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Isaac Newton's rotating spheres argument attempts to demonstrate that true rotational motion can be defined by observing the tension in the string joining two identical spheres. The basis of the argument is that all observers make two observations: the tension in the string joining the bodies (which is the same for all observers) and the rate of rotation of the spheres (which is different for observers with differing rates of rotation). Only for the truly non-rotating observer will the tension in the string be explained using only the observed rate of rotation. For all other observers a "correction" is required (a centrifugal force) that accounts for the tension calculated being different from the one expected using the observed rate of rotation.〔 See and 〕 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. Alternatively, these experiments provide an operational definition of what is meant by "absolute rotation", and do not pretend to address the question of "rotation relative to ''what''?". ==Background== Newton was 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. Such determination, he says, can be accomplished by observing the causes of motion (that is, ''forces'') and not simply the apparent motions of bodies relative to one another (as in the bucket argument). As an example where causes can be observed, if two globes, floating in space, are connected by a cord, measuring the amount of tension in the cord, with no other clues to assess the situation, alone suffices to indicate how fast the two objects are revolving around the common center of mass. (This experiment involves observation of a force, the tension). 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 (again involving a force). Alternatively, the sense of the rotation can be determined by measuring the apparent motion of the globes with respect to a background system of bodies that, according to the preceding methods, have been established already as not in a state of rotation, as an example from Newton's time, the fixed stars. In the 1846 Andrew Motte translation of Newton's words:〔See the ''Principia'' on line at (【引用サイトリンク】title=Definitions )〕 To summarize this proposal, here is a quote from Born: Mach took some issue with the argument, pointing out that the rotating sphere experiment could never be done in an ''empty'' universe, where possibly Newton's laws do not apply, so the experiment really only shows what happens when the spheres rotate in ''our'' universe, and therefore, for example, may indicate only rotation relative to the entire mass of the universe.〔 An interpretation that avoids this conflict is to say that the rotating spheres experiment does not really define rotation ''relative'' to anything in particular (for example, absolute space or fixed stars); rather the experiment is an operational definition of what is meant by the motion called ''absolute rotation''.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「rotating spheres」の詳細全文を読む スポンサード リンク
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