Concepts of Simultaneity: From Antiquity to Einstein and Beyond

14 Recent Debates on the Conventionality of Simultaneity

An important publication supporting the conventionality thesis of distant simultaneity is W. F. Edwards’s 1963 paper on the special theory of relativity based on nonstandard synchrony.1 It shows that the Lorentz transformations can be generalized by admitting anisotropic light propagation and yet be observationally equivalent to those conventionally constructed. Since spatial anisotropy of the propagation of light corresponds to nonstandard synchrony and the Lorentz transformations comprise the whole special theory of relativity , Edwards’s paper implies that no experiment in the special theory could ever disprove the conventionality thesis. It proves that the equivalence of different forms of the Lorentz transformations, obtained by different definitions of simultaneity, implies that direct observables, such as the readings on a single clock, would not be affected by adopting a different form of the Lorentz equations. But such quantities as time lapse involving two clocks separated in distance with respect to the observer, are not directly observable and involve the definition of C H A P T E R F O U R T E E N Recent Debates on the Conventionality of Simultaneity 1 W. F. Edwards, “Special relativity in anisotropic space,” American Journal of Physics 31, 482–489 (1963). 252 Concepts of Simultaneity simultaneity which is, to some extent, arbitrary. . . . One may well argue about the reality of time dilation, or the relativity of simultaneity or other indirect observables , but in the final analysis, the argument is academic unless a two-way signal velocity greater than c is discovered outside of electromagnetism. As far as any measurement is concerned, one can adopt any view he wishes consistent with the fact that the circulation speed of light is c (if, indeed, it is). For most problems the most convenient assumption to make is still that of isotropic space. Edwards’s essay attracted little favorable attention at the time it was published . In fact, it was even rejected by Martin Strauss, who, by the way, was an ardent admirer of Reichenbach. In a contribution to an international seminar on problems of relativistic physics, organized by the University of Jena in 1965, Strauss declared, with explicit reference to Edwards’s paper, that the admission of anisotropic propagation of light, within the limits of light propagation, “does not give a kinematics physically equivalent to that of special relativity (contrary to what is claimed by the author) but a kinematics either contradicting special relativity . . . or else essentially poorer in physical content.”2 One of the reasons of this deficiency is the declared unavoidable interrelation between the notions of simultaneity and velocity that led to the conceptual circularity stressed by Reichenbach. But this circularity follows only “if the traditional operational definition of velocity in terms of length and time is adopted. Yet we are free to use any other operational definition such as the definition by means of the Doppler effect, or no operational definition at all but an implicit definition by a set of axioms.” Strauss thus proposed to “give up trying to define velocity in terms of length and time and, instead, take the relation of constant velocity as a primitive notion to be characterized (‘defined implicitly’) by a set of axioms . This, by the way, is the normal procedure in axiomatics.”3 Compared with Edwards’s paper, another synchrony-free derivation of the Lorentz transformations, published seven years later, gained a much more favorable reception. In fact, the paper published in 1970 by John A. Winnie,4 2 M. Strauss, “The Lorentz group: axiomatics–generalizations–alternatives,” Wissenschaftliche Zeitschrift der Friedrich-Schiller-Universität 25, 109–118 (1966); English translation in M. Strauss, Modern Physics and Its Philosophy (Dordrecht: Reidel, 1972), pp. 130–151. See also M. Strauss, “Ist die Isotropie der Lichtausbreitung in einem Inertialsystem eine Konvention?” Monatsberichte der Deutschen Akademie der Wissenschaften 7, 626–627 (1965). 3 Op. cit., p. 139. 4 J. A. Winnie, “Special relativity without one-way velocity assumptions,” Philosophy of Science 47, 81–99, 223–238 (1970). then of Hawaii University, which derived the kinematics of the special theory of relativity without any assumption concerning the one-way velocity of light or, equivalently, concerning the numerical value of the synchronization parameter , was hailed by the conventionalists as an important contribution to the philosophy of time. Winnie’s approach was based on the simple argument that if the value of  is indeed merely a matter of convention...