Information and Energy
Entropy is a fascinating thing. Back in the days of steam engines, scientists realized that the hot steam that drove the engines would always contain some energy that couldn’t be put to useful work. Steam engines (and all engines, machines and devices) have an inherent inefficiency.
Even worse- this unusable energy had a tendency to spread. If you have a box full of gas, eventually, every molecule in that gas is going to have the same temperature as every other- you can’t use the heat energy of the gas to do work. Even if it started hot in one spot, eventually it’ll all end up uniform. Entropy spreads and everything winds down. We’ve all heard the “second law of thermodynamics”- entropy always increases*.
In James Clerk Maxwell’s thought experiment about his “demon”, he supposed that he could take a box of gas and put a thin barrier in the middle. In the barrier, there would be a door guarded by a demon. When the demon saw a hot molecule approaching from one side of the barrier, he’d open the door and let it pass. Vice versa, if a cold molecule approached from the other side of the barrier, this demon would let that molecule pass through. Eventually, one side of the box would be very hot, and one side would be cold. Entropy appears to have been reversed, and work can be done using the hot side of the box.
This was pretty weird stuff for its day. And even weirder, physicists proved that this didn’t actually violate the known laws of physics. It meant that it was possible, at least in theory, to reverse entropy. In practice, on the other hand…
It wasn’t until Quantum Mechanics looked at the problem of entropy that we found a physical basis for Maxwell’s demon. You see, entropy isn’t just the energy that can’t be put to work. It’s also the information we can’t know about a system. For Maxwell’s demon to do his sorting trick, he would have to know whether or not a molecule was hot or cold. To do that, he’d have to find some way of entangling himself with the state of the molecule (quantum entanglement is its own weirdly awesome thing). Suffice to say, this demon isn’t truly reversing entropy. It’s moving the entropy to its own state.
So the second law of thermodynamics is not only sound, but this formulation is also testable- which is what’s discussed at the link. An experimental version of Maxwell’s demon has been built, and by collecting information about particles (through entanglement), it is possible to shovel entropy under the carpet.
In terms of practical applications, we’re a little short at the moment. But it’s an extremely fascinating bit of information about our universe, proving once again that it is weirder and more wonderful than we can imagine.
*Actually, it’s that entropy tends to increase. The 2nd Law of Thermodynamics is more of a statistical statement then a physical law, and the reasons behind this get very complex, and I can’t explain them properly, so let’s just say that it is a physical law and move on.
NIGHTNIGHT by DEDDY
