The Laws of Physics

Sources

The Rules of Nature. One area of science in which the medieval philosophers excelled, discussing important and fundamental concepts of the way nature works, was physics. Because medieval philosophers believed that knowledge of the material world would bring them closer to an understanding of the divine nature of that world, the study of “physica”—the orderly part of nature subject to rules and to cause and effect—was easily accepted in church schools across Europe. The areas most frequently investigated were motion and optics. Studies of both benefited immensely from twelfth-century translations of ancient Greek sources and medieval Arabic commentaries.

Motion. Aristotle taught that all motions are made up of natural motion and violent motion. This theory explains how things move, but it did little to explain why they moved. (In fact, it was even somewhat unsatisfactory in explaining how.) In reading and commenting on Aristotle, medieval Scholastics came to some new and striking insights into the nature of motion. These ideas were passed on to later scientists, including Galileo, and underlie modern physicists’ understanding of motion. The realization that Aristotle’s explanation of motion was incomplete, or even faulty, came to medieval thinkers as they tried to understand why things kept moving once they no longer had contact with the force that made them move. Aristotle had said that all things that are moving are moved by something. A person can apply a force to make a rock move violently, that is, in a direction (up or side-to-side) in which it would not move on its own. But when the rock leaves a person’s hand, as when it is thrown, why does it keep moving in the direction it has been thrown instead of falling straight to the ground? Aristotle said that the air the rock pushes out of the way as it moves rushes around to push the rock from behind, but few medieval scientists were convinced by his explanation. Throughout the thirteenth and fourteenth centuries Scholastics debated the question, and eventually they arrived at a new solution.

Impetus. Jean Buridan, a secular teacher at the University of Paris during the first half of the fourteenth century,

ORESME ON MOMENTUM

Nicole Oresme considered Jean Buridan’s idea that impetus is slowly “used up” as objects move, developing a theory that suggests how different motions might be related. In the following passage he compared the swing of a pendulum to the motion of a stone traveling back and forth in a hole through the center of the earth:

I posit that the earth is pierced clear through and that we can see through a great hole farther and farther right up to the other end where the antipodes would be if the whole earth were inhabited; I say, first of all, that if we dropped a stone through the hole, it would pass beyond the center of the earth, going straight on toward the other side for a certain limited distance and that then it would turn back going beyond the center on this side of the earth; afterward it would fall back again, going beyond the center, but not so far as before; it would come and go this way several times, but with a reduction of its reflex motions until finally it would rest at the center of the earth. This is caused by the impetuosity or “momentum” which it acquires by the acceleration of its motion.... We can understand this more easily by taking note of something perceptible to the senses: if a heavy object ... is hung on a long string and pushed forward, it begins to move backward and then forward, making several swings, until it finally rests absolutely perpendicular and as near the center as possible.

Source: Nicole Oresme, On the Heavens, II.3.1, in “The Scholastic Pendulum,” by Bert S. Hall, Annals of Science, 35 (1978): 441–462.

may not have been the first scholar to come up with the idea of impetus (motive force), but he is credited with devising the clearest explanation of how it works. Buridan suggested that in throwing a rock, the thrower transfers something to the rock, giving to the rock an impetus that propels it along in its unnatural course until the impetus is gradually used up. The rock then moves according to Aristotle’s concept of natural motion: straight to the ground. Buridan also reasoned that if impetus is not opposed by some other force in this case the gravitas (weight) of the rock, it will maintain its impetus indefinitely, and hence continue to move. Although he did not think of it in the same way, Buridan’s conception of impetus is similar to the modern idea of momentum (the product of mass and velocity). His understanding that motion comes not from an external force, but from some innate or imparted quality of the rock itself, is important because for the first time objects in physics were considered not for their ultimate meaning (the explication of the primum movens), but for their particulars. That is, fourteenth-century natural philosophers began to ask how certain things worked rather than considering how those specifics contributed to a wider understanding of God. They became partially liberated from overarching theories that often inhibited the understanding of particulars, such things as the flight of an arrow or the swing of a pendulum. As Buridan and his followers began to pay attention to actual, not hypothetical, examples, the real world began to enter the study of physics.

Quantitative Physics. Other Parisian scholars, including Buridan’s student Nicole Oresme, began to quantify impetus and to begin to discuss quantitative understandings of terrestrial physics. Continuing to accept Aristotle’s theory that the mechanics controlling objects on earth and the mechanics of the celestial sphere were fundamentally different—an idea contrary to the modern understanding of motion—Oresme nonetheless made significant contributions to the study of mechanics. The use of algebra to create geometric diagrams or graphs was still nearly three centuries away in Europe, so Oresme and his fellow Scholastics related motions, positions, and velocities to each other through ratios. In particular, Oresme was the first scholar to realize that time should be considered as a variable along with position and velocity, and in so doing, he considered acceleration (the rate of change of velocity with respect to time), the most interesting part of motion. Building on Buridan’s idea of impetus, he related acceleration to the gravitas of the object and proposed uniform acceleration as the consequence of gravitas. While his worldview was not as modern as that of Galileo or Sir Isaac Newton, Oresme opened the door for the modern understanding of motion, in which time and acceleration are the fundamental variables.

Optics. Another significant area of investigation in the Middle Ages was the study of the behavior of light rays. The ancient Greeks had investigated how images were altered by lenses and mirrors, and their theories had been reformulated and augmented by the Arabs. Sources from both these traditions came to Europe in the twelfth century and were incorporated into a Christian framework. As one of the foundation elements of Christian theology, light is the instantiation (representation by a physical instance) of God. Just as the cathedrals were built to magnify light (and color), and thus the glory of God, optics was seen as a philosophical way in which to approach God’s truths.

Geometrical Optics. Medieval optics was based on Aristotelian foundations, which explained various perceptions as light interacting with different “exhalations” in the air. According to Aristotle the earth gives off moist and dry exhalations, and light traversing regions with more or less of these substances undergoes certain changes. This imprecise explanation was complemented and concretized by the other branch of ancient Greek optics, geometrical optics, which derives from the work of the father of geometry, Euclid, as well as from Ptolemy. Euclidean optics considers two areas: catoptrics, the study of the reflection of light from matter, usually metals; and dioptics, the study of the transmission of light through crystals, glass, or liquids. Witelo, a thirteenth-century Polish scholar, investigated dioptric relations.

The Arab Contribution. The question of how mankind sees the world was widely discussed in the thirteenth and fourteenth centuries, with scholars basing their investigations

largely on the work of Arabic physicist and physician Alhazen (Abu ’Ali al-Hasan ibn al-Haytham). His theory of physiological optics suggests that the lens is the sensible element in the eye and responds to rays entering the eye from objects in the field of vision. This explanation runs counter to the Platonic idea that vision occurs when the eye sends out rays to objects. By reversing that belief, Alhazen and others suggested that the world is humankind’s to perceive, not to create. From Euclid, Ptolemy, and Alhazen western Europeans formulated the geometry to explain direct vision, reflection, and refraction (the way in which light is bent as it passes from one medium—such as air—to another—such as glass or water).

Sources

Edward Grant, “Jean Buridan and Nicole Oresme on Natural Knowledge,” Vivarium: Journal for Mediaeval Philosophy and the Intellectual Life of the Middle Ages, 31 (1993): 84–105.

Grant, Much Ado about Nothing: Theories of Space and Vacuum from the Middle Ages to the Scientific Revolution (Cambridge & New York: Cambridge University Press, 1981).

Grant, “Scientific Thought in 14th-Century Paris: Jean Buridan and Nicole Oresme” in Machaut’s World: Science and Art in the 14th Century, edited by Madeleine Pelner Cosman and Bruce Chandler (New York: New York Academy of Sciences, 1978), pp. 105–124.

David C. Lindberg, Studies in the History of Medieval Optics (London: Variorum Reprints, 1983).

Lindberg, Theories of Vision from al-Kindi to Kepler (Chicago: University of Chicago Press, 1976).

Anneliese Maier, On the Threshold of Exact Science: Selected Writings of Anneliese Maier on Late Medieval Natural Philosophy, translated by Steven D. Sargent (Philadelphia: University of Pennsylvania Press, 1982).

A. George Molland, “Nicole Oresme and Scientific Progress,” in Antiqui and moderni, edited by Albert Zimmermann (Berlin: de Gruyter, 1974), pp. 206–220.

Nicole Oresme, Nicole Oresme and the Kinetics of Circular Motion, edited and translated by Grant (Madison: University of Wisconsin Press, 1974),

John Peckham, John Pecham and the Science of Optics: Perspectiva Communis, edited and translated by Lindberg (Madison: University of Wisc consin Press, 1970).

J. M. M. H. Thijssen and Jack Zupko, The Metaphysics and Natural Philosophy of John Buridan (Leiden & Boston: Brill, 2001).

James A. Weisheipl, “The Interpretation of Aristotle’s Physics and the Science of Motion,” in The Cambridge History of Later Medieval Philosophy: From the Rediscovery of Aristotle to the Disintegration of Scholasticism, 1100–1600, edited by Norman Kretzmann, Anthony Kenny, Jan Pinborg, and Eleonore Stump (Cambridge 8c New York: Cambridge University Press, 1982), pp. 521–536.