Bucket argument
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Isaac Newton's rotating bucket argument attempts to demonstrate that true rotational motion cannot be defined as the relative rotation of the body with respect to the immediately surrounding bodies. It is one of five arguments from the "properties, causes, and effects" of true motion and rest that support his contention that, in general, true motion and rest cannot be defined as special instances of motion or rest relative to other bodies, but instead can be defined only by reference to absolute space.
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[edit] Background
These arguments, and a discussion of the distinctions between absolute and relative time, space, place and motion, appear in a Scholium at the very beginning of his great work, The Mathematical Principles of Natural Philosophy (1687), which established the foundations of classical mechanics and introduced his law of universal gravitation, which yielded the first quantitatively adequate dynamical explanation of planetary motion.
Despite their embrace of the principle of rectilinear inertia and the recognition of the kinematical relativity of apparent motion (which under determines whether the Ptolemaic or the Copernican system is correct), natural philosophers of the seventeenth century continued to consider true motion and rest as genuinely contrary predications of an individual body. The dominant view Newton opposed was devised by René Descartes, and was supported (in part) by Gottfried Leibniz. It held that empty space is a metaphysical impossibility, that space is nothing other than the extension of matter, and that the true motion of a body consists in its transference from the vicinity of bodies immediately surrounding it to the vicinity of other bodies.[citation needed] Descartes recognized that an indicator of true rotational motion is the tendency of the parts of the body to recede from the axis of rotation.
[edit] The argument
Newton discusses a bucket filled with water hung by a cord.[1] If the cord is twisted up tightly on itself and then the bucket is released, it begins to spin rapidly, not only with respect to the experimenter, but also in relation to the water it contains.
Although the relative motion at this stage is the greatest, the surface of the water remains flat, indicating that the parts of the water have no tendency to recede from the axis of relative motion. Eventually, as the cord continues to unwind, the surface of the water assumes a concave shape as it acquires the motion of the bucket spinning relative to the experimenter. This concave shape shows that the water has acquired a centrifugal form despite the fact that the water is at rest relative to the pail, contrary to Descartes' theory.
Having established to his satisfaction that true motion can be understood only in reference to absolute space, Newton remains concerned to address the problem of how it is that we can experimentally determine the true motions of bodies in light of the fact that absolute space is not something that can be perceived. This, he says, can be accomplished by observing the causes and effects of motion as well as the apparent motions of bodies relative to one another, which are the differences of true motions. For example, if two globes, floating in space, connected by a cord that is under tension, with no other clues to assess the situation, measuring the amount of tension in the cord alone suffices to indicate how fast the two objects are revolving around the common center of mass. (This employs an effect of true motion.) Also, the sense of the rotation —whether it is in the clockwise or the counter-clockwise direction— can be discovered by applying forces to opposite faces of the globes and ascertaining whether this leads to an increase or a decrease in the tension of the cord (this employs a cause of true motion). Alternatively, the sense of the rotation can be determined by measuring the apparent motion of the globes with respect to a system background bodies that, according to the preceding methods, have been established not to be in a state of rotation.
[edit] See also
[edit] References
- ^ For Newton's original argument, see Max Born & Günther Leibfried. Einstein's Theory of Relativity. New York: Courier Dover Publications, pp. 78-79. ISBN 0486607690.
[edit] External links
- Newton's Views on Space, Time, and Motion from Stanford Encyclopedia of Philosophy, article by Robert Rynasiewicz. At the end of this article, loss of fine distinctions in the translations as compared to the original Latin text is discussed.
- Life and Philosophy of Leibniz see section on Space, Time and Indiscernibles for Leibniz arguing against the idea of space acting as a causal agent.
