Bucket argument: Difference between revisions

[[Isaac Newton]]'s rotating '''bucket argument''' (also known as '''Newton's bucket''') is a thought experiment that was designed 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 [[argument]]s 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 [[Galilean invariance|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''?"<ref name="Cohen">{{cite book |author=Disalle |first=Robert |url=https://books.google.com/books?id=3wIzvqzfUXkC&dq=centrifugal+Einstein+rotating+globes&pg=PA44 |title=The Cambridge Companion to Newton |publisher=Cambridge University Press |year=2002 |isbn=0-521-65696-6 |editor-last=Cohen |editor-first=I. Bernard |page=43 |editor-last2=Smith |editor-first2=George E.}}</ref> [[General relativity]] dispenses with absolute space and with physics whose cause is external to the system, with the concept of [[Geodesics in general relativity|geodesics]] of [[spacetime]].<ref>{{citation |title=Mach's Principle II |first1=James G. |last1=Gilson |date=September 1, 2004 |arxiv=physics/0409010 |bibcode=2004physics...9010G}}</ref>

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, despite proximity to the pail. 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 is rotating, despite the fact that the water is at rest relative to the pail. In other words, it is not the relative motion of the pail and water that causes concavity of the water, contrary to the idea that motions can only be relative, and that there is no absolute {{nowrap|motion{{hsp}}{{mdash}}{{hsp}}}}a situation which corresponds to diagram D above. Possibly the concavity of the water shows rotation relative to ''something else'': say absolute space? Newton says: "One can find out and measure the true and absolute circular motion of the water".<ref name=Cohen_Smith>{{cite book |author=Robert Disalle |chapter=Newton's philosophical analysis of space and time |editor=I. Bernard Cohen, George Edwin Smith |page=45 |title=''op. cit.'' |date=25 April 2002 |publisher=Cambridge University Press |chapter-url=https://books.google.com/books?id=3wIzvqzfUXkC&pg=PA45 |isbn=0-521-65696-6}}</ref>