Tuesday, 30 October 2012

Maxwell's demon

It often happens that scientists need to use metaphors for explaining their point to other scientists and the general public. This is how many mysterious fantastic creatures are born. Many of these creatures were fabricated during the debates on the Second Law of Thermodynamics. Undoubtedly,  the most popular of all those creatures was Maxwell demon – a product of a thought experiment created by the physicist James Clerk Maxwell, the author of the famous Maxwell's equations that accurately describe the electromagnetic field.

Since 1871, Maxwell's demon had occupied the great minds of science. Scientists still look for processes which would allow heat to pass from less hot bodies to hotter bodies. Of course, all of us (or at least those of us who still remember our physics classes) know that heat can be passed from hot to cold bodies. The paradigm called the Second Law of Thermodynamics seemed irrefutable until it was challenged by the existence of Maxwell’s demon.

The idea of Maxwell’s demon is very simple: imagine a sealed container, divided into two gas-tight parts with by the wall with door that would allow just one gas atom to pass through it. At the beginning of the experiment, the upper part of the vessel contains gas, and at the bottom part is filled with vacuum. The door in the middle of the wall dividing both parts is guarded by an imaginary microscopic janitor who is vigilantly watching over molecules. The janitor would allow fast molecules to go through the door to the bottom of the container, while the slow molecules are left in the top half. Clearly, if such a watchman is on duty at the door for a long time, the gas will be divided into two parts: in the upper part of the container there would be cold gas contained of slow molecules, and in the bottom part of the container there would be hot gas consisting of fast molecules. This mythical janitor opening and closing the door was nicknamed “Maxwell's demon”.

Maxwell's demon has such special powers that he can monitor every single molecule in its movements and recognize its speed. He opens the door only to fast or slow molecules, forcing them to move to different parts of the container. Thus, in one part of the container the temperature and pressure are getting higher than in the other which yields the unlimited supply of energy at no costs. In case Maxwell's demon remains on duty forever (or several Maxwell’s demons do shifts), the container can be turned into full-functioning “perpetuum mobile”.
Many prominent scientists argue that Maxwell's demon was merely a joke of the great physicist. Indeed, Maxwell's demon is not effective in a container with two molecules. In that case, all molecules might end up in just one part of the container to begin with. However, if the number of molecules is high, the probability of such a case is extremely small.
Maxwell’s demon keeps occupying many scientists’ minds. Not long ago, there was an article published on that issue in one scientific journal that explains the existence of Maxwell's demon in a form of a quantum oscillator, a laser which separates the molecules with high energy from the low-energy molecules.

Recently, the debate on the existence or non-existence of Maxwell's demon was enforced by the quantum entanglement theory that aims at developing supercomputers, instant Internet and even teleporting devices. In their recent paper Japanese physicists showed that quantum effects make information translation into energy to be more efficient than previously thought. Although their paper was not yet accepted for publication, its pre-print is available at Cornell University’s archive and can be downloaded here: http://arxiv.org/abs/1207.6872.

The Japanese scientists’ major innovation is the introduction of quantum entanglement into Maxwell’s experiment. In their extension of Maxwell’s experiment, there were two vessels with quantum entangled particles. By measuring the rate of only one of these particles, one can obtain information about the other. If all the particles in the experiment are entangled in pairs, this means that the demon will need only half of the time to recognize the nature of the particle than in the case of classical particles. The authors performed statistical calculations that take this into account and showed quantum entanglement would effectively increase the performance of Maxwell’s demon.

So far, no one has proved whether Maxwell’s demon exists or whether it is just a myth. But one thing is certain - Maxwell's demon might help scientists to heat up the interest in further research in natural sciences!

Happy Halloween everyone!

EL and WS

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