A liter of ordinary air weighs less than half a US penny, but it contains enough thermal energy to toss a 7-kg bowling ball more than 3 m off the ground. A gadget able to harvest that abundant energy by converting the erratic movement of colliding molecules into directed motion could be very useful indeed. Of course, the second law of thermodynamics forbids any device whose sole effect is the extraction of energy from a single thermal reservoir and its conversion into work. But that prohibition has not prevented nearly 150 years of imaginative thinking about the possibilities and impossibilities of rectifying thermal noise, changing it from random to directional.

That happy tradition traces back to James Clerk Maxwell, who in 1867 conjured up a creature—now known as Maxwell’s demon—situated near a tiny opening in a partition that separates the two chambers of a gas-filled box. By blocking the passage...

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