For a long time it was thought that Maxwell’s demon’s thought experiment was against 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 that the Scottish mathematician James Clerk Maxwell first proposed in 1867. Maxwell imagined a little devil who could open and close a door between two gas-filled chambers. By gently opening and closing the door, the demon only allows fast-moving gas particles to slide into one room. On the contrary, he allows only slow-moving particles to pass through to the other space.
Because the velocity of the particles determines the temperature of a gas, the first chamber heats up and the second cools down. The resulting temperature difference can be used to keep an engine running forever.
The problem is that the actions of the demon reduce the entropy, or degree of disorder, in this closed system without using energy. It violates the second law of thermodynamics.
Thought experiment in practice
The experiment has since been performed in practice with microscopic chambers. Small temperature fluctuations were used, so-called thermal fluctuations, where a gas particle accidentally deviates briefly from the average speed of movement. 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 more closely, a demon that works on a larger scale is also needed. Nahuel Freitas and Massimiliano Esposito, physicists at the University of Luxembourg, have now devised 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 need to make it work,” Esposito says.
Their setup starts with a CMOS inverter, a small device used in many electronic circuits and consisting 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. Another CMOS inverter with an external power source acts as the daemon. 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 one 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 minimal fluctuations and create a voltage from scratch. “It would be great if it worked,” Nahuel says. “It would also be a violation of the second law of thermodynamics.”
These kinds of systems can help researchers study thermal fluctuations that occur on small scales from quantum mechanical effects that we do not normally see on larger scales. “This interesting physics can be taken from the micro scale to the macro scale, so we can see some of these very fancy effects that we do not expect on the macro scale,” says Esposito.
This can also teach us about biological machines such as enzymes (proteins that accelerate reactions in cells) that amplify small fluctuations in their environment.
“We’re trying to figure out if Maxwell’s demon is just a fun thought experiment to demonstrate basic physics, or if something practical can come out of it,” 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.