Brownian ratchet

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The Brownian ratchet is a thought experiment about an apparent perpetual motion machine postulated by Richard Feynman in a physics lecture at the California Institute of Technology on May 11, 1962 as an illustration of the laws of thermodynamics.

The device consists of a gear with a ratchet, that vibrates under Brownian motion (hence the name) in a heat bath. The idea is that motion in one direction is allowed by the ratchet, and motion in the opposite direction is prevented. Thus, it might be reasoned, the gear will rotate with a small force continuously in one direction, without any heat gradient. This is against the principle of the second law of thermodynamics, which can be stated as "It is impossible for any device that operates on a cycle to receive heat from a single reservoir and produce a net amount of work."

Although at first sight the Brownian ratchet seems to extract useful work from Brownian motion, Feynman demonstrated through a number of detailed arguments that its operation would be self-defeating, and would in fact not produce any work. A simple way to visualize how the machine might fail is to remember that the ratchet itself will undergo Brownian motion as well. In fact, the ratchet will be a comparable size to the gear and undergo motions of similar size. So, as often as the machine ratchets forward, it slips due to the ratchet's Brownian motion. However, this idea led to the development of Brownian motors, which do produce useful work, but do not violate the laws of thermodynamics.