Four-point functions in glass-forming liquids: what information do they encode

We examine the four-point functions describing dynamic heterogeneity in glass-forming liquids and show how contributions with different physical origins can be identified and determined. Collective relaxation fluctuations constitute only one of the contributions to the four-point functions, but this contribution can be cleanly extracted in certain time regimes. This provides a way to determine the exchange time describing the persistence of faster and slower relaxing regions in dynamically heterogeneous systems.