Scientists and philosophers are fond of elaborate thought experiments. For instance, there's Schrödinger's cat, Descartes' evil genius, Wittgenstein's beetle in the box, Plato's cave, and Borel's monkeys, among many others. Usually these are mere hypothetical curiosities, ideas or metaphors that help us to wrap our minds around complex, abstract concepts. Sometimes, though, scientists find a way to bring them to life.

That's now the case with one famous thought experiment, at least: Maxwell's demon. Physicists have found a way to create a real-life version of this fearful-sounding critter at the quantum level, to test its veracity, reports Science News.

It sounds nefarious, but it's harmless enough. That is, unless you happen to be the second law of thermodynamics. Originally conceived by the physicist James Clerk Maxwell, the thought experiment attempts to credibly violate what is also known as the law of entropy, the idea that the total entropy of an isolated system can only increase over time. In other words, the second law of thermodynamics states that to maintain order in any system, it takes energy. Without energy, any system will eventually slip into disorder.

Maxwell imagined a demon, however, that was capable of manipulating things at the molecular level. Suppose, Maxwell suggested, this tiny creature could shuttle around molecules to manually decrease entropy. For example, suppose you have a bowl filled with lukewarm water. If there existed a demon that could separate the molecules that are moving slightly faster from the others, then it could create order (order, in this case, represented as warmth) out of this otherwise lukewarm collection of molecules, by creating pockets in the water that stay warmer than other pockets.

Such a demon would violate the law of entropy, because according to that law, such an ordering ought to require an expenditure of energy.

Thankfully, scientists have since calculated that Maxwell's demon is nothing special. Such an entity, even if it only exists in our minds, must still spend energy in the form of information storage, in order to know how to properly sort through molecules. So not even Maxwell's demon can violate the second law of thermodynamics, not really.

To really hammer the point home, scientists decided to see if they could create an actual demon in the lab, on the quantum level, to test and make sure that the second law also cannot be violated on the tiniest of scales. It sounds like a mad experiment, but don't worry. No gates to Hell were opened up, no one was possessed. Everything turned out okay, even for the law of entropy.

For the experiment, which was performed by physicist Benjamin Huard and colleagues, a superconducting cavity within which microwaves bounce back and forth was manipulated by light that is tuned to a particular frequency. Depending on the frequency, the system in the cavity could jump from a low-energy to high-energy state, or vice versa, thus replicating the role of Maxwell's demon.

Using a process known as quantum tomography, the researchers were able to measure exactly how much energy the demon siphoned off in order to store information about how to organize the system. They were then able to empirically confirm that the second law of thermodynamics is safe, even from Maxwell's demon, and even on the quantum level.

That's a profound result. The law of entropy was originally understood only for large systems like steam engines, so to confirm that it also applies on the quantum level seems to demonstrate its universality. This means that eventually designs for quantum machines could be developed that are more intuitive, and which operate more efficiently.

Bryan Nelson ( @@brynelson ) writes about everything from environmental problems here on Earth to big questions in space.

Not even a demon can violate the laws of nature
New research verifies that the second law of thermodynamics is safe, even from cleverly crafted thought experiments.