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# Séminaire – Histoire et philosophie de la physique

## octobre 16 @ 16h00 - 18h30

**Marco Giovanelli**(Université de Turin)

*« Appearance and reality: Einstein, Ehrenfest 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 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-Beranthread-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.

**Dennis Lehmkuhl**(Université de Bonn)

*« Einstein’s six paths to the metric tensor – and why he interpreted it differently than you do »*

John Stachel, the first editor of *The* *collected papers of Albert Einstein* and the founder of what is today called Einstein scholarship, divides the creation of the general theory of relativity (GR) into a drama of three acts. The first act centers around 1907, when Einstein was overwhelmed by the epiphany of the equivalence principle, the idea that the force of gravity and the inertia of bodies were intimately connected. The second act takes place around 1912, when Einstein entered the promised land and proceeded from scalar theories of gravity to those based on a metric tensor. And the third act finishes in late November 1915, when Einstein found what we now call the Einstein field equations, the successors of Newton’s law of gravity. Stachel further argued that the « missing link » between the second and the third act was Einstein’s so-called rotating disc argument, which allowed him to forge a connection between gravity-inertia and non-Euclidean geometry. In this talk, I shall argue that instead of being the protagonist in a drama in which the rotating disc argument is the one *eureka* moment that allowed the transition to a metric theory of gravity, Einstein, in the summer and autumn of 1912, was an adventurer walking on six different paths in parallel, all of which led him to the program of finding a theory of gravity based on a metric tensor. And yet, I shall argue, it is Einstein’s starting point, his scalar theory of gravity of early 1912, that, together with his equivalence principle, pointed him to these six paths, and determined the way he eventually saw the metric tensor. In particular, I shall argue that Einstein’s work on a scalar theory of gravity, and his multi-path journey from there to the metric tensor, equipped him with many of the interpretational moves and tools that would influence his later interpretation of GR, and made him resist seeing GR as a « reduction of gravity to spacetime geometry ». I shall decipher how Einstein saw the role of geometry in GR instead, what he himself meant by « geometry », and how his notion of geometry differed from his contemporaries and successors. I shall outline how all this led him to an interpretation of GR that saw the distinction of matter and spacetime geometry as something to be overcome rather than as something to be celebrated.