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Physicists Test Classic Paradox 'Maxwell's Demon' with Beams of Light

New experiments revisit a classic paradox of thermodynamics.
February 15, 2016, 12:30pm
Image: Oxford

In 1867, renowned physicist James Clerk Maxwell wrote a letter to fellow physicist and middle-name enthusiast Peter Guthrie Tait in which he outlined a thought experiment that seemed to allow for the violation of the second law of thermodynamics. The thought experiment has become known as Maxwell's demon and it has been widely studied in the 140 odd years since its proposal. Most of these studies have been theoretical, although a handful of experiments have actually managed to realize Maxwell's thought experiment in a lab.

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Recently, a team of physicists from Oxford became the first to create a photonic Maxwell's Demon. The results of their experiments, published in a recent issue of the Physical Review of Letters, details how the team was able to use measurements on two beams of light to create an energy imbalance between these two beams, thereby allowing work to be extracted from the system which can then be put to practical uses such as charging a battery.

To create its photonic demon, the team began with Maxwell's original hypothetical, wherein the physicist imagined that two boxes filled with gasses were placed next to one another with a demon controlling a small door between the two chambers of gas. At first the average energy (or speed) of the gas molecules in each box is the same, but as faster than average molecules approach the door, the demon lets them through into the other chamber. In this way, more of the high speed molecules are trapped in one chamber and the low speed molecules are trapped in the other. This destroys the initial equilibrium: one chamber now has a higher average energy than the other thereby creating a temperature difference between the two chambers (faster molecules generate more heat).

The demon's ability to create this temperature difference without the expenditure of work appeared to Maxwell to be in violation of the second law of thermodynamics, which states that two bodies of different temperature, when brought into contact with one another in isolation from the rest of the universe, will establish a thermodynamic equilibrium. Another way of putting this is that in an isolated system, entropy never decreases—although Maxwell's hypothetical did in fact seem to allow the entropy of the system to decrease.

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In the years since Maxwell initially proposed his hypothetical, physicists have managed to satisfactorily explain away the evident paradox of Maxwell's demon. According to some of these physicists' explanations, although Maxwell's demon is not directly doing work on the system, it is extracting information about the system by sorting the molecules. The process of extracting this information about the system is a form of work, and therefore the entropy of the system does in fact increase in accordance with the second law of thermodynamics.

Although physicists were able to show that Maxwell's paradox didn't actually violate the second law of thermodynamics, the exact nature of the relationship between the extraction of work from a system and the information about this system acquired through measurements which explained the paradox was not that well understood. This was the relationship that the Oxford team hoped to elucidate with their photonic demon.

In the Oxford experiments, physicists exchanged the chambers of gas for two beams of light. The demon was a photodetector (which measured the number of photons in each light beam) combined with a "feed forward operation," which channels the brighter beam (which has more photons) in one direction and the dimmer beam (with fewer photons) in a different direction. Each light beam hits a different photodiode (a semiconductor which converts light into an electric current) which then channels the resulting currents to a battery.

If the currents generated from the two beams were equal, they would cancel one another out in the battery. Since they are not, the physicists are able to exploit the imbalance in the energy of the light beams to charge the battery.

"Often we have more information available than thermodynamics supposes," study coauthor Oscar Dahlsten told Phys.org. As Dahlsten explained, most systems are not fully random and have some degree of predictability, meaning "we can then use demon set-ups such as this one to extract work, making use of that information."

According to the team, its experiment is the first step toward gaining a better understanding of how thermodynamics plays out on microscales. A better understanding of the link between information and thermodynamics could have a variety of real world applications, ranging from more efficient cooling and energy extraction systems to application in quantum information technologies.

"Personally I think that sort of technology will have a real impact on meeting the energy challenge facing the world," said Dahlsten. "We are already thinking of ways in which features such as entanglement can be introduced in future experiments based on this one, as our interests gravitate around quantum information."