Physics and the art of demonology

Illustration for article titled Physics and the art of demonology

Find out what Maxwell's Demon is, and how some physicists are channeling it.

The second law of thermodynamics states that over time, the entropy in a given system will inevitably increase. The only way to decrease it is to pump more energy into the system in the service of organizing it. Philosophically it reflects the decay and disarray to which everything is doomed. Practically speaking, it just says that less and less useful energy can be harvested from a closed system. There always needs to be a source of new energy.

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In the eighteen hundreds, James Clerk Maxwell thought of a way around this idea. Say in a box, or a cave, or a house full of hot gas, there lives a demon. Specifically there lives an obsessive-compulsive demon. The heat of a gas is reckoned by the average speed of its molecules. Some travel faster, and some travel slower. The demon has a grudge against the slow molecules, and so whenever one approaches a certain corner of the box, it snaps shut a trap door and blocks the molecule's path. When a faster molecule approaches, it opens the door and lets it pass. Over time, the box has a hot half and a cool half. This decreases the amount of entropy in the box, and allows work to be done using it – if a tunnel were installed that let the hot air flow back towards the cool side again, driving a turbine as it did.

Illustration for article titled Physics and the art of demonology
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Many contemporaries pointed out that this was crap, seeing as demons didn't exist, weren't obsessive compulsive, and would be using energy to sort the atoms anyway. And there the idea rested.

Until recently, when a group of scientists revived the idea of sorting atoms by their relative speed.

First they set up a magnetic trough for a number of atoms to rest in. At the top of the trough, they placed a laser beam. The atoms in the trough moved at varying speeds, but as they climbed up the out of the trough, their speed decreased, the way a ball would if it were to roll uphill. Only the most energetic make it up to the beam, and when they do, their kinetic energy is reduced to almost nothing. In other words, they're the slowest there is. At that point, the beam ‘kicks' them over, out of the magnetic trap, and into an optical trap. In this way, the coolest atoms are separated out, and the entropy of the system decreases.

Illustration for article titled Physics and the art of demonology
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This is smoke and mirrors, of course. The laser beam pumps energy into the system, and so the demon runs on practical, everyday energy, not diabolic ‘found energy.' Still, it provides a way to cool and condense gas, allowing physicists to perform more tricks on it.

Via Auburn, Physics World, Science Blogs, Optoiq, and IOP.

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DISCUSSION

Many contemporaries pointed out that this was crap, seeing as demons didn't exist, weren't obsessive compulsive, and would be using energy to sort the atoms anyway. And there the idea rested.

That's not true at all, physicists routinely use thought-experiment which analyze weird scenarios as long as they don't violate any of the basic laws of physics, so Maxwell's Demon has been a subject of much discussion since Maxwell invented it (see this book for example). The modern consensus seems to be that the solution has to do with the fact that a hypothetical Demon would need to do information-processing in order to identify faster or slower moving particles, and that in order to avoid filling up his memory he'd have to periodically erase old information, which would increase his internal entropy by an amount that would balance out the decrease in entropy in the two chambers of gas, see the discussion towards the middle of this article for details, especially the section on Charles Bennett's analysis of the problem.