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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{Ladd_1993} short-time motion of colloidal particles: theory;\\ theory.\\  \citet{1995PhyA..214..185E},  \citet{Espanol_Rubio_Zuniga_1995} for discussion of how HD disturbances propagate, and evidence for faster-than-vorticity interaction;\\ interaction. One considers incompressible and the other considers compressible.\\  \citet{Clercx_1997};\\  \citet{Kao_Yodh_Pine_1993} for high-speed experiment;\\  \citet{nderson_Mitchell_Bartlett_2002} describe an experiment where sonic communication is observed.