The first demonstration of an information-heat engine could revolutionise the way nanomachines get their power.
Here’s a fascinating piece of work. Build a tiny staircase and place a small polystyrene bead on the bottom step (a staircase is fairly straightforward to construct using electric fields).
It’s easy to see the bead being jostled around by the random motion of molecules in the surrounding air, the well-known phenomenon of Brownian motion. Most of the time, the Brownian motion tends to knock the bead down the stairs but sometimes the jostling is powerful enough to push the bead up a step.
Keep a close eye on the bead using a video camera and every time you see it go up a step, change the electric field so that it cannot drop back down again. This is like placing a barrier behind the bead. As you repeat this process, the bead will move up the staircase, driven by Brownian motion.
This is exactly the experiment that Shoichi Toyabe at Chuo University in Tokyo and a few buddies have successfully carried out. The implication is that the bead is somehow able to extract energy from the environment, which at first sight, looks like a blatant violation of the 2nd law of thermodynamics.
Of course, there’s more to this than first appears. Toyabe and co’s work is an experimental version of the famous Maxwell’s demon. Imagine a box filled with air but divided in half by a barrier. Maxwell’s demon is an imaginary being capable of opening the barrier to allow fast moving molecules through while closing it for slower ones. Eventually, the fast moving molecules end up on one side of the barrier which becomes hotter than the other side, even though no energy has been added to the system.
The question is whether or not Maxwell’s demon violates the 2nd law of thermodynamics which is that heat cannot flow from a cool to a hot system by itself. The most recent thinking is that Maxwell’s demon does not violate the 2nd law of thermodynamics because it has to measure the velocity of all the molecules before deciding which to let through and this requires energy. When this is taken into account, there is no violation.
But here’s the curious thing. There is no conventional transfer of energy into the system: no heating or accelerating of molecules or some such. Instead, information itself seems to be the medium through which energy is transferred. That kind of thinking has been a theoretical curiosity, until now. Toyabe and friends’ have actually done it, the first experimental demonstration of this kind of energy transmission. In effect they’ve converted information into energy in this system.
There’s no violation of thermodynamics here. These guys use a video camera to determine the position of the bead so when the camera’s energy budget is taken into account, everything works out as the laws of thermodynamics predict.
It’s hard to overstate the significance of what they’ve done: they’ve been able to operate a nanomachine–a stair-climbing bead–using nothing more than information as the power supply. This, they say, is an entirely new type of engine which they call an information-heat engine, and it’s not hard to imagine its potential.
It means that it’s now possible to power nanomachines using information as the medium to transfer energy, even if there is no direct contact with the nanomachine. The task now will be to shrink the sensing system. A video camera is a hefty thing to cart around. It would clearly be handy to find some microscopic way of sensing the environment and using the information gathered to power a nanodevice.
It’s not yet clear how this can be done but you can bet your bottom dollar that Toyabe and pals are working on it. The idea might even generate a little interest elsewhere.
Ref: arxiv.org/abs/1009.5287: Information Heat Engine: Converting Information To Energy By Feedback Control.
