Page "Maxwell's demon" Paragraph 10

In 1960, Rolf Landauer raised an exception to this argument.

He realized that some measuring processes need not increase thermodynamic entropy as long as they were thermodynamically reversible.

He suggested these " reversible " measurements could be used to sort the molecules, violating the Second Law.

However, due to the connection between thermodynamic entropy and information entropy, this also meant that the recorded measurement must not be erased.

In other words, to determine whether to let a molecule through, the demon must acquire information about the state of the molecule and either discard it or store it.

Discarding it leads to immediate increase in entropy but the demon cannot store it indefinitely: In 1982, Bennett showed that, however well prepared, eventually the demon will run out of information storage space and must begin to erase the information it has previously gathered.

Erasing information is a thermodynamically irreversible process that increases the entropy of a system.

Although Bennett had reached the same conclusion as Szilard ’ s 1929 paper, that a Maxwellian demon could not violate the second law because entropy would be created, he had reached it for different reasons.

On the other hand, the demon could store a limited amount of information, thus finitely limiting the entropy upon releasing it-e. g.

remember only one molecule-if the current one is faster, let it by the gate, if it is slower, stop it.