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Published online by Cambridge University Press: 14 March 2022
Moon and Spencer maintain (p. 228) that there is a divergence between Einstein's analysis of simultaneity, as set forth in his fundamental paper on relativity of 1905, and my treatment of that concept in a recent publication. They write: “Einstein decided that simultaneity is meaningless in all cases of relative motion. … Grünbaum decided that even Einstein's restriction is not sufficiently stringent and that simultaneity is a questionable concept even with stationary observers. … Grünbaum rejects Postulate VI [which states (p. 218) that ‘If A and B are not in relative motion, the time taken for a pulse of radiation to travel in a vacuum from A to B is the same as from B to A’] and is thereby unable to synchronize clocks even when they are stationary.”
This journal 23, 216 (1956)
2 A. Grünbaum, “Logical and Philosophical Foundations of the Special Theory of Relativity,” American Journal of Physics 23, 450 (1955), §2.
3 It is noteworthy in this connection that the physicist L. Jánossy, who, unlike Moon and Spencer, does wish to uphold the Lorentz transformations but believes that a philosophically satisfactory interpretation of quantum mechanics requires the non-Einsteinian assumption of the possibility of causal chains faster than light [cf. Jánossy, “The Physical Aspects of the Wave-Particle Problem,” Acta Physica Hungarica 1, 423 (1952) and “Über philosophische Fragen der modernen Physik,” Deutsche Zeitschrift für Philosophie vol. 3, pp. 358–373] is driven to invoke conceptions akin to those of Lorentz and Fitzgerald in order to avoid a contradiction between his non-Einsteinian assumption and the relativity of simultaneity inherent in the Lorentz transformations [cf. Jánossy, “Über die physikalische Interpretation der Lorentz-Transformation,” Annalen der Physik 11, 293 (1953)].
4 The Principle of Relativity (a Collection of Original Memoirs), Dover reprint, p. 40; italics are Einstein's.
The definitional rather than factual character of the outgoing and return velocities of light is also stressed by H. P. Robertson, who writes: “Einstein's synchronization insures as a matter of definition the equality of the forward and backward velocity along any given line.” [Robertson, Reviews of Modern Physics 21, 380 (1949)].
5 A further elaboration is given in A. Grünbaum, “Fundamental Philosophical Issues in the Special Theory of Relativity,” in K. Sapper (editor), Kritik und Fortbildung der Relativitätstheorie, to appear in Europe, and “Relativity of Simultaneity within a single Galilean Frame: A Rejoinder,” American Journal of Physics 24, 588 (1956).