It seems that something odd happens to the second law of thermodynamics when systems get sufficiently small. The law states that the entropy, or disorder, of the universe increases over time and it holds steadfast for large-scale systems. For instance, whereas a hot beverage will spontaneously dissipate heat to the surrounding air (an increase in disorder), the air cannot heat the liquid without added energy. Nearly a decade ago, scientists predicted that small assemblages of molecules inside larger systems may not always abide by the principle. Now Australian researchers writing in the July 29 issue of Physical Review Letters report that even larger systems of thousands of molecules can also undergo fleeting energy increases that seem to violate the venerable law.

Genmiao M. Wang of the Australian National University and colleagues discovered the anomaly when they dragged a micron-sized bead through a container of water using optical tweezers. The team found that, on occasion, the water molecules interacted with the bead in such a way that energy was transferred from the liquid to the bead. These additional kicks used the random thermal motion of the water to do the work of moving the bead, in effect yielding something for nothing. For periods of movement lasting less than two seconds, the bead was almost as likely to gain energy from the water as it was to add energy to the reservoir, the investigators say. No useful amounts of energy could be extracted from the set-up, however, because the effect disappeared if the bead was moved for time intervals greater than two seconds.

The findings suggest that the miniaturization of machines may have inherent limitations. Noting that nanomachines are not simply "rescaled versions of their larger counterparts," the researchers conclude that "as they become smaller, the probability that they will run in reverse inescapably becomes greater."

--Sarah Graham