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Ernst Mach

thc Tcchnical Univcrsi(y of Ziirich wcancd him back to Switzcrland in 1912. Threc scmestcrs later he transfcrrcd again, this timc to Berlin where hc bccamc a collcaguc of Max Planck and a regular member of thc Prussian Acadcmy of Science.

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Albert Einstein couchcd the csscntial insight of his theory of generał rclativity, thc cquivalcncc of inertial and gravitational mass, within a number of Machian idcas. Indeed, he considcred the theory as a natu-ral consequcncc of Machs earlier work and bclieved that cxccpt for accidcntal factors Mach quitc likcly would havc discovcrcd and elabo-rated the theory himsclf.

Einstein shared Machs desire to climinatc the notion of forcc froin physics, that is, he wanted to solvc dynamie problems using a merely kinematic approach.³⁴ He reasoned that if gravitalion wcrc completcly undcrstandablc in geometrie terms, and if gravitational and inertial accclcration were equivalent, then perhaps cvcn inertia could be under* stood without appealing to traditional forcc c.\planation, particularly if inertial mass itsclf could be madc dependent on thc gravitational at-traction of the totality of the stars in the univcrsc as determined by a mero funcńonal rclation. In other words, Einstein, rejccting Planck\s warning about thc misusc of kinematies, attempted to apply Mach’s rclativistic “solution” of Ncwtons buckct cxperimcnt to support his theory of equivalence. Einstein called Mach's notion, which was partly bascd on rcviving philosophical assumptions underlying Ptolemy’s as-tronomy, Mach s Principlc.³⁰

Einstein wrotc an enthusiastic lettcr to iMach on the subjcct, daicd June 25, 1913:

Rcccntly, you havc probably rcceived my ncw puhlication on relativity and gravitation which I havc 3t last finished after unending labor and painful doubt. Ncxt year at thc solar cclipsc it will turn out whether the light rays arc bent by thc sun, in other words whelher thc basie and funda* mcntal assumption of thc cquivalcncc of the accelcration of the rcference frarnc and of thc gravitational field really hołd. If so, then your inspired invcstigations into thc foundations of incchanics—despite Planck's unjust criticisms—will reccivc a splrndid confirmation. For it is a ncccssary con-scqucncc that inertia bas its origin in a kind of mutual intcraction of bodics, fully in thc sense of your critiquc of Newton’* buckct exprri-rnent/*⁷

Einstein referred to Machs principlc again in another Icttcr to Mach, which is undated, but according to Joseph Petzoldt was probably sen: in carly 1914: “I can’t quitc understand how Planck has so littlc un-derstanding for your cfTorts. His stand on my [generał relativity] theory is also one of refusal. But I can’t takc it amiss; so far, that one single epistemological argument is the only thing which I can bring forward in favor of my theory.” ³⁸

The history of Einstein's generał theory is a long and complicatcd one, but suffice it to say that Einstein changcd and amendcd scvcral fćaturcs of it, that it is widcly acccptcd today (1970), and that it still has not bcen conclusivcly proved or refuted. Einstein’s goal of climin3t-ing forcc from physics has not been accomplishcd. His understandings of mass and inertia still havc their qucstionable aspects, and latcr vcr-sions of his theory havc lcft out Mnch’s principlc altogethcr.³⁹

The common sense objcction to Einstcins theory of generał rclativity is not dircctcd against his somewhat start ling artempt to make inertial and gravitational mass cquivalent, hut against thc curious opinion that gravitational attraction is morę undcrstandahlc than inertial ac-cclcration and that geometry is capablc of cxplaining inertial mass. Nor will common sense tolcratc any abandonment or redefinition of force which in any way is likcly to rcducc thc succcssful application of force in practical life, thc utopian hopcs of Mach and Einstein not-withstanding.

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Einslcin’s adulation of Mach reachcd a high point in his obituary on thc lattcr, published in thc April 1, 1016, cdition of the Phyń\alische Zcitsc/irift.

The fact is that Mach by his historical writings, in which he followed thc devcIopmcnt of thc individual Sciences with so much !ove ... has had a great influence on our generation of natural scicntists. . . . The significancc of such spirits, as Mach, in no way mcrely lics in hasing satished ccrtain philosophical needs of the timc. ... I rcqucst thc reader to takc in hand Mach’s work The Science of Mcchanict, chaptcr two, scctions si\ and scvcn . . . One will find thcrc a mastcrful presentation of thoughts, which bv no means have yet bccomc thc common property of all physicists. . . . It is not improbable that Mach would have come across thc theory of rclativitv, if at thc timc when his mind was still young and frtsh, thc qucstion ot the constancy of thc speed of light had alrcady movcd physicists. . His

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