Thirty years ago Feynman et al. presented a parado x in the Lectures on Physics: an imagined device could let Brow nian m otion do work by allowing it in one direction and blocking it in the opposite direction. In the chapter Feynman et al. eventually show that such ratcheting can only be achieved if there is, in compliance with the basic conservation law s, some energy input from an external source. Now that technology is going into ever smaller dimensions, ratcheting Brownian motion seems to

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... e a real possibility in nanotec hnological applications. Furthermore, Brownian motion plays an essential role in the action of m otor proteins (individual m olecules that convert chemical energy into m otion) . A uthor' s address: Section of P lastic and R econstructive Surgery, D epartm ent of Surgery, M C 6035, U niversity of C hicago, 5841 South M aryland A venue, C hicago, IL 60637, U SA