For a long time, the thought experiment of Maxwell’s demon was thought to be contrary to the laws of nature. It now appears that the experiment can be performed without going against the rules of thermodynamics.
Maxwell’s demon is a thought experiment first proposed by Scottish mathematician James Clerk Maxwell in 1867. Maxwell imagined a little devil, which can open and close a door between two gas-filled chambers. By gently opening and closing the door, the demon allows only fast-moving gas particles to slip into one room. On the contrary, he only allows slow-moving particles to pass through to the other room.
Because the speed of the particles determines the temperature of a gas, the first chamber heats up and the other cools. The resulting temperature difference can be used to keep a motor running forever.
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The problem is that the demon’s actions reduce the entropy, or degree of disorder, in this closed system without expending energy. That violates the Second Law of Thermodynamics.
Thought experiment in practice
The experiment has since been carried out in practice with microscopic chambers. Use was made of minuscule temperature fluctuations, so-called thermal fluctuations, in which a gas particle accidentally deviates briefly from the average moving speed. These practical experiments all require an external energy source. As a result, the laws of thermodynamics remain intact.
To study the use of thermal fluctuations in more detail, a demon is needed that also works on larger scales. Nahuel Freitas and Massimiliano Espositophysicists at the University of Luxembourg, have now invented a demon that works on any scale. However, the demon has a lower efficiency as the scale increases. “The bigger the demon, the more energy you have to use to make it work,” Esposito says.
Their setup begins with a CMOS inverter, a small device used in many electronic circuits, which consists of two transistors. The transistors can be seen as doors, one of which opens when there is a negative voltage across the inverter, while the other opens when a positive voltage is applied. A second CMOS inverter with an external power source acts as the demon. Where the original Maxwell demon sorted particles by speed, this version sorts voltage by direction. Instead of storing each voltage on its own side of a box, it discards the negative voltages and sends the positive back to the first inverter.
In theory, even if no external voltage is applied to the system, the demon should be able to take advantage of minuscule fluctuations and create a voltage from scratch. “It would be great if it worked,” says Nahuel. “It would also be a violation of the second law of thermodynamics.”
biological machines
These kinds of systems can help researchers study thermal fluctuations, which arise at small scales from quantum mechanical effects that we can’t usually see at larger scales. “This interesting physics can be taken from the microscale to the macroscale, so we can see some of these very fancy effects that we don’t expect at the macroscale,” Esposito says.
This can also teach us about biological machines such as enzymes (proteins that speed up reactions in cells), which amplify small fluctuations in their environment.
“We’re trying to understand if Maxwell’s demon is just a fun thought experiment to demonstrate basics of physics, or if something practical can come out of it,” he says. John Bechhoefer† He is professor of stochastic thermodynamics at Simon Fraser University in Canada. “You might think of some biological machines as a Maxwell demon. So hopefully by trying to understand all the different aspects of it, we can get a better understanding of the idea,” he says.