Abstract
This paper addresses and proposes to resolve a longstanding problem in the philosophy of physics: whether and in what sense Albert Einstein’s Chasing the Light thought experiment was significant in the development of the theory of special relativity. Although Einstein granted this thought experiment pride of place in his 1949 Autobiographical Notes, philosophers and physicists continue to debate about what, if anything, the experiment establishes. I claim that we ought to think of Chasing the Light as Einstein’s first attempt to problematize the very idea of the electromagnetic ether frame, and that it thereby contributed to his eventual adoption of one of special relativity’s two foundational axioms: the “light postulate”. This interpretation requires the assumption that Einstein had presupposed special relativity’s other axiom, the “principle of relativity”, when initially considering Chasing the Light. This argument is novel insofar as it provides evidence that such a presupposition by Einstein is both conceptually and historically plausible. Moreover, this paper directly challenges John D. Norton’s compelling claim that Chasing the Light is best understood as a refutation of emission theories of light propagation; while both interpretations of the experiment are conceptually coherent, I argue that the interpretation found in this paper is supported more straightforwardly by historical evidence.
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Notes
Technically, the notion of “frozen” or “standing” light may be incoherent given that light is a transverse wave, and should therefore oscillate transverse to the direction of motion even when observed from a theoretical, “absolute” frame of reference. However, this technicality does not materially impact any aspect of the “ether” problem discussed here, and I continue to refer to “frozen” or “standing” light for the sake of simplicity throughout this exposition.
Aside from Peacock and Norton, few commentators even acknowledge the general confusion concerning the precise import of Chasing the Light. Adolf Grünbaum, who notes that the thought experiment has a “flimsy character”, is another notable member of this small group (Grünbaum 1973, pp. 371–375).
Henceforth, the “propagation of light” is assumed to be in a vacuum.
Norton himself conceives of Chasing the Light as including a third concern in addition to the two discussed here, because he views the question “[f]or how should the first observer know or be able to determine that he is in a state of fast uniform motion?” as a separate objection the thought experiment attempts to raise for the concept of a frozen light wave (Norton 2013, p. 125). However, I do not believe this question, on its own, is meant to pose a third concern about the very possibility of observing frozen light. Rather, I believe it is an expression of Einstein’s commitment to the principle of relativity, and that the question simply asks what the phenomenon of frozen light entails for this principle. I express and argue for my position on this matter in Sects. 5 and 6, but in the meantime, I explicate Norton’s account of Chasing the Light on the basis of concerns (1) and (2) for the sake of clarity and organization.
Einstein’s “Autobiographical Sketch” contains only the following passage on Chasing the Light: “[I]f one chases a light wave with the speed of light, then one would have before one a time independent wave field. But such a thing appears not to exist…” (Einstein 1955, p. 10). Additionally, Wertheimer’s report on his interview with Einstein recounts Chasing the Light more as a series of open-ended questions than as a systematic thought experiment: “First came questions such as…[w]hat if one were to run after a ray of light? What if one were riding on the beam? If one were to run after a ray of light as it travels, would its velocity thereby be decreased? If one were to run fast enough, would it no longer move at all? To young Einstein this seemed strange” (Wertheimer 1959, pp. 214–215).
Banesh Hoffmann (1982 in Holton and Elkana 1982, pp. 93–97) has previously suggested that Einstein tacitly presumed some form of the principle of relativity when questioning the odd notion of frozen light, but that due to some psychological block he did not apply it more broadly such that Chasing the Light could cause a conceptual threat to ether theory.
“Minkowski geometry” refers to the geometrical structure used to describe the spacetime of special relativity, which includes mathematical analogs for both the principle of relativity and the light postulate.
I derive this claim from a well-known set of lecture notes by David Malament (2009), in which he explains that Minkowski and non-relativistic Euclidian geometries differ only in the types of metrical structures that they contribute to affine spaces.
References
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Acknowlegements
I thank Professor Bryan W. Roberts at The London School of Economics and Political Science for his invaluable counsel as I crafted this paper’s arguments. I also thank the reviewers, as well as my family and friends, for their thoughtful feedback as I refined this paper for publication. Finally, I thank Professor John D. Norton for helping me to develop a complete understanding of his perspective on this issue, and for providing recommendations on how I could improve my response.
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Sussman, N.F. Quick thinking: how Einstein did (and did not) refute the ether frame of reference. Synthese 199, 5995–6008 (2021). https://doi.org/10.1007/s11229-021-03056-0
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DOI: https://doi.org/10.1007/s11229-021-03056-0