deletions | additions       diff --git a/Payam.tex b/Payam.tex  index 242fca4..775e908 100644  --- a/Payam.tex  +++ b/Payam.tex 
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\item Dimensional analysis argument not fully understood: I think $t_{\rm h}\sim \ell/c_{\rm s}$ on physical grounds, but Payam says $t_{\rm h}\sim \ell^2\rho/\eta$ from dimensional analysis. What is the interpretation of Payam's $t_{\rm h}$? Are the two compatible?  %  \begin{itemize}  \item No! They The two $t_{\rm h}$s  are measuring different things. Payam's $t_{\rm h}$ is considering the {\it  diffusion of momentum information about }of  a particle (I think this is particle's momentum:  the characteristic time in thedecay of the  velocity autocorrelation). autocorrelation.  My $t_{\rm h}$ is aboutpure  information flow. flow via sound waves.  \item Why does the hydrodynamic disturbance travel diffusively rather than at the sound speed?