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

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 Varicak 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 lesserknown private correspondence with Varicak, 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'.