Maxwell’s demon faces the heat

Test of second law of thermodynamics shows that manipulating energy comes at a price

Anyone trying to circumvent the physical laws governing heat is going to get burned.

A new experiment reveals how a device that robs a closed system of heat to make it more orderly, an action forbidden by a bedrock law of physics, inevitably pays a price by becoming hotter and more disordered. It’s a real-life demonstration of a nearly 150-year-old thought experiment known as Maxwell’s demon. If this demon really could skirt the second law of thermodynamics — which states that the entropy, or disorder, of an isolated system can never decrease ­— then it would be possible to create a perpetual motion machine.

The demonstration described in a paper to be published in Physical Review Letters is the first to monitor both a system and the demon that’s working to reduce the system’s entropy. “It’s a really nice experiment,” says Eric Lutz, a theoretical physicist at the University of Erlangen-Nuremberg in Germany. The work confirms theoretical research showing that information and heat are intertwined: The demon heats up because it must discard the information it learned to manipulate the system. A demonlike device could eventually perform functions like refrigeration, but this experiment proves the contraption would consume energy just like the kitchen appliance.

Nineteenth century Scottish physicist James Clerk Maxwell was very familiar with the second law of thermodynamics. It explains why heat always flows from hot to cold until everything reaches a stable temperature, a state of maximum entropy. Steam engines work by exploiting the heat transfer to drive a turbine.

In an 1867 letter, Maxwell introduced a scheme that seemed to game the system. He envisioned a microscopic entity that monitored gas molecules bouncing around two neighboring containers. This “demon” would increase the temperature difference between containers, and thus decrease the total entropy, by allowing only fast-moving molecules to cross into the hotter container and slow-moving molecules to enter the colder container. The sorting would enable the demon to perpetually run an engine.

Molecular sorting isn’t the only way to decrease entropy — stealing heat works too. The laboratory version of Maxwell’s demon created by Jonne Koski, a physicist at Aalto University in Finland, and colleagues essentially tricked an electronic circuit into forfeiting heat.

Without the demon, electrons in the circuit progressed from high to low energy, as if rolling down a gentle slope. As the electrons rolled downhill, they released energy in the form of heat into their environment, increasing the system’s temperature and entropy.

At one point along their path, though, the electrons had to briefly borrow some of that energy to scale a small bump — which isn’t a big deal as long as they gave it back when rolling down the bump. But the demon, in the form a charge-manipulating device, was monitoring that obstacle. Whenever an electron scaled the bump, the demon introduced a charge that transformed the bump into a pothole. The electron then had to consume even more energy to escape the hole. Once the electron left, the demon brought back the bump for the next electron. The cumulative effect of electrons overcoming the demon-created obstacle course drained heat from the environment, leading to lower temperature and lower entropy. A scientist with no knowledge of the experiment would be shocked to find the system seemingly violating the second law.

But there’s no need to rewrite the textbooks, because Koski’s demon pays a price. The researchers found that as the demon fooled with electrons, it heated up. In fact, it warmed so much that the total entropy of the system and the demon increased. The heat is a by-product of the demon’s inability to store information about the system it’s monitoring. Unlike making observations and recording them, erasing information always requires some use of energy, a principle first articulated by physicist Rolf Landauer in 1961. Since Koski’s demon can keep tabs only on one electron at a time, it must discard its knowledge of past electrons — an entropy-increasing process that more than compensates for the entropy lost by the system. “The demon has to heat up more than the system cools,” Koski says.

Some physicists say that while the experiment is compelling, they’re not convinced it captures the essence of Maxwell’s original demon concept. Nonetheless, a device similar to Koski’s demon could prove useful for cooling nano-sized devices — even if it has to play by the rules.

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