Friday, September 25, 2009

Maxwell's Demon as the Observer

For examples in which observation changes the obeserved, check out the idea of Maxwell's Demon.

Maxwell conceived of a system in which there were two chambers. A demon controlled a valve between the two chambers. The demon could observe the velocity gas molecules that approached the valve. If the molecule were energetic, the demon would open the valve if the molecule was in the left chamber (say) and was traveling to the right chamber. If it were in the right chamber, the demon would keep the valve closed. Similarly, the demon would do the same for slow or less energetic molecules traveling to the left. Fast molecules would travel to the right chamber and stay there and slow molecules would travel to the left chamber and stay there. Thus the right chamber would become hot and the left chamber would become cold. This temperature difference could be used to drive a heat engine such as a steam engine.

Thus Maxwell's demon could obtain energy for free and could create a perpetual motion machine in violation of the Second Law of Thermodynamics. It took may years of research to discover why Maxwell’s Demon could not exist and the Second Law would not be violated. It turned on the analysis of what “observation” meant. If it meant observing light that had interacted with the molecules then the amount of energy that would be used in these interactions would overcome any advantage that the Demon could generate. The Second Law was preserved.

Thus there needs to be a definition of what “observation” and “observer” menas and how observation can be affected.

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