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Aristotelian physics
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The Greek philosopher Aristotle (384 BC – 322 BC) developed many theories on the nature of physics. These involved what Aristotle described as the four elements, as well as a variety of other principles that differ significantly from modern ideas about the laws of physics. He spoke intimately of the relation between these elements, of their dynamics, how they impacted on the earth, and how they were, in many cases, attracted to each other by unspecified forces.
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The Greek philosopher Aristotle (384 BC – 322 BC) developed many theories on the nature of physics. These involved what Aristotle described as the four elements, as well as a variety of other principles that differ significantly from modern ideas about the laws of physics. He spoke intimately of the relation between these elements, of their dynamics, how they impacted on the earth, and how they were, in many cases, attracted to each other by unspecified forces. He was also responsible for a related theory of moving energy.
Aristotelian physics
Aristotle taught that the elements that composed the Earth were different from those that made up the heavens and Outer space.
He also taught that dynamics were mostly determined by the characteristics and nature of the substances that the object that was moving was composed of.
Elements
Aristotle believed that there were four main elements or compounds that made up the Earth: earth, air, water and fire.
He also held that all the heavens, and every particle of matter in the universe, was formed out of another, fifth element he called 'aether' (also transliterated as "ether"), which was supposedly weightless and "incorruptible". Aether was also known by the name 'quintessence' - literally, "fifth substance".
Heavy substances such as iron and metals were considered to be primarily consisting of the "element" of earth, with a limited amount of matter from the other elements. Other, less heavy and/or dense objects were thought to be less earthy, and composed of a greater mixture of the other elements.
Humans were shaped with all of the substances, with the exception of aether, but the relative proportion of elements was unique to each person, and there was no standard amount of each within the human body.
Dynamics
Aristotle held that each of the four worldly elements would each seek each other and cluster together, and that this seeking of other similar elements would have to be hindered to be stopped, as it was as natural as two magnets rejecting each other, or rain falling from the clouds. For instance, because smoke was mainly air, it would rise to meet the air in the sky. He also taught that objects and matter could only move so long as a form of energy was forcing it in a given direction. Therefore, if all the applied forces on Earth were removed, such as the throwing of a rock, then nothing could move. This idea had flaws that were visible even at the time the concept was formulated. Many people questioned how an object such as an arrow could continue to move forward after it had left the power released by the string, and continue to sail forward. Aristotle proposed an idea that arrows, etc., created a type of vacuum behind them that forced them forward., which was consistent with his interpretation of motion as an interaction of the moving object and the medium through which it moves. Since the turbulent motion of air around an arrow is very complicated, and still not fully understood, any discrepancy with observation could be swept under the rug.
Because Aristotle placed the medium at the center of his theory of motion, he could not make sense of the notion of a void central to the atomic theory of Democritus. A void is a place free of anything, and since Aristotle asserted that motion required a medium, he came to the conclusion that a void was an incomprehensible idea. Aristotle believed that the motion of an object is inversely proportional to the density of the medium. the more tenuous a medium is, the faster the motion. If an object is moving in void, Aristotle believed that it would be moving infinitely fast, so that matter would instantly fill up any void at the moment it formed.
Gravity
The Aristotelian theory of gravity was a theory that stated that all bodies move towards their natural place. For some objects, Aristotle claimed the natural place to be the center of the earth, wherefore they fall towards it. For other objects, the natural place is the heavenly spheres, wherefore gases, steam for example, move away from the centre of the earth and towards heaven and to the moon. The speed of this motion was thought to be proportional to the mass of the object.
Medieval criticisms
During the Middle Ages, the Aristotelian theory of gravity was first criticized and/or modified by John Philoponus and later by Muslim physicists. Ja'far Muhammad ibn Musa ibn Shakir (800-873) of the Banu Musa wrote the Astral Motion and The Force of Attraction, where he discovered that there was a force of attraction between heavenly bodies, foreshadowing Newton's law of universal gravitation.
Ibn al-Haytham (965-1039) also discussed the theory of attraction between masses, and it seems that he was aware of the magnitude of acceleration due to gravity and he discovered that the heavenly bodies "were accountable to the laws of physics". Abu Rayhan al-Biruni (973-1048) was the first to realize that acceleration is connected with non-uniform motion, part of Newton's second law of motion. During his debate with Avicenna, al-Biruni also criticized the Aristotelian theory of gravity for denying the existence of levity or gravity in the celestial spheres and for its notion of circular motion being an innate property of the heavenly bodies.
In 1121, al-Khazini, in The Book of the Balance of Wisdom, proposed that the gravity and gravitational potential energy of a body varies depending on its distance from the centre of the Earth. Hibat Allah Abu'l-Barakat al-Baghdaadi (1080-1165) wrote a critique of Aristotelian physics entitled al-Mu'tabar, where he negated Aristotle's idea that a constant force produces uniform motion, as he realized that a force applied continuously produces acceleration, a fundamental law of classical mechanics and an early foreshadowing of Newton's second law of motion. Like Newton, he described acceleration as the rate of change of velocity.
In the 14th century, Jean Buridan developed the theory of impetus, based on Avicenna's theory of mayl and the work of John Philoponus, as an alternative to the Aristotelian theory of motion. The theory of impetus was a precursor to the concepts of inertia and momentum in classical mechanics.
In the 16th century, al-Birjandi discussed the possibility of the Earth's rotation. In his analysis of what might occur if the Earth were rotating, he developed a hypothesis similar to Galileo Galilei's notion of "circular inertia", which he described in the following observational test:
Life and death of Aristotelian physics
The reign of Aristotelian notions of physics lasted for almost two millennia, and provide the earliest known speculative theories of physics. After the work of Alhacen, Avicenna, Avempace, al-Baghdadi, Jean Buridan, Galileo, Descartes, Isaac Newton, and many others, it became generally accepted that Aristotelian physics were not correct or viable.
Despite this, Aristotle's physics were able to live into the late seventeenth century, and perhaps longer, as they were still taught in universities at the time. Aristotle's model of physics was the main academic impediment for the creation of the science of physics long after Aristotle's death.
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.
Since Aristotle did not believe that motion could be described without a surrounding medium, he couldn't treat air resistance as a complicating factor. 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 mass 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 masses 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 .
On the surface of the moon, 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 modified, but not invalidated, by Albert Einstein, who developed a new picture of gravitation, in the framework of his general theory of relativity. See gravity for a much more detailed complete discussion.
See also
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