Gravitation
by Jacques Léon
February the 17th 1998
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Gravitation |
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by Jacques Léon |
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February the 17th 1998 |
Pour obtenir la version
française cliquez ici
Gravitation has always fascinated human beings since its immaterial nature is somewhat magic. Before the Newtonian revolution, gravitation was considered an intrinsic feature of bodies. According to Aristotle’s philosophy, nature was made of four basic elements referred to as air, fire, water and earth. The very principles that presided to the motion of each of these elements were supposed to be embedded in them. Fire, for instance, supposedly of the same nature as heavenly bodies, was irresistibly attracted by them in order to join them. This explained the upward movement of fire. On the contrary, earth and all the objects made of this element (remember that back to ancient Greece, earth and minerals were the main raw material used by human beings) tended to move downwards, down to the ground. The world as described by Aristotle and his predecessors looked like an onion, i.e. a hierarchical world composed of multiple layers of increasing level of perfection. At the bottom, the Earth, absolute imperfection; at the top, the stars and the heavenly spheres, absolute perfection. In such a vision the Earth necessarily sits in a central position. So the Aristotelian universe is a geocentric one.
Newtonian and classical mechanics have shed a new and rational light on gravitation, including it in the great edifice of the laws of nature. Using a very simple formula, human beings were able to compute the trajectories of the planets as well as artificial satellites thereby penetrating the so-called gods' wills. However, the sound revolutions of XXth century physics didn’t spare the Newtonian vision of gravitation. From the status of universal force it became a mere revelation of the curvature of a supposedly « elastic » space-time and eventually dissolved in the uncertainties and undeterminism of quantum physics. Today, three centuries after the publication of the Philosophae Naturalis Principia Mathematica (Mathematical Principles of Natural Philosophy) by Isaac Newton, it is worth asking ourselves about the intimate nature of gravitation.
I don't pretend to answer this tough question in a few pages. I would rather attempt to describe as simply as possible the key ideas that led the physicists on this hot topic. The plan of this presentation is not original and only follows the course of history. It comprises four parts :
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Send me your comments or questions at jacques.leon@cegetel.fr or leondwek@aol.com
