“But One Must not Legalize the Mentioned Sin”. Phenomenological vs. Dynamical Treatment of Rods and Clocks in Einstein’s Thought Marco Giovanelli Abstract The paper offers a historical overview of Einstein’s oscillating attitude towards a ‘phenomenological’ and ‘dynamical’ treatment of rods and clocks in relativity theory. Contrary to what it has been usually claimed in recent literature, it is argued that this distinction should not be understood in the framework of opposition between principle and constructive theories. In particular Einstein does not seem to have plead for a ‘dynamical’ explanation for the phenomenon rods contraction and clock dilation which was initially described only ‘kinemat- ically’. On the contrary textual evidence shows that, according to Einstein, a realistic microscopic model of rods and clocks was needed to account for the very existence of measuring devices of identical construction which always measure the same unit of time and the same unit of length. In fact, it will be shown that the empirical meaningfulness of both relativity theories depends on what, following Max Born, one might call the ‘principle of the physical identity of the units of measure’. In the attempt to justify the validity of such principle, Einstein was forced by different interlocutors, in particular Hermann Weyl and Wolfgang Pauli, to deal with the genuine epistemological, rather then physical question whether a theory should be able or not to described the material devices that serve to its own verification Keywords: Albert Einstein, Relativity Theory, Rods and Clocks, Hermann Weyl, Wolfgang Pauli, Constructive Relativity, Conformational Holism Intrinsically Brobdingnag and Lilliput are precisely the same; it needs an intruding Gulliver—an extraneous standard of length—to make them appear different A. S. Eddington Preprint submitted to Studies in History and Philosophy of Modern Physics 01/04/2014 1. Introduction In the last years, mainly thanks to Harvey Brown’s seminal work (for the most part collected in Brown, 2005), increasing attention has been drawn to Einstein’s ‘self-confessed sin’ of treating rods and clocks as ‘primitive’, ‘self- sustained’, or ‘unstructured’ entities. Brown rightly insisted that Einstein was admittedly aware of the fact that rods and clocks are actually ‘structured dynamical entities’, ‘composite bodies’, the behavior of which depends on the structural properties of the forces responsible for the microstructure of matter. Therefore—as Einstein put it paradigmatically in an often quoted passage from his self-written ‘obituary’ for the volume edited by Paul Arthur Schilpp for the library of Living Philosophers—the ‘sin’ of treating rods and clocks as ultimate elements incapable of further explication should not be ‘legalized’ (Einstein, 1949a, 59). It was a temporary expedient, necessary only because physics at that time did not posses the adequate conceptual tools to construct rods and clocks explicitly from the fundamental physical quantities in the theory, from fields alone, or in worst case scenario, from fields and particles together (Barbour, 2007, 587) Einstein’s waving attitudes towards the role, indispensable or provisional, of rods and clocks in both of his theories, has been usually cast in the well-known opposition between ‘constructive’ and ‘principle’ theories, which Einstein had explicitly introduced in 1919 (Einstein, 1919b), but addressed in several occasions starting at least from 1907 (see Howard, 2005, for more details). In Einstein’s original stance toward special relativity as a ‘principle theory’, the geometry of space-time appeared to be ‘defined’ through the behavior of ‘rods’ and ‘clocks’, whose contractions and dilations were postulated, without introducing a realistic microscopic model of their material constitution. However, when a suitable ‘constructive theory’ of matter would eventually be at hand, rods and clocks will be thought as rather complicated physical systems obeying fundamental dynamical laws. Symmetry properties of space-time will turn to be nothing but a codification of the symmetries of the laws governing matter. In particular ‘length contraction’ and ‘time dilation’ in special relativity should be described in the final analysis as a consequence of the dynamical laws that govern rods and clocks. Brown’s non-standard approach to special relativity has been challenged from different sides. John Norton (Norton, 2008) and Michel Janssen (Janssen, 2009), to mention only the more influential reactions, have defended the correctness of what one might call the ‘mainstream’ view that Lorentz invariance reflects the symmetry of Minkowski space-time, so that rods and clocks exhibit their characteristic behavior because the laws of nature governing their functioning conform to symmetries of space-time. However, at first glance, this approach seems to fail to make sense, or at least to address, the many passages in which Einstein, until the end of his life, insisted again and again that rods and clocks should be treated as solutions of the dynamical equations, and not introduced separately as structureless entities. This paper has not the ambition to solve the vexed question of the “arrow of 2 explanation in special relativity” (Gorski, 2010), whether it is the behavior of matter and fields which explains the symmetries of space-time or if it is the other way around. It would rather attempt to take cue from this debate to offer an overall historical survey of Einstein’s repeated concerns about a tension between ‘phenomenological’ and ‘dynamical’ treatment of rods and clocks. The paper will therefore follow as close as possible the historical development (or rather, as we shall see, the surprising constancy) of Einstein’s stance towards this issue in published and unpublished writings. In particular the paper will document the context in which Einstein, probably, at the beginning of 1917, seems to have firstly expressed in private correspondence his concerns about the use of complicated material systems as measuring devices (section2). It will consider the circumstances in which Einstein in the 1920s, pressed by the epistemological objections raised by different interlocutors, in particular Hermann Weyl (Scholz, 2007) and the young Wolfgang Pauli (Stachel, 2005), felt the urgency to articulate his point of view in public writings (section3). Thereby, he outlined a two- stage epistemological strategy, waving between a provisional and a principled attitude toward the use of rods and clocks in relativity theory (section4), to which he remained faithful until the end of his life (section5). Even if other interlocutors were worth being taken into consideration (in particular Arthur Stanley Eddington), we decided to concentrate on the German debate, which offers for obvious reasons, a particularly rich dialogical network made of private correspondences, public confrontations, and philosophical disputes. Some of this material has been analyzed in some excellent historical and philosophical literature (Stachel, 1989; Howard, 1994; Ryckman, 2005; Howard, 2007; Fogel, 2008) to only mention a few. However, as far I can see, there is no overall historical-critical overview of Einstein’s stance towards this issue over the years. This is unfortunate since the question wether the behavior of measuring devices should be derived from the theory equations and or stipulated independently has been often discussed in the recent philosophical debate by appealing to a limited group of passages written by Einstein in disparate occasions. As, I will try to show, if inserted in their proper historical setting, Einstein’s dissatisfaction toward his initial ‘operational’ treatment of rods and clocks and his correspondent appeal to a ‘dynamical’ explanation seem never actually address the question in which the contemporary debate is mostly interested: wether the space-time lives a ‘parasitic’ existence at the expense of the dynamical phenomena of rods contraction and clock dilation, or whether on the contrary rods contract and clocks slowdown because they adapt to the structure space- time. In particular there is no textual evidence that Einstein ever presented in his arguments in favor of dynamical explanation of rods and clocks in the context of his opposition between constructive and principle-theories, so that constructive relativity would finally ‘dynamically’ explain, what relativity as a principle theory had described only ‘kinematically’. It is undeniable that Einstein expressed early on the conviction that a dynamical account of rods and clocks was needed in special as well as in general relativity. However, not in order to explain rods contractions and clock dilations. What needed to be ‘dynamically’ explained, was precisely the very opposite fact 3 that, put it in a somehow provocative way, there exist clocks which do not slow down and rods which do not shorten in any circumstances. More precisely, in special as well as in general relativity, it is more or less tacitly assumed that identically constructed rods and identically constructed clocks, which measure respectively the same length and show the same rate of ticking when they are held side by side at one location at a certain time, they will always agree on their readings, if carefully put next to each other at rest again whatever their intermediate histories might have been. In other terms a dynamical explanation should account for the very existence of ‘good’ rods and clocks of ‘identical construction’ which always measure the same true