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\item Why do the swimmers float on top?  \item For now consider the simple case of a single inert particle in a concentration gradient. Say it moves towards the source of the chemical. It will follow some mean trajectory, but with plenty of scatter. This is an out-of-equilibrium process.  \begin{enumerate} \begin{enumerate}[a)]  \item Which part is out of equilibrium?  \item What does the scatter in trajectories tell us about the system? Anything useful? Use linear-response theory (is this justified? Why?).  \end{enumerate}