séminaire HistPhilPhys

Résumé : For decades the zig-zag motions of pollen particles suspended in a liquid, first observed under the microscope by the botanist Robert Brown in 1827, evaded satisfactory explanation. Being largely studied by chemists and city hygienists, they were an unlikely candidate to convince the skeptical physicists of the existence of atoms. While Boltzmann sought to ground atomism in the second law of statistical thermodynamics, precisely this happened in the years 1905-9 through theoretical work by Albert Einstein and Marian von Smoluchowski, and experiments of Jean Perrin. But these discoveries, often quoted in the debates between realists and constructive empiricists, did not happen in a philosophical vacuum. The emergence of a quantity of a new kind, fluctuations, required a combination of experimental research in several domains, including radioactivity research, with a theoretically sophisticated and philosophically informed understanding of statistical mechanics and probability. Then it was also possible, as Perrin did in his influential book, to find evidence for atomism in other phenomena. While much of the literature focuses on Einstein and Perrin, I will analyze the contributions of Smoluchowski and his Vienna colleague Egon von Schweidler. This will also provide a physics background to the thesis of Vienna Indeterminism that I have advocated long ago as an intellectual tradition of an empiricist indeterminism that informed the debates about causality and probability within Logical Empiricism.

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