Concepts of Simultaneity: From Antiquity to Einstein and Beyond

8 The Reception of the Relativistic Conception of Simultaneity

Einstein’s definition of distant simultaneity had far-reaching consequences. It led not only to a new conception of time, which in itself would have been a major innovation, but also to a radical break with classical physics and philosophy . First, the definition implies the renunciation of the previously generally accepted idea that there is no upper limit to physically attainable velocities , for if arbitrarily high velocities were admitted, the classical notion of distant simultaneity, obtained by an instantaneous transmission of information , would not have to be discarded. Second, the acknowledgment of a finite upper bound of such velocities is incompatible with classical mechanics , for according to Newton’s second law of motion a constant force, acting for a sufficiently long time, could accelerate a given mass to an arbitrarily high velocity. The new conception of distant simultaneity entails, therefore, the abandonment of classical mechanics and of all philosophical systems based on it. These consequences were not recognized immediately with the publication of Einstein’s 1905 paper. His 1905 method of deriving the Lorentz transformations , which explain all relativistic effects, from his definition of disC H A P T E R E I G H T The Reception of the Relativistic Conception of Simultaneity tant simultaneity was soon superseded by mathematically simpler methods that did not explicitly involve the notion of distant simultaneity. This notion consequently lost much of its prominence in the thinking of the physicists , who were more interested in the experimental consequences and empirical confirmations of the theory than in its conceptual foundations. Philosophers, before the early 1920s, did not engage themselves with the foundations of relativity either. The mathematically minded philosopher Hugo Bergmann, when he reviewed a text on relativity written by Alexander Brill1 in 1913, complained that despite the fact that philosophical conceptions , like space and time, play an important role in the foundations of the theory, philosophers had not yet acquainted themselves with the new theory .2 This is not to say, however, that the concept of time played no role in the philosophical literature of those days. Metaphysicians, like F. H. Bradley, B. Bosanquet, J. Royce, or G. Gentile, dedicated a large part of their writings to the problem of time and especially to the question of its reality. J. M. E. McTaggart’s attempt at refuting the reality of time, published at first in the 1908 issue of Mind, is still today a subject of lively debates. Samuel Alexander began his Gifford Lectures with the statement that “all the vital problems of philosophy depend for their solution on the solution of the problem what Space and Time are”; and in his monumental work Space, Time and Deity he wrote that “to realize the importance of Time as such is the gate of wisdom.”3 The writings of M. Guyau and J. Ward stressed more the psychological aspects of the idea of time. In all these writings, however, the notion of simultaneity was hardly given serious attention, if at all. The outstanding exceptions were an essay by the philosopher Louis Dominique Joseph Armand Dunoyer, who called the notion of simultaneity “la porté philosophique de la théorie [relativiste ]”4 and, of course, the treatise Durée et Simultanéité5 in which Henri Bergson challenged the relativistic conception of simultaneity for reasons to be discussed. Reception of the Relativistic Conception of Simultaneity 149 1 A. Brill, Das Relativitätsprinzip (Leipzig: Teubner, 1912). This is the second textbook on relativity ever published. The first was Max von Laue’s Das Relativitãtsprinzip (Braunschweig: Vieweg, 1911). 2 H. Bergmann, “Rezension,” Zeitschrift für Philosophie und philosophische Kritik 150, 211–212 (1912). 3 A. Alexander, Space, Time and Deity (London: Macmillan, 1920), vol. 1, p. 36. 4 L. Dunoyer, “Einstein et la relativité,” La Revue Universelle 9, 179–188, 814–835 (1922). 5 H. Bergson, Durée et Simultanéité (Paris: Alcan, 1922); Duration and Simultaneity (New York: Bobbs-Merrill, 1965). 150 Concepts of Simultaneity The following discussion of the acceptance of Einstein’s early papers on relativity is confined, of course, only to responses that refer to the notion of distant simultaneity or to the related notion of the synchronization of spatially separated clocks. Because this synchronization involves the question of whether the “one-way” velocity of light or of any other signal can be determined experimentally, our discussion also involves issues related to the problem of...