Aristotelian Physics - Life and Death of Aristotelian Physics

Life and Death of Aristotelian Physics

The reign of Aristotelian physics lasted for almost two millennia, and provides the earliest known speculative theories of physics. After the work of Galileo, Descartes, and many others, it became generally accepted that Aristotelian physics was not correct or viable. Despite this, the scholastic science survived well into the seventeenth century, and perhaps even later, until universities amended their curricula.

In Europe, Aristotle's theory was first convincingly discredited by the work of Galileo Galilei. Using a telescope, Galileo observed that the moon was not entirely smooth, but had craters and mountains, contradicting the Aristotelian idea of an incorruptible perfectly smooth moon. Galileo also criticized this notion theoretically – a perfectly smooth moon would reflect light unevenly like a shiny billiard ball, so that the edges of the moon's disk would have a different brightness than the point where a tangent plane reflects sunlight directly to the eye. A rough moon reflects in all directions equally, leading to a disk of approximately equal brightness which is what is observed. Galileo also observed that Jupiter has moons, objects which revolve around a body other than the Earth. He noted the phases of Venus, convincingly demonstrating that Venus, and by implication Mercury, travels around the sun, not the Earth.

According to legend, Galileo dropped balls of various densities from the Tower of Pisa and found that lighter and heavier ones fell at almost the same speed. In fact, he did quantitative experiments with balls rolling down an inclined plane, a form of falling that is slow enough to be measured without advanced instruments.

A heavier body falls faster than a lighter one of the same shape in a dense medium like water, and this led Aristotle to speculate that the rate of falling is proportional to the weight and inversely proportional to the density of the medium. From his experience with objects falling in water, he concluded that water is approximately ten times denser than air. By weighing a volume of compressed air, Galileo showed that this overestimates the density of air by a factor of forty. From his experiments with inclined planes, he concluded that all bodies fall at the same rate neglecting friction.

Galileo also advanced a theoretical argument to support his conclusion. He asked if two bodies of different weights and different rates of fall are tied by a string, does the combined system fall faster because it is now more massive, or does the lighter body in its slower fall hold back the heavier body? The only convincing answer is neither: all the systems fall at the same rate.

Followers of Aristotle were aware that the motion of falling bodies was not uniform, but picked up speed with time. Since time is an abstract quantity, the peripatetics postulated that the speed was proportional to the distance. Galileo established experimentally that the speed is proportional to the time, but he also gave a theoretical argument that the speed could not possibly be proportional to the distance. In modern terms, if the rate of fall is proportional to the distance, the differential equation for the distance y travelled after time t is

with the condition that . Galileo demonstrated that this system would stay at for all time. If a perturbation set the system into motion somehow, the object would pick up speed exponentially in time, not quadratically.

Standing on the surface of the moon in 1971, David Scott famously repeated Galileo's experiment by dropping a feather and a hammer from each hand at the same time. In the absence of a substantial atmosphere, the two objects fell and hit the moon's surface at the same time.

With his law of universal gravitation Isaac Newton was the first to mathematically codify a correct theory of gravity. In this theory, any mass is attracted to any other mass by a force which decreases as the inverse square of their distance. In 1915, Newton's theory was replaced by Albert Einstein's general theory of relativity. See gravity for a much more detailed complete discussion.