In the late 1700s, James Maxwell came up with a dilemma that, although just meant as a though experiment, was a troublesome idea to physicists. He came up with a clever arrangement of systems of matter at different temperatures that might possibly violate the second law of thermodynamics, which would be troubling because, well, it's supposed to be a law, and we don't like nature going around breaking its own laws, since when that happens, it means we got the law wrong in the first place.
The experiment is as follows:
- We have two systems of different temperatures that are isolated from each other and any other system they could exchange heat with. Since the energy has nowhere to go, they will remain at this state. (This is in accordance with the first law)
- We bring the two systems into contact and have a small door, guarded by a small demon, which only allows heat to flow one direction. That is, the demon checks if a particle moving from one system to the other is one of the 'hot' particles and allows it to move through in one direction on that condition.
Here we have to stop a minute and realize, as many did after him, that the situation where this demon requires no energy to do its census and rearranging of every molecule in the systems is impossible. Measuring the particles requires interacting with them, and so the demon itself is part of the large system here and the energy it uses sorting is part of the overall equation. This is where my experiment today is different!
I am performing this demonstration in Minecraft, which is a made up world that very clearly marches to the beat of a different Grand Unified Theory, and so I can create a zero-entropy demon. My demon will be played by a fence gate and a pressure plate.
Here we can see a set of two sheep pens with my wonderfully rainbow-colored sheep in them. All but the red sheep are in pen two (the further pen) representing a system at higher energy than pen one (the nearer), where there is just one sheep. In the Middle is the gate with a pressure plate which will cause the gate to open located on the side of higher energy (read: more sheep). This means that energy (sheep) can only flow from the higher energy to the lower energy pen1. This may seem obvious, but it's also the situation Maxwell tried to set up in his famous experiment.
Let's see what happens!
Long story short, it worked! I left the sheep alone for a little while, as the time of day change demonstrates nicely, and when I returned they were all stuck in the near pen. In case it was in any doubt, this says some strange things about the state of entropy in Minecraft. Perhaps things like cheap cold fusion are easy in this world. Maybe I just need to wait for a hydrogen block and a deuterium block...
 I have chosen to ignore second-order sheep effects, like a sheep stepping on the plate and allowing another sheep from the lower energy side to pass back into the higher energy side.