Uh oh: entangled particles break second law of thermodynamics

Looks like today's the day to head out to the garage and build yourself a perpetual motion machine, since Japanese physicists have just shown how to smash the classical second law of thermodynamics to smithereens using quantum entanglement.

The second law of thermodynamics is the one that says you can't get something for nothing. There's no free energy, you can't make a perpetual motion machine, and the entropy of a system must always increase. That last thing about entropy is basically saying that energy always tries to even itself out: if you have a container of hot water and you pour cold water into it, you end up with lukewarm water. If you want it to go the other way (heating or cooling a container of lukewarm water), you have to put energy into the system from the outside.

So that's all straightforward, then, but a Scotsman named James Maxwell in 1867 challenged this law in the following thought experiment: suppose you have a container of lukewarm water. This water is full of molecules moving at many different speeds; some are "hot" molecules moving fast and others are "cold" molecules moving slowly, but the average temperature of the system is just "warm." Now, divide the container into two halves, and put a tiny little molecule-sized door in the divider. Rig up the door such that whenever a particularly fast molecule approaches, it gets passed through to one side of the divider, and whenever a particularly slow molecule approaches, it gets passed through to the other. Eventually, this door will have sorted all the molecules into a fast side and a slow side, and you'll have a container with hot water and cold water instead of lukewarm water without having to input energy into the system. Violation alert!

As it turns out, this thought experiment isn't just a thought experiment: you can actually do something functionally similar in a lab. In 2010, scientists showed that it was possible to move a tiny plastic bead up a staircase using the random motion of air molecules, using a door technique similar to what Maxwell had thought up. You put the tiny bead on a tiny staircase and watch it getting pushed around by air molecules randomly bumping into it. Eventually, it'll randomly get pushed up a stair, and you shut an electrical door* behind it, holding it there. Over time, you can get this bead to climb the entire stairway without adding energy to the system.

After much scratching of heads, physicists realized that these experiments depend entirely on having a lot of accurate information about the system in question. In Maxwell's experiment, you need to be measuring the velocity of the molecules all the time, and in stair-climbing bead experiment, you need to be measuring the position of the bead. All of this measuring takes energy, which neatly balances out the apparent "free" energy that comes out of the system.

The key realization here is that these experiments are effectively converting information into energy. So with the plastic bead, you're taking information on the location of the bead and converting that into the energy required to push the bead up the steps, which preserves the second law of thermodynamics. Information literally becomes power.

That's a fairly awesome concept to consider, but things start to get problematic when you weave quantum mechanics into the system, which is what physicists from Tokyo University and the University of Kyoto have done. Let's go back to Maxwell's experiment sorting hot and cold molecules. Now, imagine that you start off with all of the molecules on one side entangled with all of the molecules on the other side. Entanglement, remember, is when you've got two molecules that are connected on a quantum level such that measuring one of them can give you information about the other one. That "give you information" bit is important, since we've seen how information and energy can be fundamentally equivalent, and this is where the crazy bit is: with a container of entangled molecules, you only have to spend the energy to measure half of them to get information about all of them, meaning that you can do the little door-sorting trick and create a hot side and a cold side of the container twice as efficiently.

From what I understand (and I'll be honest: understanding these equations is a walk in the park, if the park is full of people throwing giant Greek symbols at your face), the energy difference between the quantum version of this experiment and the classical one comes from the difference in energy required to perform measurements of quantum states as opposed to classical ones. Using quantum entanglement, you can get more energy out of a system than classical physics would seem to allow, but the difference comes from the energy required to measure all of the quantum information necessary to make it work.

All of this stuff is still only working on paper, but in principle there's no reason that it wouldn't work in real life, or at least as close to real life as quantum entanglement ever gets. And what's it good for? Well, the authors of the paper say that there may be potential applications for "controlling quantum-correlated thermodynamic systems," so get ready for all of those quantum-correlated thermodynamic systems that you've been using to soon get way more controllable.

Paper, via Tech Review

Via Tech Review

*From what I can make out, you don't have to worry about energy you use to make the doors work because they're isolated from the system that you're looking at: the doors don't transmit energy to the bead, or in Maxwell's experiment, to the water molecules.

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