Studies in History and Philosophy of Science Part B: Studies in History and Philosophy of Modern Physics
Length matters: The Einstein–Swann correspondence and the constructive approach to the special theory of relativity
Introduction
Considerable attention has been drawn lately to the distinction, attributed to Einstein, between principle and constructive theories, and to the methodological importance it may have to the scientific practice. Viewed as part of the context of discovery, however, this distinction is rarely acknowledged as having any philosophical significance, with the exception of Howard (2004) who urges us to regard it as one of Einstein's most valuable contributions to 20th-century philosophy of science. In this paper I would like to demonstrate this significance, suggesting that while the principle–constructive distinction furnishes the physicist with an important methodological tool, it also carries a philosophical weight, to the extent that it serves as a demarcation mark in debates that may seem purely epistemological or metaphysical.
Admittedly, while Einstein was not the first to introduce the distinction between principle and constructive theories to theoretical physics, he definitely popularized it when reflecting along his career on the conception of STR. Expressing the novelty of the theory, Einstein ultimately chose the principle view over the constructive view, but his ambivalence with respect to this choice (and his misgivings about what he regarded as its unfortunate implications on the foundations of quantum mechanics) are well known (Brown, 2005a, Brown, 2005b; Janssen, 2000, Schilpp, 1949). Also well documented is the attempt, made by Einstein's contemporaries Lorentz and FitzGerald, to think about the kinematical phenomena of electromagnetism in constructive dynamical terms (see, e.g., Janssen, 1995). Other physicists who expressed, along with Einstein (as some believe), dissenting constructive views of STR are less known in this context. They include Weyl, Pauli, and Eddington in the 1920s, Swann in the 1930s and the 1940s, and Janossy and Bell in the 1970s.
In a recent monograph entitled Physical Relativity, Brown (2005a) adds his voice to this distinguished list of unconventional voices, arguing that the universal constraint on the dynamical laws that govern the nature of non-gravitational interactions, namely, their Lorentz-covariance, is the true lesson of STR. My modest goal in this paper is to examine one, presumably contentious, issue within Brown's controversial view. This issue is purely historical, and concerns Einstein's attitude towards the constructive approach to STR. Reading Brown, 2005a, Brown, 2005b one gets the impression that Einstein's ambivalence with respect to his choice in the principle view to STR warrants annexing him to the constructive camp. Here I shall suggest an alternative interpretation, that also sheds new light on the way Einstein saw the scope of the dichotomy between principle and constructive theories.
In order to achieve this goal I shall take my cue from a rarely cited correspondence between Einstein and the physicist Swann, mentioned only briefly in Stachel (2002) and in Brown (2005a). In this correspondence Swann presents Einstein with his constructive approach to STR, wherein rods and clocks are not introduced as primitive building blocks, or as “independent objects”, but are taken instead to be material bodies obeying the Lorentz-covariant laws of the quantum theory of matter. Einstein, in response, argues cryptically that any such constructive formulation of STR must, like the quantum theory, contain a fundamental measure of length. My main concern will be to examine how the postulation of a fundamental measure of length (which, according to Einstein, is an inevitable consequence of the constructive approach) bears on the issue I have set forth to investigate.
The paper is organized as follows. Section 2 offers an introduction to the distinction between principle and constructive theories. In Section 3 I discuss the Einstein–Swann correspondence on the constructive approach to STR, and in 4 Reading Einstein, 5 Einstein and the constructive approach to STR I set the record straight with respect to the putative historical consequences of this correspondence on the constructive programme. Before concluding, in Section 6 I mention briefly some aspects that arise in current research on quantum gravity that demonstrate the methodological asymmetry between the constructive and the principle approaches to theoretical physics. These aspects are discussed in more detail in a sequel to this paper (Hagar, 2008).
Section snippets
Einstein
In his famous letter to the London Times from November 28, 1919, Einstein mentions a distinction between two types of scientific theories, namely “constructive” and “principle” theories1:
We can distinguish various kinds of theories in physics. Most of them are constructive. They attempt to build up a picture of the more complex phenomena out of the materials of the relatively simple formal scheme from which
The Swann–Einstein correspondence
In this section I shall discuss a historical anecdote, mentioned briefly in Brown (2005a, pp. 119–120), that regards a short correspondence Einstein had in early 1942 with the physicist W.F.G. Swann concerning the constructive approach to STR. I will use this correspondence to draw some interesting conclusions on Brown's project.
Reading Einstein
Admittedly, Einstein's letter to Swann is not one of his famous letters. Nor is it one of his most transparent. In effect, while mentioned only twice in the vast literature on the foundations and history of STR, I believe that this letter was misunderstood on both occasions. In this section we shall display these misunderstandings, and then set the record straight.
The physics of despair
A recurrent theme in Brown's project is the claim that Einstein expressed, throughout the years from 1907 and at least until 1949, a certain “unease” with respect to his choice in the principle view of STR22
Length matters
Setting these historical issues aside, and regardless of what Einstein did mean in his letter to Swann, there exists an interesting philosophical question, namely, what are the consequences of explaining geometry with a dynamical theory of matter, or, in other words, of pursuing a constructive approach in the context of spacetime theories?
Taking our cue from the discussion in Section 5, we can now identify at least two possible (albeit speculative) consequences. The first says that any
Concluding remarks
Defending them with textual evidence, in this paper I have made the following historical observations:
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Einstein's famous principle–constructive distinction must be handled with care. While it may apply to many interesting cases in the domain of the constitution of matter, it may carry unexpected consequences when applied to theories of spacetime structure.
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Einstein's letter to Swann can be interpreted as a warning against the consequences of pursuing a constructive approach to spacetime theories
Acknowledgments
Early versions of this paper were presented at the first &HPS conference in Pittsburgh (October 2007) and the UMN IAS Symposium on Time and Relativity (October 2007). Thanks to Harvey Brown for several discussions throughout the years and for interesting comments on this paper, Giovanni Amelino Camelia for email correspondence, Itamar Pitowsky for insightful suggestions, Bill Demopoulos for constructive skepticism, and Meir Hemmo for particularly stimulating brainstorming sessions on the
References (58)
Experimental metaphysics2
Studies in the History and Philosophy of Modern Physics
(2007)Einstein on locality and separability
Studies in the History and the Philosophy of Science
(1985)A quantum gravity phenomenology: Status and prospects
Modern Physics Letters
(2002)- et al.
Critical notice: Presentism and relativity
British Journal for the Philosophy of Science
(2003) - Bell, J. S. (1976). How to teach special relativity. Progress in Scientific culture, 1. Reprinted in Bell (1987) pp....
Speakable and unspeakable in quantum mechanics
(1987)- Bell, J. S. (1992). George Francis FitzGerald. Physics World, 5, 31–35. Abridged by Denis Weare from a lecture in...
Physical relativity
(2005)Einstein's misgiving about his 1905 formulations of STR
European Journal of Physics
(2005)- et al.
Minkowski spacetime—A glorious non-entity