Appearance and reality: Einstein and the early debate on the reality of length contraction

European Journal for Philosophy of Science 13 (4):1-30 (2023)
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Abstract

In 1909, Ehrenfest published a note in the Physikalische Zeitschrift showing that a Born rigid cylinder could not be set into rotation without stresses, as elements of the circumference would be contracted but not the radius. Ignatowski and Varićak challenged Ehrenfest’s result in the same journal, arguing that the stresses would emerge if length contraction were a real dynamical effect, as in Lorentz’s theory. However, no stresses are expected to arise, according to Einstein’s theory, where length contraction is only an apparent effect due to an arbitrary choice of clock synchronization. Ehrenfest and Einstein considered this line of reasoning dangerously misleading and took a public stance in the Physikalische Zeitschrift, countering that relativistic length contraction is both apparent and real. It is apparent since it disappears for the comoving observer, but it is also real since it can be experimentally verified. By drawing on his lesser-known private correspondence with Varićak, this paper shows how Einstein used the Ehrenfest paradox as a tool for an ‘Einsteinian pedagogy’. Einstein’s argumentative stance is contrasted with Bell’s use of the Dewan-Beran thread-between-spaceships paradox to advocate for a ‘Lorentzian pedagogy’. The paper concludes that the disagreement between the two ways of ‘teaching special relativity’ stems from divergent interpretations of philosophical categories such as ‘reality’ and ‘appearance’.

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Marco Giovanelli
Università di Torino

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References found in this work

Autobiographical Notes.Max Black, Albert Einstein & Paul Arthur Schilpp - 1949 - Journal of Symbolic Logic 15 (2):157.
Minkowski space-time: A glorious non-entity.Harvey R. Brown & Oliver Pooley - 2006 - In Dennis Geert Bernardus Johan Dieks (ed.), The ontology of spacetime. Boston: Elsevier. pp. 67--89.
How to teach special relativity.John S. Bell - 1976 - Progress in Scientific Culture 1.
Drawing the line between kinematics and dynamics in special relativity.Michel Janssen - 2009 - Studies in History and Philosophy of Science Part B: Studies in History and Philosophy of Modern Physics 40 (1):26-52.

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