Modeling in physics: a story of scales

HiPhiS (History & Philosophy of Science) lecture by Annick Lesne, theoretical physicist, CNRS research director at LPTMC Paris-Sorbonne and IGMM Montpellier. Free admission.

Summary

The starting point for this presentation is to show how understanding a real system using models depends fundamentally on the scales at which it is observed, those of the phenomena we wish to predict, and the intrinsic characteristic scales of the system. I will illustrate this point with several examples that are now considered classic: diffusion processes, the DNA molecule, and the coast of Brittany. We will see that this perception of different scales is put to the test for fractal structures and, more generally, for systems exhibiting scale invariance; and conversely, how the detection of scale invariance is limited by the scales of observation. Clearly distinguishing between different scales and levels of organization is also essential for considering the concept of emergence and studying the complex systems in which it manifests itself, such as a crowd or a dune.