Coarse Graining = Understanding

Hans Christian Oettinger

ETH Zurich, Department of Materials

Institute of Polymers, HCI H 543

CH-8093 Zurich, Switzerland

Coarse graining is NOT a necessary evil required to solve problems which are computationally too large. Coarse graining rather is the art to simplify to the essentials and hence to provide understanding. Systematic coarse graining procedures are therefore needed, which are the topic of statistical equilibrium and nonequilibrium thermodynamics. This does not sound controversial but it immediately becomes massively so when illustrated for rarefied gases. We show that any coarse graining step should be accompanied by an increase in irreversibility and is potentially accompanied by the same kind of historical confusion that was created by the passage from reversible to irreversible equations.