| Themes > Science > Physics > About Physics, Generalities > A Brief History and Philosophy of Physics > Earliest Beginnings, and the Greeks |
People have always been acutely aware of the regularities in nature: the sun rises every day; the moon appears at the same place in the sky roughly every twenty-seven days, about the same as a woman's menstrual cycle; the seasons always follow in the same order; the pattern of the "fixed" stars (all the heavenly bodies except for the planets, sun, moon and comets) repeats itself at the same time every year; snowflakes all have six points; a dropped stone always falls. In fact, the very well-being of a family depended until recent times on knowing when to plant, or when to move camp for the next season's game. This obvious order begged for explanation, and the earliest people attributed it to a range of gods and goddesses who controlled the world. With the Greeks, for example, Gaea was the earth goddess, Zeus threw lightning bolts, and Apollo drove the fiery chariot of the sun once per day across the heavens. "Science" is the attempt to give a rational, rather than religious or magical, explanation for the order in nature. People in different parts of the world began to develop science at different times, with different emphases. As one example, as early as 36 B.C. [Cole, p.46] the Mayan people of what is now Mexico and Central America used a calendar with an accuracy equivalent to knowing the length of the year to within six seconds, and plotted the movement of the sun, moon and planets. They also used a "place system" for numbers (like our decimal place system) at the time when the Romans were still using a new symbol for every new power of ten they encountered, and the Mayans employed the zero centuries before Europe. (The zero was used in India from about 850 A.D.) Although the Mayans had recorded much of their customs and learning on hundreds of books made of beaten-bark paper, very little remains today. Their Spanish conquerors systematically destroyed almost all of this "heathen" literature. The first European attempts to provide a rational explanation for the workings of nature began with the Greeks, about 600 B.C. For example, Pythagoras (582-500 B.C.) and his followers belonged to a religious fraternity dedicated to the study of numbers. They believed that the world, like the whole number system, was divided into finite elements, an early precursor to the idea of atoms ("atom" means "indivisible"). Their discovery of irrational numbers such as Ö2, which could not be expressed as a ratio of whole numbers, was a serious threat to this system, and history tells us that they killed the Pythagorean who released this secret to the world. The Greeks Leucippus (~440 B.C.), Democritus (~420 B.C.) and Epicurus (342-270 B.C.) put forward the hypothesis that matter was composed of extremely small atoms, with different materials being composed of different combinations of these atoms. Aristarchus of Samos (310-230 B.C.) is the first person known to have proposed that the earth rotates once per year around the sun, rather than the intuitive explanation that the sun rotates around the earth. He also attempted to calculate relative sizes for the earth, moon and sun. However, it was not considered necessary by the Greeks to test such hypotheses experimentally; all that most of them were looking for was a self-consistent explanation of the world based on a small number of philosophical principles. Aristotle is generally credited with providing the most comprehensive of such explanations. He believed that there were four earthly elements: earth, water, air and fire. Each had its natural place determined by its weight. Earth, being the heaviest, "wanted" to be at the centre of the universe. Water was above the earth, with air above water, and then fire. This order makes intuitive sense. Solid ("earthy") bodies sink in water; if you release air under water the air bubbles to the surface; and flames leap upward during burning. (Wood could float even though it was a solid body, because it contained both earth and fire; the fire was released on burning.) The farther a body was from the earth, the more perfect it became. Hence the moon was the least perfect of the heavenly bodies, as could be seen by its uneven appearance, while the fixed stars were the most perfect of all, and were composed of a fifth element (the "quintessence") which had no weight at all. In Aristotle's physics, a moving body of any mass had to be in contact with a "mover", something which caused its motion, or it would stop. This mover could either be internal as for animals, or external as in the case of a bowstring pushing on an arrow. The arrow was kept in flight by air displaced from the front rushing to the back to fill the vacuum left by the arrow. Since Aristotle said that a vacuum was impossible ("nature abhors a vacuum"), this explanation of an arrow's motion was again internally consistent. However, because the stars were without mass, once they were put in motion by a "prime mover" they could continue to move by themselves. The Greeks spent much effort trying to explain the motion of the sun, moon, planets and stars. Since this motion also played a major role in the development of modern science, it is worth discussing in some detail. The stars are so far from us that their relative motions cannot be observed except over timescales of a few centuries. Therefore, to someone standing on the earth the stars appear to be fixed in a vast sphere, concentric with the earth. This sphere rotates at constant speed about the earth at a rate of just more than once in twenty-four hours, returning to almost the same position at a given time of day once every year. Similarly, the sun and moon appear to lie on spheres, which rotate about the earth once per day and once every 27 days, respectively. The motions of the planets appear much more complicated to an earthly observer. We now know that the planets are all on orbits with different average distances from the sun, and orbital periods that increase the farther the planet is from the sun. For example Venus, Earth's nearest and brightest planetary neighbour, has a period of 225 days, compared to Earth's 365. This means that as Venus makes its annual pilgrimage through the night sky as viewed from Earth, it occasionally moves backwards relative to the fixed stars, in "retrograde motion", as its orbit carries it opposite to the direction the earth is moving. (Hence the name "planet", meaning "wanderer".) The Greeks usually described this motion using a device invented by Eudoxus of Cnidus (409-356 B.C.), who was apparently the first Greek to use quantitative observation to develop a mathematical description. Noting that the motion of the planets was periodic, he developed a system of spheres each of which carried a planet, with each sphere centred on the earth but with its axis of rotation fixed in a larger sphere. This explanation fitted with the Greek belief that the circle was the most perfect geometrical form. However, this system was approximate at best. Apollonius of Perga (~220 B.C.) suggested, instead, that each planet was attached to a small sphere which, in turn, rolled on a large sphere centred on the earth, with the larger one rotating roughly once per day. The large sphere accounted for the daily motion of the planet, while the small one (the "epicycle") explained the retrograde motion. A later addition was the use of the "eccentric", which allowed the centre of rotation of the large sphere for each planet to lie away from the centre of Earth. As the accuracy of the mathematical description increased, so did the need for reliable observations. This was recognized by Hipparchus of Nicea (190-120 B.C.) who had studied the observational records of the earlier Greeks and Babylonians, with the latter dating back to the seventh century B.C. In this process, Hipparchus discovered the "precession of the equinoxes"; that is, that it takes the sun about 20 seconds more to return to its position at the equinox every year than it does to return to its position among the fixed stars. To satisfy the need for accurate data, Hipparchus catalogued the position and brightness of 1080 stars. By the time of Ptolemy (85-165 A.D.), who observed at Alexandria in Egypt, the system of system of epicycles and eccentrics required eighty circles to describe the known periodicities of the heavens. Of course, the Greeks did not restrict their science to physics. For example, the Hippocratic oath sworn by doctors today takes its name from Hippocrates of Cos (~460-377 B.C.). Aristotle's most lasting contribution to science was in biology, where he classified about 540 animal species, and carried out careful dissections of at least 50 different animals. Archimedes (287-212 B.C.), scientist-engineer, has been described as one of the three greatest geniuses of all time [Kramer]. He invented the Archimedean screw for raising water, discovered the principle of buoyancy of a body in a liquid, and calculated an accurate value for p, among other accomplishments. In light of his future influence on the course of European science, it is of interest to look at Aristotle's attitude towards the role of women. In his "Generation of Animals" he says, "Wherever possible and so far as possible the male is separate from the female, since [he] is something better and more divine in that [he] is the principle of movement for generated things, while the female serves as their matter ... We should look upon the female state as it were a deformity, though one which occurs in the ordinary course of nature." . This attitude was not shared by all Greeks. For example, Pythagoras admitted women to his school equally with men. |
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