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Dynamical activity and fluctuations far from equilibrium

MSC : Frédéric van Wijland, Paolo Visco.

Fluctuations in equilibrium systems are described by the fluctuation-dissipation theorem. It relates the response of the systems to an infinitesimal perturbation which drives it out-of-equilibrium to its equilibrium correlations. The proportionality coefficient between response and correlation is the temperature, independently of the perturbation. Analogous relations have been recently put forward for out-of-equilibrium systems. Interestingly, no temperature, nor any other proportionality relationship between correlation and fluctuations has emerged. However, a physical observable called dynamical activity seems to play a central role. This quantity represents a measure of the propensity of the system to move away from its actual microscopic configuration. We are trying to understand how direct measurements (as e.g. in microscopic systems such a living cell) of both the response and the correlations can yield informations about the activity. We are also trying to find out a way of directly measuring the activity, for example in colloidal systems, where it might exhibit signatures of a glassy state.

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