Maxwell's demon

According to the second law of thermodynamics, a closed system will always increase in entropy until it reaches a state with as high as possible entropy. The entropy will never go down. It is therefore not possible that two bodies with an equal temperature in thermal contact with each other, and together forming a closed system, evolve to a state where one body has a higher temperature than the other.

James Clerk Maxwell imagined an imaginary creature in a thought experiment to contradict the second law of thermodynamics. It was his friend William Thomson (later Lord Kelvin) who named the being Maxwell's intelligent demon.

Suppose that you have a box filled with a gas or a fluid at some temperature. This means that the average speed of the molecules is a certain amount depending on the temperature. Some of the molecules will be going faster than average and some will be going slower than average. Suppose further that a partition is placed across the middle of the box separating the two sides into left and right. Both sides of the box are now filled with the gas or fluid at the same temperature. Maxwell imagined a molecule sized trap door in the partition with his minuscule creature poised at the door who is observing the molecules.

When a slower than average molecule approaches the door he makes certain that it ends up on the left side (by opening the tiny door if it's coming from the right) and when a faster than average molecule approaches the door he makes sure that it ends up on the right side. So after these operations he ends up with a box in which all the slower than average gas molecules are in the left side and all the faster than average ones are in the right side. So the box is cold on the left and hot on the right. Then one can use this separation of temperature to run a heat engine by allowing the heat to flow from the hot side to the cold side: A perpetuum mobile.

This imaginary situation seemed to contradict the second law of thermodynamics. Perpetual motion is motion of bodies that continues indefinitely. Promises. Machines were built. None delivered as expected. Scientists were perplexed until in 1929 Szilárd pointed out that a real-life Maxwell's demon would need to have some means of measuring molecular speed (in the form of photons for example), and that the act of acquiring information would require an expenditure of energy. And the demon itself (plus the trap door mechanism) would gain entropy from the gas as it moved the trap door. Since the demon and the gas or fluid are interacting, we must consider the total entropy of the gas or fluid and the demon combined. The expenditure of energy by the demon will cause an increase in the entropy of the demon, which will be larger than the lowering of the entropy of the gas. Thus the total entropy of the system still increases. Or?

Researchers at an Aalto University laboratory, in collaboration with Stony Brook University and Tokyo University, have been successful with the famous thought experiment that has haunted the scientific world for more than 150 years. Experiments show how an electronic Maxwell's demon converts information to energy with the help of nano technology. This achievement could lead towards a more energy-efficient future.