Information-powered device manages to cheat the laws of thermodynamics

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The laws of thermodynamics tell us that all work requires energy. But a recent demonstration used only information, not energy, to control electric potential, apparently violating the laws of thermodynamics. Here's how they did it.

The second law of thermodynamics tells us that, over time in a closed system, differences in pressure, temperature, and chemical potential will even out to a general equilibrium. It also suggests that, in a closed system, entropy will never decrease, meaning it always involves a net loss of useful energy to do work.

Legendary Scottish physicist James Clerk Maxwell played around with the law in an 1871 thought experiment that later became known as Maxwell's Demon. He imagined gas molecules in thermal equilibrium placed in an isolated container. This has a divider in the middle of it, and there are trapdoors on the divider. The molecules then start moving around, but unless one of the trapdoors is opened when a molecule's trajectory would take it toward it, the gases will never leave their side of the divider.


Maxwell imagined what he termed a "finite being" that had senses far more advanced than ours. Its heightened senses would allow it to perceive which molecules were moving faster and which were moving slower and where all the different molecules were. Based on this, it could open the trapdoors at precise moments to let the slower molecules pass to one side and the faster molecules to move to the other. Because the velocity of a molecule corresponds to temperature, the side with the slower molecules will become colder while the side with the faster molecules will become hotter. That seemingly violates the second law of thermodynamics.


One of the best responses to this was in 1929 by Leó Szilárd, who pointed out there's no violation if you expand the system to include this "finite being", which by then was known almost exclusively as Maxwell's Demon. The demon would have to expend lots of energy in figuring out the exact trajectories and velocities of all the different particles, and that would cause a net increase in entropy in the entire system. Szilárd realized that there's an equivalence between energy and information, coming up with a simple formula that described the minimum amount of work needed to store a single binary bit, which is also the maximum amount that can be released when the bit is erased.

Japanese researchers have created a simple device that uses this equivalence. They built an electric field that varies sequentially so that it resembles a sort of tiny, particle-sized spiral staircase. The difference in electric potential between different steps of the staircase is small enough that fluctuating particle can occasionally move from one step to another. Most of the time, it would jump down a step, but every once in a while it would jump up a level. The researchers exerted a little control by placing a barrier in the way of the particle, preventing it from moving back down a level once it had moved up. If repeated long enough, the particle would get all the way up the staircase.


That may not appear to involve any thermodynamic trickery, but the devil - or, in this case, the Maxwell's Demon - is in the details. The electric field caused the particle to rotate in one direction which, under normal circumstances, would cause it to slowly move back down the potential energy steps of the spiral staircase. This couldn't happen because of the barriers the researchers put in the way, but normally the particle would just keep rotating on its same step forever. However, the particle was placed in an aqueous solution that worked against that effect, and every once in a while it started rotating in the opposite direction, which allowed it to move up a step.

All the while, the researchers tracked the particle's motion using a video camera, which allowed them to know when it had rotated against the field. Whenever this occurred, they immediately put the barrier in place, keeping the particle on the higher level before it could rotate back down. This rather circuitous process allowed them to increase the potential of the particle without ever actually imparting it with additional energy. Thus, the researchers have become Maxwell's Demon.


Of course, there's no violation of the laws of thermodynamics here, as the energy needed to run all the macroscopic devices far, far outstrips the microscopic gains in electric potential. That said, the microscopic gains are a real breakthrough - on the nanoscale, the researchers had tapped into a full quarter of the information's energy content, by far the most ever accessed in one experiment, and the first real practical demonstration of the energy-information equivalence.

Belgian physicist Christian Van den Broeck says right now this is purely of scientific interest, but he does think it could have practical applications within 30 to 40 years. He suggests that, at least on nanometer scales, one could actually use bits of information to boil water molecules. And, perhaps even more intriguingly, it's possible that nature is already converting information to energy somewhere. It appears that Maxwell's Demon really might be out there somewhere.


[Nature Physics via Physicsworld]