- 12 Arguments against the Conventionality Thesis
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The thesis of the conventionality of intrasystemic distant simultaneity states that there are no logical or empirical reasons to prefer any particular value, or range of values, of the Reichenbach synchronization parameter , except, of course, the condition that it is conﬁned (for causality reasons) to the open interval between 0 and 1. The thesis has been challenged by numerous philosophers and physicists, in fact, by so many that it would require a separate volume to give a comprehensive, let alone exhaustive, account of their arguments. We will therefore review only the historically most important or methodologically most signiﬁcant examples and ignore technical details. To refute the conventionality thesis it sufﬁces to show that it is possible by a convention-free method, that is, by a method that, in particular, does not presuppose standard synchrony, to establish either (1) distant simultaneity or (2) synchronization of distant clocks or alternatively to measure (3) the one-way velocity of light or (4) that of any physical object of nonzero mass. Each of these methods, if successful, would single out a unique value of . The validity of the last two methods is because, in the notation of chapter 9 and equation (9.2), knowledge of the one-way velocity of light cAB in C H A P T E R T W E L V E Arguments against the Conventionality Thesis the direction from point A to point B equals c/2 and cBA in the opposite direction equals c/[2(1 )], where c denotes the round-trip light velocity, which is measurable by only a single clock. From equation (9.1) it also follows that the one-way velocity AB of any physical object equals c/[c (2 1)] and BA in the opposite direction equals c /[c (2 1)], where is the velocity of the object measured in standard signal synchrony.1 In the case c the last two expressions are, of course, identical with the preceding two expressions. A supposedly convention-free experimental veriﬁcation of even only a qualitative anisotropy of the velocity of light, such as suggested in 1904 by Wilhelm Wien2 and by Alfred Schweizer,3 or with technically more advanced instrumentation, like electronically synchronized Kerr cells by Maurice Jacob4 in 1927 or Alessandro Amerio5 in 1947, would already be a serious challenge. It would not only invalidate standard signal synchrony, based on Einstein’s “equal-time stipulation,” it would also invalidate the conventionality thesis according to which any value of , including 1/2, should be acceptable. Formally , this is because, if the one-way velocity of light in one direction, say cAB, differs from cBA, their ratio and therefore also /(1 ) would differ from 1. But this implies that 1/2 and excludes standard synchronization. Reichenbach himself, shortly after having asserted the earlier-quoted “simultaneity–velocity circle,” discussed an experimental proposal to establish convention-free distant simultaneity by the so-called “galvanometric method.” This proposal was made by Friedrich Adler, formerly a fellow student with Einstein at the ETH in Zürich. It consists of a galvanometer connected at both ends via switches with an electric battery so that to close the Arguments against the Conventionality Thesis 221 1 The last two equations can be derived as follows. Let a light pulse and a particle be emitted (locally) simultaneously from A toward B. The difference between their arrival times at B, measurable by a local clock, must be the same whether calculated in standard or in nonstandard synchrony . Hence (AB/c) (AB/v) (AB/cAB) (AB/vAB). Substituting cAB c/2 yields the abovementioned value for vAB. An analogous calculation yields the above-stated value for BA. 2 W. Wien, “Über einen Versuch zur Entscheidung der Frage, ob sich der Lichtäther mit der Erde bewegt oder nicht,” Physikalische Zeitschrift 5, 585–586, 604–605 (1904). 3 A. Schweizer, “Über die experimentelle Entscheidung der Frage, ob sich der Lichtäther mit der Erde bewegt oder nicht,” Physikalische Zeitschrift 5, 809–811 (1904). 4 M. Jacob, “Procédé expérimental permettant de comparer, à un instant donné, la vitesse de la lumière dans un sense et dans le sense opposé,” Comptes Rendus de l’Académie des Sciences 184, 1432–1434 (1927). 5 A. Amerio, “Un’ esperienza sulla teoria della relatività,” Atti della Accademia Nazionale dei Lincei 2, 736–739 (1947). 222 Concepts of Simultaneity circuit the two instants of momentarily closing the switches must be simultaneous . Adler therefore claims that a deﬂection of...

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