Bucket Argument - Background

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 Newton's 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. See the Principia on line at Andrew Motte Translation pp. 77–82.

Despite their embrace of the principle of rectilinear inertia and the recognition of the kinematical relativity of apparent motion (which underlies whether the Ptolemaic or the Copernican system is correct), natural philosophers of the seventeenth century continued to consider true motion and rest as physically separate descriptors 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 because space is nothing other than the extension of matter, or, in other words, that when one speaks of the space between things one is actually making reference to the relationship that exists between those things and not to some entity that stands between them. Concordant with the above understanding, any assertion about the motion of a body boils down to a description over time in which the body under consideration is at t₁ found in the vicinity of one group of "landmark" bodies and at some t₂ is found in the vicinity of some other "landmark" body or bodies.

Descartes recognized that there would be a real difference, however, between a situation in which a body with movable parts and originally at rest with respect to a surrounding ring was itself accelerated to a certain angular velocity with respect to the ring, and another situation in which the surrounding ring was given a contrary acceleration with respect to the central object. With sole regard to the central object and the surrounding ring, the motions would be indistinguishable from each other assuming that both the central object and the surrounding ring were absolutely rigid objects. However, if neither the central object nor the surrounding ring were absolutely rigid then the parts of one or both of them would tend to fly out from the axis of rotation.

Here is an everyday experience of the basic nature of the Descartes experiment: Consider sitting in your train and noticing a train originally at rest beside you in the railway station pulling away. Initially you think it is your own train accelerating, but then notice with surprise that you feel no force. Thus, it is not your own train moving, but the neighboring train. On the other hand, you would confirm your own train is accelerating if you sensed g-forces from the acceleration of your own train.

For contingent reasons having to do with the Inquisition, Descartes spoke of motion as both absolute and relative. However, his real position was that motion is absolute.

A contrasting position was taken by Ernst Mach, who contended that all motion was relative.

When, accordingly, we say that a body preserves unchanged its direction and velocity in space, our assertion is nothing more or less than an abbreviated reference to the entire universe.

— Ernst Mach; as quoted by Ciufolini and Wheeler: Gravitation and Inertia, p. 387