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Newton's Bucket Experiment RONALD LAYMON IN THISPAPER I WILt ARGUEthat Newton did not intend his bucket experiment to be interpreted as showing either that absolute space exists or that a relativistic mechanics is not possible. The intended function of the bucket experiment is more modest. It is to be used, as Newton clearly states, as an illustration of a case where absolute rotations can be distinguished from relative motions, given Newton's dynamical theory. It can also be used, again as Newton noted (although this time not so clearly), to refute Descartes's system of dynamics. It is not to be used to show, for example, that no set of relative motions (such as, with respect to the stars) could be used as a suitable reference frame for this particular case. A suitable reference frame is one that would provide a set of motions and water surface behavior that are coherent with the then accepted causal notions. I will also challenge the usual analysis of the "logic" of the bucket experiment, which is that Newton showed experimentally that the curvature of the water surface is independent of the relative motion of the water with respect to the sides of the bucket. I will show that Newton's argument is that the experiment demonstrates that the degree of surface curvature is inversely related to this relative motion. Therefore, relative motion with respect to the bucket sides cannot be true motion since true motion even for Descartes is directly proportional to its effects. This clarification of the logic of the bucket experiment (as a refuter) will allow for a more adequate appreciation of the parallels between Newton's early work in dynamics and his Principia. After I develop and substantiate these claims I will briefly consider Mach's criticisms given my reading of the bucket experiment. Finally, I will consider some of Newton's direct arguments for the existence of absolute space. The only purpose of this latter examination will be to contrast these direct arguments with the indirect support that the bucket experiment gives to the existence of absolute space. The thesis, contrary to mine, that Newton held that the bucket experiment does show the existence of absolute space and the untenability of a relativistic mechanics is one of the standard items in the repertoires of many historians and philosophers of science. In close association with this interpretation of Newton is the claim that the bucket experiment is unsuccessful with respect to its purported purpose, that Mach showed why, and that therefore Newton was in error. In a recent paper, for example, Ian Hacking gives this version of Newton's error. In Principia,we are to imagine a universe with nothing in it but a bucket of water that starts to spin. This hypothesis can make no sense to a relativist, yet we know what would happen if it were true. Although the "spin" would not be visible, the water would gradually start to rise up the side of the bucket. Hence, even if there were nothing else in the world, there would be a [399] 400 HISTORY OF PHILOSOPHY difference between spin and rest, and so, said Newton, relativism is refuted. It seems to have been left to Mach... to protest that such an interpretation depends on imposing laws from our universe on to this "abstraction. ''1 Ernest Nagel's influential textbook states the offense this way. Newton's argument was severely criticized by Ernst Mach, who showed that it involved a serious non sequitur. Newton noted quite correctly that the variations in the shape of the surface of the water are not connected with the rotation of the water relative to the sides of the bucket. But he concluded that the deformations of the surface must therefore be attributed to a rotation relative to absolute space. However, this conclusion does not follow from the experimental data and Newton's other assumptions, for there are in fact two alternative ways of interpreting those data: the change in the shape of the water's surface is a consequence either of a rotation relative to absolute space or of a rotation relative to some system of bodies different from the bucket...
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