Exorcizing Maxwell’s Demon

Abstract

P hysicists have been haunted by the idea of Maxwell's demon for almost 150 years. The beast, conjured in a thought experiment by James Clerk Maxwell, sorts particles in a gas according to their speeds, thus transferring heat from the colder, evenly mixed gas to the region containing the hotter, high-speed particles. At first sight, the demon appears to violate the second law of thermodynamics. But the paradox can be resolved by re-alizing that work must be performed on the demon for it to do its job properly. This description isn't entirely satisfy-ing, however, as it introduces an external—not necessarily physical—entity to do work on the demon. Jukka Pekola and colleagues of Aalto University in Finland [1] have now exorcised such nonphysical beings by realizing a nanodevice equivalent to a Maxwell's demon (Fig. 1), but one whose op-eration doesn't depend on an external entity. This so-called autonomous device, also known as an information machine, is completely self-contained. So far, autonomous demons have been a purely theoretical concept; this new experimen-tal system provides a way to test formulations of fundamen-tal axioms of thermodynamics and descriptions of informa-tion processing. In the second half of the 19th century, the second law of thermodynamics was still relatively new. The law had been restated to describe specific situations, including, for in-stance, that heat cannot be transferred from cold to hot with-out doing work (Clausius statement), and that heat engines cannot operate with 100% efficiency (Carnot statement) [2]. But physicists questioned whether the second law was true for a system whose properties were governed by the average behavior of many particles or if it held for each individual particle. To illustrate the average quality, Maxwell proposed, in an 1867 letter to his colleague Peter Tait, a thought exper-iment that allowed a violation of the Clausius statement [3]. Maxwell imagined two boxes, filled with a gas of particles and separated by a common wall. A " neat fingered being, " capable of measuring each particle's velocity, sat by a little door in the wall. This being could sort particles by opening and closing the door, allowing only fast particles to go to the