Dynamics of complex systems in Cauchy cavities

We demonstrate a generalized invariance property: that the ratio between the intrinsic dynamics of complex media and the perceived timescale of scattered intensity fluctuations depends only on macroscopic properties and not on the detailed circumstances of geometry-dependent light-matter interaction. We show that the necessary conditions can be established in a diffusive Cauchy cavity where the relation between these two timescales can be controlled at will. This approach provides the flexible means to study the intrinsic dynamics of systems with arbitrary geometry and in various conditions of interaction.