# Aristotelian theory of gravity: Wikis

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The famous philosopher Aristotle, depicted in a painting by Rembrandt Harmensz. van Rijn.

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. 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.

The main principles of Aristotle's physics are:

1. natural places: each element would like to be at a different position relative to the center of the Earth, which is also the center of the universe.
2. gravity/levity: to achieve this position, objects feel a downward or upward force.
3. rectilinear motion: motion in response to this force is in a straight line at constant speed.
4. velocity density relation: the speed is inversely proportional to the density of the medium.
5. vacuum is impossible: Motion in a vacuum is infinitely fast.
6. all pervading ether: all points in space are filled with matter.
7. infinite universe: space cannot have a boundary.
8. continuum theory: Between atoms is vacuum, so matter cannot be atomic.
9. quintessence: Objects above the Earth are not made of Earthly matter.
10. incorruptible and eternal cosmos: Sun/Planets are perfect spheres, and do not change.
11. circular motion: Planets move in perfect circular motion.

Aristotle's principles are not correct in any approximation, and they do not accurately describe anything in our universe. Contemporaries of Aristotle like Democritus, Aristarchus and Archimedes, rejected these principles in favor of atomism and heliocentrism, but their ideas were not widely accepted. Aristotle's principles were difficult to disprove merely through casual everyday observation, but later development of the scientific method challenged his views with experiments, careful measurement, and more advanced technology such as the telescope and vacuum pump.

## Aristotelian physics

Aristotle taught that the elements that composed the Earth were different from those that made up the heavens and Outer space.[1] 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.[1]

### Elements

Aristotle believed that there were four main elements or compounds that made up the Earth: earth, air, water and fire.[a][2] 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")[2], which was supposedly weightless and "incorruptible".[2] Aether was also known by the name 'quintessence' - literally, "fifth substance".[3]

Page taken from the 1837 edition of Aristotle's Physics, a book written about a variety of subjects including philosophy and physics

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.[3] 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.[3]

### 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.[1] Therefore, if all the applied forces on Earth were removed, such as the throwing of a rock, then nothing could move.[1] 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.[1], 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.[4]

### Gravity

The Aristotelian theory of gravity was that stated that all bodies move toward their natural place. For some objects, Aristotle claimed the natural place to be the center of the earth, and thus they fall toward it. For other objects, the natural place is the heavenly spheres, and as such, gases--steam for example--move away from the center of the earth and toward 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 Mūsā ibn Shākir (800-873) of the Banū Mūsā wrote the Astral Motion and The Force of Attraction, where he discovered that there was a force of attraction between heavenly bodies,[5] foreshadowing Newton's law of universal gravitation.[6]

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".[7] Abū Rayhān al-Bīrūnī (973-1048) was the first to realize that acceleration is connected with non-uniform motion, part of Newton's second law of motion.[8] 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.[9]

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.[10] 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.[11] Like Newton, he described acceleration as the rate of change of velocity.[12]

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",[13] which he described in the following observational test:

"The small or large rock will fall to the Earth along the path of a line that is perpendicular to the plane (sath) of the horizon; this is witnessed by experience (tajriba). And this perpendicular is away from the tangent point of the Earth’s sphere and the plane of the perceived (hissi) horizon. This point moves with the motion of the Earth and thus there will be no difference in place of fall of the two rocks."[14]

## 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.[3] 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 modern 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 [15]. 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[16]. 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[15].

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

${dy\over dt} = y$

with the condition that y(0) = 0. Galileo demonstrated that this system would stay at y = 0 for all time. If a perturbation set the system into motion somehow, the object would pick up speed exponentially in time, not quadratically [16].

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 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.

## Notes

a ^  The term 'earth' refers to a pure element that Aristotle theorized, not the actual planet Earth, which is known by modern science to be composed of a large number of chemical elements. The same applies to the other terminologies used. 'Air' refers to a pure element of air, opposed to the air that is found in the Earth's atmosphere, which is also made up of many chemical elements.

## Notes

1. ^ a b c d e
2. ^ a b c "www.hep.fsu.edu" (PDF). Retrieved 26 March 2007.
3. ^ a b c d "Aristotle's physics". Retrieved 6 April 2009.
4. ^ Land, Helen The Order of Nature in Aristotle's Physics: Place and the Elements (1998)
5. ^ K. A. Waheed (1978). Islam and The Origins of Modern Science, p. 27. Islamic Publication Ltd., Lahore.
6. ^ Robert Briffault (1938). The Making of Humanity, p. 191.
7. ^ Duhem, Pierre (1908, 1969). To Save the Phenomena: An Essay on the Idea of Physical theory from Plato to Galileo, p. 28. University of Chicago Press, Chicago.
8. ^  .
9. ^ Rafik Berjak and Muzaffar Iqbal, "Ibn Sina--Al-Biruni correspondence", Islam & Science, June 2003.
10. ^ Mariam Rozhanskaya and I. S. Levinova (1996), "Statics", in Roshdi Rashed, ed., Encyclopedia of the History of Arabic Science, Vol. 2, p. 614-642 [621-622]. Routledge, London and New York.
11. ^ Shlomo Pines (1970). "Abu'l-Barakāt al-Baghdādī , Hibat Allah". Dictionary of Scientific Biography. 1. New York: Charles Scribner's Sons. pp. 26–28. ISBN 0684101149.
(cf. Abel B. Franco (October 2003). "Avempace, Projectile Motion, and Impetus Theory", Journal of the History of Ideas 64 (4), p. 521-546 [528].)
12. ^ A. C. Crombie, Augustine to Galileo 2, p. 67.
13. ^ (Ragep 2001b, pp. 63-4)
14. ^ (Ragep 2001a, pp. 152-3)
15. ^ a b Galileo Galilei, Dialogue Concerning the Two Chief World Systems.
16. ^ a b Galileo Galilei, Two New Sciences.

## References

• Ragep, F. Jamil (2001a), "Tusi and Copernicus: The Earth's Motion in Context", Science in Context (Cambridge University Press) 14 (1-2): 145–163
• Ragep, F. Jamil (2001b), "Freeing Astronomy from Philosophy: An Aspect of Islamic Influence on Science", Osiris, 2nd Series 16 (Science in Theistic Contexts: Cognitive Dimensions): 49-64 & 66-71
• H. Carteron (1965) "Does Aristotle Have a Mechanics?" in Articles on Aristotle 1. Science eds. Jonathan Barnes, Malcolm Schofield, Richard Sorabji (London: General Duckworth and Company Limited), 161-174.