Closing the Loop in Neuroprosthetic Control

Reversible processes make the thermodynamist’s life simpler, but the experiments can require a lot of waiting. In 1997, Jarzynski derived a remarkable equality that relates the work done in irreversible processes to the equilibrium, or reversible, free energy difference of the system, no matter how far from equilibrium the irreversible paths might go. The catch, however, is that the more irreversible the paths measured, the larger the number of trials that must be performed to achieve a reasonable standard deviation. Liphardt et al. (p. 1832; see the Perspective by Egolf) now report an experimental verification of Jarzynski’s equality, based on hundreds of stretches of a single molecule of RNA between its folded and unfolded conformations.