Medieval Physics: Transition and Stagnation

Responsibility for the growth of the science of physics moved from Greek physicists to Middle Eastern physicists and Western European philosophers. Greek culture stagnated and intelligent thought began to shift to the Middle East. Western Europe contributed little significant thought to the field of physics before the Scientific Revolution later on, as the West became consumed with religious thought and Scholasticism. This post will cover the scientific contributions of Middle Eastern scientist and polymath Alhazen, as well as the ideas of Western European philosophers and theologians.

Alhazen is arguably the greatest of all of the medieval scientists, at least of the Arabic scientists. Alhazen was often called “Ptolemy the Second,” or, in medieval Western Europe, “The Physicist.” He is most well-known for his contributions to the science of optics, along with physical science and the further development of the scientific method. His most well-known and greatest work is his seven-volume Book of Optics. It had a great influence on scientists for a good time to come, including Johannes Kepler.

The most important and influential theory this work dealt with was the intromission theory of vision, which was also suggested by Aristotle. The dominant theory, however, was the extramission, or emission, theory, purported by Ptolemy, and by Euclid in his Optica. The emission theory of vision was that vision was caused by light emitted from the eyes. Alhazen, however, suggested with experimental evidence in his Book of Optics that vision was actually caused by rays of light entering the eyes, or the intromission theory which is dominant today. Today, of course, we know that light is transmitted through photons which are emitted by a light source, reflected by visible objects, and picked up by a detector like the eye. His argument against emission theory was that rays from our eyes could not reach distant stars the moment we open our eyes, and therefore we wouldn’t be able to immediately see them like we can. Alhazen also pointed out that eyes could be dazzled by looking directly at a very bright light, which likely wouldn’t happen if the eyes emitted rays in order to see.

Not only did Alhazen also prove that rays of light travel in straight lines, he explained the effect of a pinhole camera. This is the effect where an entire image is projected through an aperture, rather than just what can be seen by looking directly through the aperture. Alhazen showed this by being the first to project an entire outdoor image to a screen indoors using a “camera obscura,” which is a room or box with a hole in one side. This aperture allows the light of the surroundings to pass through and be projected onto a screen on the inside.

Alhazen also contributed significantly to the development of the modern scientific method, because he relied on experimentation and controlled testing to explore his scientific inquiries. He was also one of the first scientists to apply mathematics to his theories and experiments. This was a critical inclusion, as it shaped the logical direction of modern science for centuries to come.

Western philosophers became aware again of the works of the Ancients through translations from Arabic to Latin. Earlier, Greek works had been translated to Arabic. This translation allowed the proliferation of Ancient thought throughout the Western world. Ancient philosophy which did not conflict with Christian theology was once again relevant and widely discussed. Where Ancient philosophy did not conflict with Christian theology, the two were reconciled to create the Scholastic school. Especially important to Western European scholasticism were the ideas of Aristotle, which formed the basis of Western philosophy for centuries to come. This embracement of the ideas of Aristotle over Plato marked a revolution in the ideas of Christianity, because, prior to Scholastics such as Saint Thomas Aquinas, Plato was the favored Ancient Greek philosopher of Christianity.

An important medieval physical theory was the theory of impetus. The theory of impetus was based on Aristotelian dynamics, and was introduced in the sixth century by John Philoponus. It was a precursor to modern knowledge of inertia. Initially, it attempted to explain motion against gravity. Philoponus’s theory was that an object can gain forced motion through violent action but that this impressed motion is only temporary and that “natural motion” (gravity) will again take hold. In the fourteenth century, Jean Buridan gave the name impetus to the theory. He also gave the theory the mathematical formula impetus = weight x velocity. His theory was actually very similar to modern ideas of momentum. Buridan made sure to apply his theories to Christianity, as he considered his idea an extension of Aristotle’s theories but was dissatisfied with their lack of Christian ideology. During the Renaissance and the later Scientific Revolution, experimentation would come to form modern ideas of inertia. It is important to note that Asian philosophers BCE formed very accurate ideas of inertia long before any Western philosophers or scientists did. For example, the Mojing stated that “The cessation of motion is due to the opposing force…If there is no opposing force…the motion will never stop. This is as true as that an ox is not a horse.”

John Philoponus

The medieval era was one of slowed intellectual progress, especially in the sciences. However, following the medieval era many of the greatest intellectual minds came to fruition in the Renaissance. These included Galileo, Copernicus, and Kepler. In the next post, the ideas of these scientists will be discussed. Finally, the ideas of Isaac Newton, the last great Renaissance scientist, will be discussed, concluding our journey from the Greeks to the Renaissance.

For a list of sources, or for further reading, see the sources page.

Archimedes and Ptolemy: Two of the Last Great Greeks

As we near the end of the most significant era of Ancient Greek innovation, there are at least two more Greek philosophers who must be discussed. Archimedes and Ptolemy made a huge impact on the world for centuries to come. Many of Archimedes’ ideas, principles, and inventions are still in use today. Ptolemy did not make many lasting contributions to science, but did make one significant contribution (which he is most known for) which shaped ideas of astronomy for centuries.

Archimedes (287 BC – 212 BC) was a Greek from Syracuse, Sicily. He is widely known for his contributions to physics and engineering, along with his additions to the fields of astronomy and mathematics. He is understood to be one of the greatest mathematicians of all time, and many consider him to be the first scientist. Archimedes was important in the hydrostatics branch of physics, and also created many machines and explained the principle of the lever.

Arguably Archimedes’ most lasting invention is what is today known as the Archimedes screw. Also known as a screwpump, it is used to transfer water from low-lying bodies to irrigation ditches. It is argued that other unknown Greek engineers may have invented it and it was just attributed to Archimedes. However, the story is that Archimedes was contracted by King Hiero II to design the largest ship in classical antiquity for Syracuse. Since the ship was so large and would begin to leak water through the hull, Archimedes supposedly developed the screw to remove this water. The Archimedes screw consists of a spiral around a center shaft encased in a hollow pipe. The screw is operated either manually or by a windmill. The lower part of the spiral picks up some of the water in the body of water it is inserted into, and the turning of the screw carries it up to the top of the screw, where it drops out into the irrigation ditch. Some have even said that this invention was used to irrigate the Hanging Gardens of Babylon. The screw is still used today in various machines, in its original form and in some variants. Also, using the screw in reverse by pouring water into it can power an electrical generator.

Archimedes has created other, more popularly-known inventions. For example, it is said (controversially) that he aligned an array of mirrors on a beach in such a manner as to combust the tar on the decks of enemy ships, burning them. However, the truth of this account is disputed due to the fact that the beach was facing a direction which would not have been optimal for this sort of feat, and that later attempts to recreate it have failed. Another invention of his was the Claw of Archimedes, which he designed in order to defend his home town of Syracuse. The Claw was a large crane with a grappling hook which would be dropped on a boat and then swung upwards, lifting it out of the water or sinking it.

The lever was explained much more thoroughly by Archimedes than any before him. His “Law of the Lever” is that “Magnitudes are in equilibrium at distances reciprocally proportional to their weights.” He is famous for stating “Give me a place to stand on, and I will move the Earth.”

One of his greatest contributions to physics was Archimedes’ principle. Upon his formulation of it, he is said to have shouted “Eureka!”, or “I have found it!”, immortalizing the phrase. The principle is that “Any object, wholly or partially immersed in a fluid, is buoyed up by a force equal to the weight of the fluid displaced by the object. In mathematical terms, density/density of fluid = weight/weight of displaced fluid. With regard to floating objects, Archimedes stated that “Any floating object displaces its own weight of fluid.” It is said that Archimedes used his principle to determine if solid gold was more dense than a golden crown to be given to King Hiero II, but this story, too, is somewhat dubious due to the fact that Archimedes would probably have had to measure the displaced water extremely accurately.

There is much more that could be said about Archimedes, but it is also important to discuss Ptolemy. Ptolemy (90-168) is most known for his treatise known today as the Almagest. This treatise contains the great body of his theories which continued to survive for centuries afterwards. It was his Almagest which contained the geocentric Ptolemaic system of the cosmos. According to Ptolemy, the “celestial realm” is a sphere, similar to that described by Aristotle. It moves as a sphere, and the Earth is a sphere. The Earth is at the center of the cosmos and does not move. In relation to other stars, based on distance, the Earth has no appreciable size and must be treated as a geometric point. The order of the solar system was: Earth, the Moon, Mercury, Venus, the Sun, Mars, Jupiter, Saturn, and then the sphere of fixed and unmoving stars. This was followed by the sphere of the “prime mover” described by Aristotle. Each planet is moved by a system of its deferent sphere and its epicycles. The epicycles required an eccentric deferent and an equant point. The Ptolemaic system was widely accepted for centuries until Copernicus’ heliocentric model came to be more dominant.

In the next post, we will consider the contributions of medieval physicists, including Arabic scientists and the idea of impetus. These laid the foundations for the modern ideas of the Scientific Revolution which began towards the end of the Renaissance era and included Kepler, Copernicus, Galileo, Bacon, Newton, Leibniz, Descartes, and others.

See sources page for a list of sources.

Natural Philosophy: Aristotle

Aristotle (384-322 BC) established the philosophical basis of physics with his “natural philosophy,” and is also considered one of the greatest philosophers in history.

“In all things of nature there is something of the marvelous.” – Aristotle

As such, much of his work in physics is speculative but offers a great deal of insight. He did contribute real research to several areas of science, and had incredible foresight for an intellectual from the age of classical antiquity.

Significantly, Aristotle espoused a certain belief in inductive reasoning which was not found in his more deductive teacher, Plato. Therefore, Aristotle’s philosophical method at least more closely resembled the current scientific method that did his teacher’s. According to Aristotle, he studied phenomena which were caused by “particular,” which was then a reflection of the “universal,” or the set of physical laws. Aristotle also described “science” as “… either practical, poetical, or theoretical.”

One of the many fields to which Aristotle contributed was the field which he called “natural philosophy.” He regarded “natural philosophy” as a “theoretical” science. Aristotle devoted most of his life to the natural sciences, contributing original research to physics, astronomy, chemistry, zoology, etc. Aristotle expressed an early teleological belief in saying that natural things tend to certain goals or ends. Teleology is the philosophical belief that there are certain final causes in nature. According to Istvan Bodnar, in The Stanford Encyclopedia of Philosophy, “Nature, according to Aristotle, is an inner principle of change and being at rest (Physics 2.1, 192b20–23). This means that when an entity moves or is at rest according to its nature reference to its nature may serve as an explanation of the event.” Essentially, Aristotle believed that reference to the innate qualities of an object (whether it is at rest or in motion naturally) would assist in determining what caused an event. Of course, we now know that objects are set in motion when acted upon by a net force and tend to stay in motion until another net force acts upon it. However, Aristotle made an important point with this idea: that forces acting upon objects can either set them in motion or make them tend towards rest.

One of Aristotle’s more famous ideas of natural philosophy is his addition of the celestial “aether” to the four natural elements suggested by Empedocles. The “aether” is, according to Aristotle, the “greater and lesser lights of heaven.” By this, Aristotle meant the stars of the universe which were visible to him in the night sky. The other four natural elements (fire, earth, air, and water) are able to change and mix, according to Aristotle. This is an early precursor to modern ideas of phase transition. These elements are capable of “generation and destruction,” as opposed to the aether, which is unchanging. Aristotle concludes that these bodies cannot be composed of the four elements, because they are not capable of change. Fire, earth, air, and water are terrestrial elements while aether is a celestial element. The most important point is that Aristotle redefined natural elements to include early ideas of phase transition.

Aristotle also made an important attempt to explain gravity. His theory was that all bodies move toward their “natural place.” Natural places are also based largely on composition (of the natural elements). For example, since many commonplace liquids are composed at least partially of water, they move towards sea-level, where the oceans are, or down towards the ground, where ground water can be found. Or, since smoke is air-like, it moves up into the atmosphere where air is. This was the way in which Aristotle described general motion. Aristotle also believed that vacuums did not exist, but that if they did, terrestrial motion in a vacuum would be infinitely fast.

Aristotle described celestial motion in terms of crystal spheres, which carried the sun, moon, and stars in unchanging endless circular motion. In Metaphysics, Aristotle says “that there must be an immortal, unchanging being, ultimately responsible for all wholeness and orderliness in the sensible world. And he is able … to discover a good deal about that being…” This is his concept of the “unmoved mover,” which is capable of moving other things without being moved. An “unmoved mover” is reminiscent of the modern idea of gravity, which is not actually a physical object but does cause motion without any other motion being necessary.

Highly significant was Aristotle’s argument that the Earth was actually spherical. He believed that the Earth was a small sphere. Since he could see stars in Egypt and Cyprus which he could not see further north, he concluded that this was because the Earth is a sphere and is small because such a significant change in the sky would not happen unless on a small sphere. Based on his theory that the earth element tends towards a center, just as all water heads down to seal level towards the concentrically spherical ocean and all air tends to move upwards to form a concentrically spherical atmosphere, he theorized that the Earth was a sphere. Further evidence Aristotle used to support his round Earth theory was that the shadow the Earth imposes on the Moon during a lunar eclipse is round.

Clearly, Aristotle made some significant contributions to the field of physics. He made several further contributions to physics, cosmology, and astronomy. Aristotle defined the scope with which Western culture would observe nature and theorize about physics for centuries. There would have been no Newtonian physics without the strides made by Aristotle. There are several more contributions to physics which could be covered here, but due to limited space, they must be discounted for now. In the next post, the ideas of Archimedes will be discussed, perhaps along with those of some later Greeks.

For further reading, see the sources page.