
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.
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:
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.
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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 body’s 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!
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 colours 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 mediaeval 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.