A microscopic version of a diving board has been driven to cheat the second law of thermodynamics 95 per cent of the time. The finding doesn鈥檛 challenge the validity of the law, but underscores how different the rules of the microscopic world can be.
The behaviour of our world is constrained by this physical law. Among other things, it sets the minimum energy expended for changing the state of something, such as putting an idle motor into a steady and controlled state of motion.
But everything is different when you go small, says at 脡cole normale sup茅rieure de Lyon (ENS Lyon) in France. He and his colleagues, and , also at ENS Lyon, devised an experiment that shows this dramatically. Their micro-sized device seems to break the second law of thermodynamics because it can move between states at a seemingly impossible low energy cost almost every time.
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The device was simple: a conductive cantilever only a few micrometres in size. It could vibrate, behaving a little like a tiny diving board. And by changing the voltage on a nearby electrode, the researchers could switch it between one of two 鈥渙scillation states鈥�: one in which the cantilever was approximately straight as it vibrated, and one in which it vibrated while angled upwards.
The second law of thermodynamics dictates how much work, or energy, the process of pushing the cantilever from one state to another should cost. But the team worked out how to use the electrode to violate this law almost every time the cantilever switched between states.
Barros says that the key to doing this was to make the cantilever small enough for tiny fluctuations in ambient temperature 鈥� which are always naturally happening 鈥� to influence its movements. The team worked out how to use those temperature fluctuations to help nudge the cantilever between states. Because the temperature fluctuations did some of the work, it appeared that the electrode needed to expend unusually little energy to shift the cantilever鈥檚 state.
鈥淭his is really a point that distinguishes the 19th-century way of looking at thermodynamics, when it was developed and applied to steam engines and cars, and the late 20th, early 21st-century 鈥榮tochastic thermodynamics鈥� that applies to small systems,鈥� says at Simon Fraser University in Canada. He says that averaging over many repeated experiments will never challenge the laws of thermodynamics, but within each individual run there is room to engineer one outlier after another.
In a previous experiment by another research group, an electron was coaxed into similarly violating the second law 65 per cent of the time, but the new experiment is a much more dramatic example of this more nuanced understanding of thermodynamics. Bellon says that he has been working on perfecting this experiment, and achieving the necessary precision, for almost 20 years.
However, his team鈥檚 work doesn鈥檛 open the door for extracting impossible amounts of energy from the cantilever. This is because the researchers didn鈥檛 build a mechanism that would selectively tack some energy-hungry process onto every second law violation. To do so would require invoking 鈥�Maxwell鈥檚 demon鈥�, another idea for how to cheat thermodynamics that dates to the 19th century. While researchers are still studying how the Maxwell鈥檚 demon idea can be made a reality, Ciliberto says that something a lot like it could actually already exist in biology, where molecular motors are just the right size to take advantage of the same thermal fluctuations as their experiment.
Journal reference: Physical Review Letters,
