What is Maxwell's demon, and how was it resolved?

Maxwell's demon is a thought experiment from the Scottish physicist James Clerk Maxwell in 1867. A tiny imaginary being sorts fast and slow gas particles and seems to lower entropy without any cost. The standard resolution, completed by Charles Bennett in 1982, is that the demon must record information about the particles and eventually erase it, and the erasure produces at least as much disorder as the sorting removes. [1]

Maxwell imagined a room of gas split into two compartments by a wall with a small door. The average speed of the particles corresponds to temperature, but at any moment some particles move faster than others. A demon at the door lets fast particles pass into the right-hand compartment and slow ones into the left. The left side becomes cold and the right side hot, so heat flows from cold to hot without apparent work. That seems to contradict the second law of thermodynamics, in the form stated by Clausius, and to make the isolated system more orderly instead of less. [1] [2]

Two advances were crucial. In 1948 Claude Shannon showed that the information content of a message can be quantified as information entropy. Then in 1961 Rolf Landauer showed that any logically irreversible operation, such as erasing a memory, must convert a minimum amount of work into heat dumped in the environment, with a matching rise in entropy. Landauer summed this up as "information is physical." [1] [2]

In 1982 Charles Bennett combined these ideas. He recognized the demon as an information-processing machine that must store a record of each particle to decide when to open the door. A finite memory eventually fills, and clearing it costs work. According to Landauer's principle, the entropy increase from erasure more than compensates for the decrease from sorting, so the second law holds. The University of Bristol describes Bennett's insight as being that the demon's difficulty is not placing the molecules but that learning where they are fills its memory. [1] [3] [2]

The framing is often described as three steps: the demon measures a molecule's speed and records it, acts on the molecule by opening or closing the door, and then resets its memory. [4]

An APS Physics commentary notes that this account is not fully satisfying, because it raises the question of who or what erases the demon's memory, and it does not give a recipe for building a demon. Physicists at Aalto University in Finland reported a nanoscale device using two coupled single-electron boxes that acts as a self-contained, or autonomous, demon with no external control. It cooled the system while the demon warmed, in line with a simple model based on mutual information, the correlation between system and demon. The commentary stresses that this does not make the second law breakable. Instead, the law must be formulated so that some entropy production is attributed to the information the demon gains. [2]

Other laboratory realizations followed in the 21st century: a light-powered gate in 2007, a demon using information to move a bead uphill in 2010, and in 2016 an experiment applying the idea to light instead of gas, which was able to charge a very small battery. [1]