Scientists Recreate ‘Maxwell’s Demon’

Scottish scientists have built a model that mimics Maxwell’s demon — a hypothetical creature that moves systems from equilibrium without using energy.

The molecular model of the demon, made by David Leigh and colleagues, at the University of Edinburgh, is a rotaxane — a molecular ring threaded onto a central axle containing binding sites where the ring can attach.

In his classic experiment of 1867, physicist James Clerk Maxwell conjured a little demon guarding a trapdoor between two separate gas-filled compartments. The creature only lets fast-moving molecules pass from left to right, and slow-moving molecules from right to left. Over time, one compartment fills with fast-moving molecules and becomes hot, while the other contains slower-moving molecules and is cooler.

Such spontaneous changes in temperature, or movement away from equilibrium, the scientists said, violate the second law of thermodynamics.

In the new rotaxane machine, information about the location of the molecular ring is used to move the system from equilibrium. But gathering and then implementing that information costs energy — so in the real world, the second law of thermodynamics remains valid.

The complex study appears in the journal Nature.