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\item Why does the hydrodynamic disturbance travel diffusively rather than at the sound speed?  \item Some reading about propagation of hydrodynamic information:\\  \citet{10.1103/PhysRevLett.70.1339} short-time motion of colloidal particles: theory;\\  \citet{1995PhyA..214..185E},  \citet{Espanol_Rubio_Zuniga_1995} for discussion of how HD disturbances propagate and evidence for sound-waves;\\  \citet{Clercx_1997};\\