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A Reboot of the Maxwell’s Demon Thought Experiment—in Real Life

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Antoine Naert has reconstructed a prop straight out of the Industrial Revolution. But don’t expect steam-powered engines or decorative brass gears. Naert, a physicist at ENS Lyon in France, and his team have a unique take : Their contraption consists of a soundproofed glass container placed on a vibrating platform that shakes some 300 steel beads inside it like a quiet maraca.

The device looks less steampunk and more 21st-century science fair project. But make no mistake—it’s Naert’s reinterpretation of a 19th-century thought experiment known as Maxwell’s Demon. To look for violations of the second law of thermodynamics, in 1867 Scottish physicist James Clerk Maxwell proposed the concept of a “demon” that could interact with microscopic particles. In a nutshell, the second law states that without the input of additional energy, heat always flows from a hot region toward a cold one. Imagining a demon that could disrupt the flow forces physicists to contemplate what the second law actually means.

“We can get inside the working of the second law by such thought experiments,” says Naert. “And we built it for real.”

Maxwell’s contemporaries had uncovered the second law while investigating the most efficient way to convert heat—from burning coal, for example—into the motion of pistons and turbines. But this principle turns out to have far broader implications than for just steam engines. It’s why ice always melts in a drink at room temperature, but the drink never turns into ice. Put another way, the second law indicates that certain processes in nature can proceed only in one direction. Such irreversible processes distinguish the past from present—what physicists call “the arrow of time.” “​​This is really the principle that describes why we get old,” says Naert.

The second law “is so much of our experience,” says physicist Harvey Leff, a professor emeritus at California State Polytechnic University, Pomona. “We observe it when we put our hand near a fire and are like, ‘Oops, that’s hot,’ and move our hand away.” Any exceptions, like a refrigerator where heat flows from cold to hot, requires a power source.

But what does it mean for something to be hot or cold? To answer that question for steam and other gases, 19th-century physicists developed the concept of temperature to describe the average speed of numerous particles bouncing around in random directions—some faster, some slower. (Later, they would discover the particles were atoms and molecules.) Hotter temperatures mean particles moving at a faster average speed.

The goal of a steam engine is to convert the chaotic motion of hot water vapor into motion in a defined direction, such as the vertical motion of a piston. To create orderly motion, the second law says that you need to keep the gas in two regions at different temperatures. If the gas were all at the same temperature, the vapor particles would move in random directions, and that random motion would not push the piston in a specific direction.

Maxwell sought potential violations of the second law, as scientists do when someone proposes that a principle should apply to all of nature. Specifically, he tried to devise a theoretical engine that exploited gas at a single temperature. In 1867, he presented a thought experiment, commonly depicted as a box with two compartments containing the same gas at a single temperature. The gas molecules bounce around at a range of speeds following a specific distribution.

But what if a little demon resided at the partition between the two compartments and sorted the molecules according to speed? One compartment would end up with faster molecules on average, corresponding to a hotter temperature, while the other compartment would contain slower molecules on average, corresponding to a colder temperature. The demon would have created two regions of different temperatures. Heat would again flow from hot to cold, making it possible to generate motion in a particular direction, which someone could use to move a piston.

Thus, the demon seems to have created an engine from a gas at one temperature, violating the second law. “Maxwell’s Demon was one of the greatest threats to the second law,” says physicist Nicole Yunger Halpern of the University of Maryland.

For more than a century, physicists have contemplated the thought experiment, and in recent years have even converted it into real-life machines. Naert’s device emulates Maxwell’s Demon, except instead of randomly bouncing molecules, he uses randomly bouncing steel beads. The beads shake around the container to hit a rotating blade in different directions. Naert has engineered it such that the blade turns a dynamo to generate an electric current—but only when the blade rotates in a particular direction. He can then use that current to turn a motor, for example. Like Maxwell’s Demon, the contraption turns chaotic motion—analogous to heat—into orderly motion.

It’s surprising that the device could generate orderly motion at all, according to Yunger Halpern, because the machine is so large. Most real-world constructions of Maxwell’s Demon use microscopic particles such as atoms or electrons .

To be clear, Naert’s device does not violate the second law of thermodynamics, nor does Maxwell’s Demon. Physicists, pondering the demon over decades, have delivered multiple explanations for why it doesn’t. One is that in order to sort the beads, the demon has to be cooler than the rest of the gas, says Naert. Thus, the container of gas particles is not a single temperature, which contradicts the premise of the thought experiment. In the case of Naert’s device, the rapidly bouncing steel beads are at one temperature, whereas the electronic component that converts the beads’ motion into the rotation of a blade is another temperature.

So why recreate Maxwell’s Demon? Physicists have used the thought experiment to explore common concepts in wildly different contexts. For example, in the 20th century, it led physicists to discover the physical nature of information. In order for the demon to sort molecules by speed, it needs some way of knowing the particles’ speed. The demon would need to store that knowledge and erase that information. From these ideas, physicists figured out that information isn’t just some abstract concept that we humans harness to communicate. It’s the physical state of some object, like representing the voltage across a transistor as a bit of information—a key concept now fundamental to the study of computing.

In addition, the second law of thermodynamics signifies the statistical nature of the universe. Its building blocks are not stars, planets, humans, or bacteria—they’re the atoms and molecules that make us up. You can think of the atoms in the universe as a deck of cards, constantly being shuffled and reshuffled. By the end of the reshuffling, the deck will have no semblance of order. But instead of dealing with a deck of 52 cards, the universe has a deck on the order of 10 82 atoms.

Or if you want to be more manageable, consider the 10 24 molecules in a cup of coffee. If you drop a sugar cube into that coffee, those sugar molecules have so many more ways of redistributing themselves throughout the coffee than staying in cube form. Or consider someone who releases perfume in a room. That perfume will rush to fill the space. This illustrates the concept of entropy, often described as “disorder.” The most likely arrangement of atoms has the highest entropy. A deck of cards sorted according to the four suits has lower entropy, for example, than one that is not. Similarly, dissolved sugar molecules cannot re-cube, and the perfume cannot rush back into the vial, without some external intervention requiring energy.

Ultimately, the second law of thermodynamics says that energy moves around in nature to increase entropy. “If you ask what physics is, you might just say it is the study of energy,” says Leff. “What’s happening as far as I can see is that energy keeps redistributing itself.”

However, as people invent new technology, it’s not always clear how the second law applies. For example, seemingly straightforward concepts like temperature get complicated. Naert’s steel beads are at room temperature in the conventional sense, defined according to the average speed of their constituent molecules. This is the same temperature that you might associate with how it would feel to touch the bead. But Naert has identified another property of his system, which he interprets as a different type of temperature, defined not by the speed of its constituent molecules, but that of the glass beads bouncing around. It’s mathematically analogous to conventional temperature, as both involve the speed of discrete particles, but has no relation to whether you will burn or cool your hand when touching it. Naert plans to work with theorists to better understand what this type of temperature means, along with measuring and understanding the role of entropy in his device.

In addition, physicists have had to revisit the second law as researchers build smaller and smaller devices, such as quantum engines —made of a few atoms. They want to know, for example, whether the second law limits these quantum engines in the same way as conventional macroscopic engines, says Yunger Hapern.

Naert’s personal motivation to build this machine was intellectual curiosity, but he thinks that studying the second law in macroscopic contexts could potentially lead to more efficient machines for harvesting energy from ocean waves, for example, as it illustrates the conversion of chaotic macroscopic motion into orderly motion that could be used to charge a battery or move a turbine. In addition, he sees his device as a teaching tool. “This is incredibly close to the original idea from the 19th century,” he says. But because he uses beads instead of molecules, “you can see everything because it’s in centimeters.” With his new device, Naert has invited Maxwell’s Demon to confuse and enlighten us at a new scale.

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