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A device based on a 155-year-old thought experiment has been realised at the largest scale yet, and it may help us understand how entropy, or disorder, is produced. It could also someday be used to harvest energy from motion.

Maxwell鈥檚 demon is a thought experiment first proposed by Scottish mathematician James Clerk Maxwell in 1867. He imagined a tiny demon sorting gas particles by temperature into two chambers, heating up one and cooling down another. In theory, this temperature differential could drive a perpetual engine, but in practise, it is impossible 鈥� it would decrease the amount of entropy in the system, which is forbidden by the second law of thermodynamics.

However, physicists now understand that a device similar to Maxwell鈥檚 demon can be created if you are simply willing to put more energy into the system than you take out of it, which obeys the second law of thermodynamics but means that the demon cannot function as a perpetual engine. at 脡cole Normale Sup茅rieure de Lyon in France and his colleagues made one such device to see whether it would still work at relatively large scales.

Their device consists of a canister 5 centimetres across sat atop a shaker and containing small stainless steel beads. As the beads bounce around, they act like the particles in a gas, moving randomly around the canister and rebounding off of a small rotor. A motor only allows the rotor to spin in one direction 鈥� if the beads are pushing it in the allowed direction, it spins and generates an electrical current, and if they are pushing it in the other direction, it stays still. The motor stands in for Maxwell鈥檚 demon, and without it the rotor would simply jiggle back and forth randomly.

The electricity to power the shaker is far more than what can be gotten back from the rotor鈥檚 motion, but it鈥檚 not hard to imagine a situation where the shaking is provided naturally, such as in a river or on a bridge people are crossing.

鈥淚f you have to pay to create this fluctuating environment, then of course it doesn鈥檛 work 鈥� you can鈥檛 get around the second law of thermodynamics,鈥� says at Simon Fraser University in Canada, who was not involved in this work. 鈥淏ut if someone else gives you this fluctuating environment and you don鈥檛 have to pay for it, then it could make sense.鈥�

This experiment produced only about 4 microwatts of power, tiny compared to the 10 watts required to keep the beads shaking. If this engine could be made larger and the shaking provided for free, the researchers say it could potentially be used to harvest energy from its environment.

It could also be useful in fundamental research, particularly in understanding how information 鈥� in this case information about which direction the rotor is being pushed 鈥� is turned into entropy, says Naert.

鈥淚f we can measure the information which is exchanged between the 鈥榙emon鈥� and the surroundings, then we can measure the entropy creation rate,鈥� he says. 鈥淭he properties of that process are completely unknown, so it鈥檚 very exciting to explore.鈥�