XII. THE PHYSICS OF THE SUBLUNAR REGION

Note that medieval 'physics' was not necessarily addressed to the same questions and purposes as modern 'physics.'

[ Compare Thomas Kuhn's concept of 'paradigms' and 'paradigm shifts.' Kuhn says that different paradigms are 'incommensurable.']

Matter, Form, and Substance

Material objects as matter + form. 2 aspects to form:

'substantial form' (essential properties)

'accidental form' (accidental properties)

Primary matter: matter, apart from its form. Matter without properties. (This cannot actually exist.)

Corporeal form: invented by later commentators. Gives matter its 3-dimensionality. [This is essential to its being a body.]

Elemental form: 4 elemental forms distinguish the 4 elements from each other. Each elemental form is associated with a combination of the basic qualities (hot vs. cold, and wet vs. dry). E.g., when matter has the hot & dry elemental form, you get the element 'fire'.

Elements can be transmuted, if the matter loses one form and gains another.

Actual matter must have at least elemental form. It can, in addition, get other forms imposed on it, as when a bunch of matter gets formed into a statue. [Note that there are different levels of composition here: the statue composed of marble, which is composed of the 4 elements, which are composed of primary matter. At each level, you have a form imposed on the stuff of the lower level, in order to get it to the higher level.]

Combination and Mixture

Elements get combined into "mixta" (mixtures). Almost all the substances we encounter (wood, granite, etc.) are mixta.

Mixta are not simply one element next to another. Rather, the elements are completely mixed. The new substance has a new form, whose properties are determined by the properties of the component elements & their proportion.

Medievals had debates about the relationship between the elemental forms and the form of the mixtum--

Do the elemental forms still exist in the mixture? Perhaps only potentially? In a weakened form?

Doctrine of minima: The smallest parts of a mixtum that still have the properties of that mixture. Different from atomism:

atomists thought there was void space between atoms

the atoms could not in principle be divided.

all atoms were of the same stuff.

atoms lacked the properties of macroscopic materials.

None of these things is true of minima.

Alchemy

[ Alchemy first began in ancient Egypt. It became popular in the middle ages, as another thing the Europeans got from the Arabs.]

Metals are compounds of sulfur & mercury, formed under the influence of heat inside the earth.

You get more or less 'perfect' metals, depending on the purity of the sulfur & mercury, their proportions, and the degree of heat.

'Noble metals' : the more perfect (& valuable) metals, such as gold, silver, platinum. Less corruptible. Gold is the most perfect metal.

Less perfect metals like iron or copper are 'base' metals.

Main goal of alchemy: to transform base metals into gold.

Interesting substances:

The 'philosopher's stone' : a legendary substance so perfect that it could transform base metals into gold. Alchemists tried to figure out how to make one.

[ Aqua regia : a liquid that dissolves noble metals, particularly gold. (Roger Bacon thought gold dissolved in aqua regia was the elixir of life.) Actually a mixture of hydrochloric & nitric acid.]

Techniques used by alchemists:

Alchemists developed chemical processes:

Solution

Calcination (heating a substance, without melting, in order to drive off its volatile components)

Fusion (melting things; also, using heat to combining 2 substances into one, as in alloys)

Distillation

Putrefaction (decomposition caused by, in modern terms, enzymes & bacteria)

Fermentation (chemical changes caused by enzymes, esp. caused by bacteria, molds, and yeasts; produces alcohol, among other things)

Sublimation (converting a solid directly to a gas)

They produced apparatus for doing these things: furnaces, various kinds of flasks & other containers, and the alembic (apparatus for distilling).

This was the beginning of chemistry later on. However, it also developed into a kind of mystical, worldview (modern day 'alchemy' is a sort of new age phenomenon).

Change and Motion

There's lots of change in the Aristotelian world:

1) Generation & corruption (coming into & going out of existence)

2) Alteration (change of qualities)

3) Augmentation & diminution (change of size, or quantity)

4) Local motion (change of place)

Medievals had a whole different conceptual scheme from classical physics, which is complex and hard to get into. We'll just look at a few parts of it.

The Nature of Motion

First question to ask in any subject: what exists (relevant to that subject)? Then you can ask what properties they have, how they're related to each other, and so on.

In what way does motion exist? 2 views:

"flowing form" view: there's just the object and its successive places.

"flow of a form" view: there's also another thing called the 'motion' of the object.

William of Ockham: held the first view. One way to view this dispute:

"Every motion is produced by a mover.": for Ockham, this really means "Every thing that is moved, is moved by another thing." The substantive 'motion' is like a fictitious entity [sort of like 'the average man'].

[ Important : "Ockham's Razor": Principle enunciated by Ockham, "Entities must not be multiplied beyond necessity." Later became a central principle of scientific method, but with different sorts of applications.]

John Burridan supported the 2^nd view. Argument:

God could make the whole cosmos rotate. This would not be possible on the 'flowing form' view, because there is no succession of different places.

[ A more modern way of formulating the issue: Suppose an object existed for a single moment of time. Could it, at that moment, be in a state of motion (i.e., having an instantaneous velocity)? The modern physical view, where state of motion is defined in terms of a derivative, seems to be "no."]

The Mathematical Description of Motion

Ancients lacked a mathematical description of motion. They saw distance & time as measurable quantities, but not 'velocity'.

Dynamics (about causes of motion) vs. kinematics (just about the nature of motion).

The Merton group: group of scholars from Merton College, Oxford. (1300s)

Developed concepts of

velocity: uniform & non-uniform

acceleration

Discovered theorems about motion.

Foundation of mathematical concept of velocity:

Qualities have intensity. This is a concept of degree. For example, 'hotness': different degrees of temperature.

They also have extension or quantity. This depends on how widely distributed the quality is. Example: concept of the 'quantity of heat' in a body. (Note modern distinction between temperature & heat.)

The same is true of motion, because motion can be viewed as a kind of quality of a body.

'Intensity' of motion = velocity.

Nicole Oresme develops technique for graphing qualities:

A horizontal line represents the subject, with a vertical line representing intensity of the quality. Shape of the graph shows distribution of the quality through the subject.

Total quantity is the area of the graph.

Example: how to represent a heated rod.

Example: motion of a rotating rod.

Example: A body moving as a unit, but nonuniformly.

Subject line becomes time, rather than spatial extension.

Pictures on p. 299 show various kinds of motion. (Note: velocity is a scalar here, not a vector. They didn't have vectors.)

So, quantity of motion is the area of the graph: which is the distance traversed.

This is cool: Makes it possible to apply geometry to claims about motion. Samples:

The 'Merton Rule' or 'mean speed theorem'. If a body accelerates uniformly from v1 to v2, then it covers the same distance as a body travelling at (v1+v2)/2 over the same time. See picture on p. 300.

A second theorem: In a uniformly accelerated motion, the distance a body travels in the 2^nd half of its motion = 3 times the distance it travels in the first half.

This was done as pure mathematics.

Importance: establishes a conceptual framework & the idea of mathematical treatment of motion, which will be used by later thinkers such as Galileo & Newton.

The Dynamics of Local Motion

All motion requires a mover. 2 kinds of motion:

Natural (caused by nature of the body)

Forced (caused by external body)

Problem: In natural motion, the mover does not seem sufficiently distinct from the thing moved.

Problem: How to explain projectile motion. Why does an object keep moving when you throw it.

Aristotle: The object is pushed along by the medium.

Alternative: the thrower endows the object with an internal motive force, or "impetus".

This is distinct from the motion itself.

Impetus is 'corrupted' [or dissipated] by resistance.

This could explain motion of the heavens.

Measured by quantity of matter x velocity

The Quantification of Dynamics

Modern summary of Aristotle's view of falling bodies:

v F/R

Leads to the conclusion that a void is impossible.

Criticized by Philoponus, on empirical grounds. There is very little difference in velocity between heavy & light objects. He decided that, in a vacuum, a body would have a certain natural velocity, determined by weight. But in air, the resistance of the medium slows down heavier bodies more (?), leading to the observed results.

Thomas Bradwardine criticizes alternative accounts on mathematical grounds:

v F/R. Problem: predicts that if F=R, the body still moves.

v F - R. Problem: fails to predict that doubling F and R leaves the velocity unchanged.

Bradwardine's solution: To double the velocity, you have to 'double' the ratio F/R (but this really means squaring it, in modern terms).

[In modern terms: v log (F/R)]

The Science of Optics

Ancient optical theories were of 2 kinds:

Intromission theories: Something is taken into the eye, from the external object. This was held by people more interested in physical/physiological aspects of vision. (Aristotle)

Extramission theories: The eye emits something out towards the objects. Held by people interested in mathematical aspects. (Euclid, Ptolemy)

Alhazen: (Muslim scientist, ~1000 a.d.)

Attacked extramission theories. Arguments:

How could bright objects injure the eye?

Implausible that the eye could emit something filling up all the space up to the fixed stars.

Appropriated the visual cone, but used with an intromissive theory.

Used a concept of incoherent radiation for this purpose. (Radiation emanates from each point on the luminous body, in all directions.)

Each point of the eyeball receives rays from every point on the visible object. Why doesn't this lead to just a jumble in the eye? Answer: only the rays striking the eyeball perpendicularly are seen.

This influenced Western scholars after Alhazen's work was translated, around 1200.

Roger Bacon took on Alhazen's theory.

Attempted to reconcile intromission & extramission theories, by proposing that extramitted rays prepare the medium to receive the rays from the objects.