Maxwell's demon experiment could be made real without breaking physics

A thought experiment called Maxwell’s demon, long hypothesised to break the laws of physics, could be made using simple electronic devices at macroscopic scales – without upsetting the laws of thermodynamics. 

A 155-year-old thought experiment about the laws of thermodynamics could help shed light on how biological cells process fluctuations in their environments.

Maxwell’s demon is a thought experiment first proposed by Scottish mathematician James Clerk Maxwell in 1867. He imagined a tiny demon controlling a gate between two chambers full of gas. The demon then opens the door to allow fast-moving gas particles into one chamber and slow-moving ones into the other. Because the speed of the constituent particles determines a gas’s temperature, the first chamber heats up and the other cools down. The resulting temperature differential could drive a perpetual engine.

The trouble is, the demon’s actions decrease the entropy, or level of disorganisation, in this closed system without expending any energy – which violates the second law of thermodynamics.

Practical versions of Maxwell’s demon that harness thermal fluctuations have been demonstrated, but they require an external energy source, leaving the laws of thermodynamics intact. However, studying such thermal fluctuations in more detail will require a demon that can be implemented at many different scales.

Jose Nahuel Freitas and Massimiliano Esposito at the University of Luxembourg have come up with a type of demon that works at any scale, albeit with lower efficiency the bigger it gets. “The bigger the demon, the more energy one has to spend to make it work,” says Esposito.

Their set-up starts with a small device called a CMOS inverter, which is used in many electronic circuits and consists of two transistors. The transistors can be thought of as doors, one of which opens when a negative voltage is fed into the inverter, while the other opens when a positive voltage is fed in. A second CMOS inverter acts as the demon: while the original Maxwell’s demon sorted particles by speed, this version sorts voltages by their direction. But, rather than storing each voltage on its own side of a box, it discards the negative voltages and sends the positive ones back into first the inverter.

In theory, even no external voltage is applied to the system, the demon should be able to take advantage of tiny thermal fluctuations and create a voltage from nothing. “That would be super good if you could do it,” says Nahuel. “It would also be a violation of the second law of thermodynamics.”

In practice, the demon would require an external power source. One way to implement this is to simply attach the system to a second CMOS inverter with an external voltage applied to it, which would act as the demon and control the input voltage of the original inverter.

This type of system could help researchers study thermal fluctuations, which are governed by probability at small scales in a way that we don’t generally see at larger scales. “This interesting rich physics of the micro-scale can be brought to the macro-scale, so we might see some of these very fancy effects that we don’t expect at the macro-scale,” says Esposito.

The implications might also extend to biological “machines” such as enzymes, which amplify small effects in their environments to accomplish a particular goal. “There’s a general effort of trying to understand, is this concept of a Maxwell demon just a nice thought experiment to demonstrate some basic principles of physics or is there something practical that can come out of it?” says John Bechhoefer at Simon Fraser University in Canada.

“Some biological machinery might be able to be thought of as a Maxwell demon… so by trying to understand all the different aspects of them I think there’s a hope that we get closer figuring that out,” he says.



  1. I have netted third demon nears pace infinity.

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