Criticism of Newton’s Alleged Proof of Absolute Motion

Abstract

After referring to Newton’s alleged proof of absolute space by the rotating bucket experiment and after quoting from Newton’s Principia the description of the experiment with two spheres tied by a string and revolving around the common center of gravity, Boscovich made the following comment:

On this Newton’s method I remarked that it appears suspect to me and not appropriate to achieve its purpose. I pass over the fact that even here the existence of the inertial forces as well as the way by which the motions are produced by them must be assumed; about it below. The very tension of the string can be defined only by the distance of the spheres, but we cannot measure this distance with certainty unless we assume that it remains unaffected by the translatory motion, the assumption which in that immense void is risky or at least doubtful. But what above all makes everything useless, this method as well as the previous one and any other one, is the fact that if the parallel and equal motions in the same plane are communicated to us and to those bodies, all the motions mentioned above would remain altogether the same according to the accepted principles of Mechanics; also the application of forces by us would produce altogether the same motions described above. Indeed, if already some common motion in a certain plane were present, the absolute motion, composed of this common motion and the motion mentioned above which we detect in that experiment, would be different from that mentioned above. Also the part [of the revolving motion] which we regard as moving backwards, could really move forwards and vice versa. If this common motion is truly faster than the respective circular motion in that plane in the same plane, then these two bodies will not be in absolute motion in that plane as the experiments indicate. If we consider two points of the diameter perpendicular to the direction of the common motion where in one of these points the circular motion is added to the common motion, in other point it is opposed to it, while still being overcome by it, the direction of absolute motion thus in both points would agree with the direction of the common motion. From this it seems to me absolutely evident that absolute motion cannot be differentiated in any way from the relative one.