This was a theory that devised a way the Second Law of Thermodynamics might be circumvented. That law says that heat energy cannot be created, it can only be transferred.
In 1847, James Clerk Maxwell came up with this idea –
Say you had a box with two sides, separated by a gate. A demon would watch over the gate, and when a fast-moving atom would be moving towards it (in a single direction, say left-to-right), he would open the gate to let it through to the right side. When a slower-moving atom was moving from right-to-left, he would let that through to the left side.
Over time, this means that one side of the box (the right side, based on our directions above) would, over time, contain more fast-moving atoms, and the left side would contain more slow-moving atoms. Thus, the right side would be “hotter.”
Essentially, the demon increased order in the universe (reduced entropy) and generated heat energy without actually doing any work. He organized his way to energy.
The significance is that this bridges the gap between physics and information science. In effect, the demon created energy (physics) by increasing organization (information science). One of the criticisms of the theory, in fact, is that determining the speed of the atoms would require the demon to expend energy, which again links the fields of physics and information science.
(And if there’s one thing the scientific community loves, it’s when branches of science come together. This is called “convergence.” I read an entire book about it: Convergence: The Idea at the Heart of Science)
Interestingly, the computing term “daemon” is from Maxwell’s Demon. Maxwell’s version sorted atoms in the background. Computer daemon’s also do information processing in the background, hence the name.
Even more interestingly, Maxwell never used the word “demon.” He first articulated the theory in a 1867 letter to a colleague. There, he referred to the sorter as “a finite being.” The transition to “demon” was added by others after the fact, though sources differ on who changed the word and exactly why they did it.