01-logos-hodgson-pp13-40 6/12/03 9:31 AM Page 13 Peter E. Hodgson Galileo the Scientist Introduction The life and achievements of Galileo form a subject of enduring interest. He is certainly one of the greatest scientists of all time and indeed has been called the founder of modern science. He showed that natural phenomena obey mathematical laws and thus, Galileo laid the foundations of quantitative dynamics and used it to give the first accurate account of the motions of falling bodies and projectiles. He improved the telescope and used it to discover the moons of Jupiter, the mountains on the moon, the phases of Venus, and the spots on the sun. All this combined to throw doubt on Aristotelian cosmology and to support the heliocentric theory of Copernicus. More than any scientist, Galileo was responsible for initiating the transition from the Aristotelian science of the Middle Ages to the mathematical science of the following centuries. Galileo lived at a critical moment in the development of science. According to the popular account, the ancient Greeks made the first steps toward a scientific understanding of the world. The Greek writings were inherited by the Muslim civilization and then logos 6:3 summer 2003 01-logos-hodgson-pp13-40 6/12/03 9:31 AM Page 14 logos transmitted to the new universities in the Middle Ages through translations done mainly in Spain. Thereafter, an authoritarian Church controlled the intellectual development of Western Europe and prevented any independent thought or scientific development. It was only during the Renaissance that the authority of the Church was challenged by men like Galileo who insisted on the greater value for science of experimentation and observation than reliance on ancient texts. This is dramatized by the story that Galileo dropped two balls of different masses from the Leaning Tower of Pisa and showed that, contrary to Aristotle’s theory,they reached the ground simultaneously. Thereafter, science developed as a free and inde- pendent search for truth. The reality is, of course, different and highly instructive. The familiar story, still heard today, that there was no science worth speaking about in the long period from the time of the ancient Greeks to the flowering of genius in the Renaissance has long been disproved by modern scholarship. Galileo himself was not only a highly original scientist but remained a devout Catholic throughout his life.1 He had a sound grasp of theology and saw clearly that the new knowledge of the world gained by the scientific method was in no way inconsistent with the teaching of the Church, since both come from God. He also saw that some of the new knowledge raised important problems of scriptural interpretation that could be resolved within the context of traditional Catholic theology.It is now recognized that Galileo’s views on the interpretation of Scripture are basically correct, and he was particularly anxious to prevent the tragedy that actually happened—the condemnation by the Church of a genuine scientific breakthrough. He was, however,overconfident concerning his scientific arguments, which were still at that time inconclusive, at least to nonscientists. In view of the delicate theo- logical questions raised by the heliocentric theory,it was not unrea- sonable for Church authorities to ask Galileo to moderate his claims until a definite proof was forthcoming. The main protagonists were 01-logos-hodgson-pp13-40 6/12/03 9:31 AM Page 15 galileo the scientist all motivated to defend the truth, but they were strongly influenced by their intellectual backgrounds and possessed personal character traits that exacerbated their misunderstandings. Before considering the achievements of Galileo, it is useful to sketch the understanding of the physical world that existed before his time. The youthful Galileo was attracted to mathematics and avidly studied the works of Archimedes. His interest in hydrostatics was stimulated by Archimedes’ solution of the problem of King Hiero’s crown, which led to Galileo’s first publication, The Little Balance (1586). Nature, he realized, is written in the language of mathe- matics. Galileo was further stimulated by his experiments on the relation between musical tones and the length, weight, and tension of strings. His work on the centers of gravity of solids led to his appointment as the chair of mathematics at Pisa. His emphasis on mathematics shows the influence of Plato, who was widely influen- tial in the early Middle Ages due to the writings of Augustine. Plato held that terrestrial phenomena are imperfect copies of abstract mathematical forms existing in the transcendent realm of ideas. Thus, mathematical relations are only approximately realized in nature. It was Galileo’s greatest achievement to show how nature fol- lows mathematical laws, but he went beyond Plato in requiring exact correspondence, within the limits of experimental uncertainties. It is important to distinguish between the professional Aris- totelians in the universities, who infuriated Galileo by insisting on the literal text of Aristotle and refusing to listen to Galileo’s argu- ments, and the open-minded Jesuits at the Collegio Romano, who so strongly influenced the young Galileo in his formative years. These Jesuits followed Aristotle in many respects and taught “a somewhat eclectic Thomism containing elements deriving from Scotist,Averroist and nominalist thought”2 that may be described as scholastic Aristotelianism. Therefore, although he bitterly attacked the professional Aristotelians (particularly their views on mechanics 01-logos-hodgson-pp13-40 6/12/03 9:31 AM Page 16 logos and cosmology), Galileo retained a basic adherence to Aristotelian natural philosophy throughout his life. Aristotle thought of nature as a process, an organism, and held that the main object of science is to see how it is related to man. For him, the aim of science was to obtain certain knowledge by under- standing the causes of natural phenomena. His cosmology was based on direct commonsense experience, and this is why it has such a strong appeal, even today. Aristotle emphasized the primacy of the senses, which takes precedence over any theory. Who can doubt that the earth is solid and immoveable, with the sun, the stars, and planets moving around it? Astronomers studied the motions of the stars and the planets, and Ptolemy was able to describe them quite accurately by compound- ing circular motions in the form of cycles and epicycles. This was a purely mathematical description, and it was not maintained that the cycles and epicycles corresponded to anything real. In contrast, Aristotle sought a more physical cosmology in terms of real entities. Johannes Kepler finally unified these two approaches in the early 1600s.3 At the center of Aristotle’s cosmology is the immovable earth. Surrounding it are a number of concentric crystalline spheres bear- ing the moon and the inner planets Mercury and Venus, then the sun, and finally the outer planets Mars, Jupiter, and Saturn. Enclos- ing all is the sphere of the fixed stars, and outside this is nothing at all. There were differing views about the reality of the crystalline spheres: Aristotle believed there are fifty-five in all, made of a pure, unalterable, transparent, weightless, crystalline solid. The whole set of spheres rotates once a day,thus accounting for the diurnal motion of the sun and the stars. Seen against the background of the stars, the paths of the planets sometimes show a retrograde or looped motion, and this was accounted for by fixing the planets to secondary spheres linked to the main ones. In this way,the Aristotelian cosmology was able to give an account of all observable celestial motions, including the prediction of eclipses. 01-logos-hodgson-pp13-40 6/12/03 9:31 AM Page 17 galileo the scientist Guided by direct experience, Aristotle made a sharp distinction between terrestrial and celestial matter: terrestrial matter is change- able whereas celestial matter is unchangeable. There are four types of terrestrial matter: earth, air,fire, and water,and each seeks its nat- ural place. Celestial matter is the quintessence (or fifth essence)— pure and unchangeable—and naturally moves on the most perfect curve, the circle. On the earth, natural motion is linear: the falling of earth and water and the rising of air and fire. These motions accelerate as each body approaches its natural place. Unnatural motion, such as the flight of an arrow,requires the continuing action of a mover. Aristotle’s physics was based on direct observation and accounted for many natural phenomena in a reasonable and coher- ent way.As a result, it was widely accepted for two thousand years. One of the weakest parts of Aristotle’s physics is his theory of projectile motion. He had no concept of force and denied the notion of inertia.Aristotle believed that because projectile motion is unnat- ural it requires the continued action of a mover,and this must be the medium. He therefore suggested that the thrower communicates both motion to the medium and the power to move. Buridan, a fourteenth-century philosopher, rejected this theory because it cannot explain the continuing motion of a spinning wheel and also because it is common experience that the medium resists the motion of the projectile. Instead, Buridan proposed that the thrower gives the projectile impetus that carries it along after it has left the hand of the thrower. This is related to one of the arguments against the motion of the earth. According to Aristotle, a projectile thrown vertically upward from a moving earth will fall behind and hit the ground west of its starting point, contrary to experience. The impetus theory, however, predicts that it retains an eastward impe- tus throughout its motion, and so returns to the same point as observed.