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Ernst Mach

When qui(c modern authors let themsclves be led asiray by the Ncwto-nian argument* which arc dcrivcd frorn the bucket ot water, to distinguish bctwccn rclativc and absolutc motion, they do not reflect that the system of the world is only givcn oncc to u*, and the Ptolcmaic or Copernican vicw is our interpretation, but both arc cqually actual. Try to hx Ncwto_n’s bucket and rotatc the hcavcn of fixcd stars and then provc the abscncc of i ccntrifugal forccs.^M

Newton's cxpcrimcnt with the rotating vcsscl of water simply informs us that the rclative rotation of the water with rcspcct to the sides of the vcsscl produccs no noticcablc ccntrifugal forces, but that such forccs arc produced by its relativc rotation with rcspcct to the mass of the carth and the other cclcstial bodics. No one is compctcnt to say how the cxpcriment would rurn out if the sides of the vcsscl incrcascd in thickncss and mass till they wcrc ultimatcly scveral leagues thick. 1 be one cxpcrimcnt only lics before us, and our business is, to bring it into accord with the other facts known to us, and not with the arbitrary fictions of our imagination.*⁶

Rclatively, not considcring the unknown and ncglcctcd medium of spacc, the motions of the uniserse arc the same whether we adopt the Pcolcmaic or the Copernican modę of vicw. Roth v:cws arc, indced, equally correct, only the lattcr is morę simplc and morę practtcaiy^>

Newtons bucket cxpcrimcnt and Mach’s criticisms of it havc re-maincd of interest to physicists today. Thcrc havc also been a number of attacks on Machs stand. His defense, for cxamplc, of the Ptolcmaic approach as “cqually correct” has run afoul of the fact that the stars would have to cxcced the speed of light if under our present under-standing they would be construcd as revolving around the carth. Equally telling in this age of spacc travcl is the likelihood that obscrva-tion from other plaucts would much morę casily fit the Copernican theory than the Ptolcmaic one®⁷

The plausibility of Machs criticism was contingent on accepting his phenomenalism, his definition of physical spacc and timc as “rclations," and on overlooking dcfccts in his causal theory and failure to define what hc mcant by “rclations.”

Even if we allow that the stars rcvolvc around a stationary bucket, and not the opposite, it is still not elear how the stars can cxcrcise any causal influence on the behavior of the bucket or the water inside it. The stars arc too far away to act as cithcr a contact or as a gravita-tional or magnetie causc. Mach’s solution of coursc was to rcducc what hc mcant by a causc to a merc observablc relational constancy. For most pcople thcrc arc many “constant rclations” which in no sense are causes and which at best arc mcrcly “incidcntally co-prcscnt,” but

Mach’s phcnomenalism macie no allowance for noncausal constant rclations.⁰* Whcn Mach claimcd that all scnsations wcrc “rclativc” he meanr that somc kind of relation could bc found bctwccn all scnsations such that no scnsation was cntircly “unrclatcd” or “absolute." The only “constant relation” hc was able to find bctwccn the water swirling in the bucket and an outsidc appcarancc was with the stars. Hence, he suggested that they wcrc rcsponsiblc for the strange water concavity in the bucket.⁶⁰

Newton, of coursc, would have admitted that the concave circular water motion in the bucket was “rclative” in that it could bc causally “relatcd” to the untwisting of the bucket cord and spcculativcly "re-latcd” to the stars. The same motion was also “absolute,” however, in that it was caused by a particular force (the untwisting of the cord), and hcnce, was not mercly rclativc (i.c., uncauscd) and bccausc obscr-v3tion and measurement from the most distant stars would rccord the motion and thus provc that it was not mercly rclativc (i.e., local). In other words, Newton hcld that the terms “relative” and “absolute” wcrc not mutually exclusive and that “absolute” mcant a particular tvpc rather than the absence of relation. To be surę, Newton used the word “absolute” in difTerent ways; noncrhclcss, thesc uses seem to be rcmarkably compatiblc with each other. On the other hand, it would be hard to argue that Mach either correctly understood Newton’s vari-ous definitions of “rclative" and “absolute” or successfully refuted Newton*s bucket argument. But we should not ovcrlook or forget the basie point. Mach and Newton wcrc arguing over apples and oranges. Mach identified the physical world with scnsations, and Newton identified it with what lay beyond or outside of scnsations. The two mon were simply not referring to the same things.

“Progress” is strange, howcvcr. Mach misunderstood Newton’$ ideas and rejected theorctical physics, and a brilliant theoretical physicist, who was evcn less philosophically sophisticated than either Newton or Mach. was so stimulated by MacłTs phenomenalistic criticisms as to devclop a revolutionary physical theory that legitimately refuted many of Newton*s physical ideas or reduced them to special case application. but which, philosophically, was closer to the views of Newton than Mach, even though the author of the theory for many years did not realize it.

Newton’s theory of causal esplanation with its distinedons between “physical" and “mathematical” causes, and causal “agents” and “laws,"

*°3