It so happens that “demons” are a nickname gifted to certain thought experiments in physics, philosophy, computer science, and other areas. Here are some of them.
Maxwell’s Demon
The Scottish physicist and mathematician James Clerk Maxwell first mentioned this demon in a letter to Peter Guthrie Tait (another physicist and a pioneer in thermodynamics) in December 1867.
The demon was mentioned again in a letter to John William Strutt (a physicist and third Baron Rayleigh) in 1871, before Maxwell included it in his book on thermodynamics Theory of Heat in 1872.
Being a very religious person, he didn’t use the term “demon” but called it a “finite being.”
In Maxwell’s head, this “finite being” would control a portal between two chambers filled with gas. As each gas molecule came near the portal, this “being” would open and close the doorway quickly to allow fast-travelling molecules to pass through one way, and only slow-moving molecules to pass through the other way. This would make one chamber heat up whilst the other cooled down, because the kinetic temperature of gas relies on the speeds of its molecules.
This would decrease the total entropy of the two chambers, which would break the second law of thermodynamics (this law states that a system left to itself evolves into a state of equilibrium, or high entropy / more dispersed and disorganised, not lower / more ordered.
Maxwell’s “demon” or thought experiment was only monikered a demon in the journal Nature in 1874 by William Thompson (Lord Kelvin, a Scottish physicist, mathematician and engineer). He meant the term demon in the same way Greek mythology used the term daemon, i.e. a supernatural entity working in the background and not something evil!
Other scientists used forms of Maxwell’s demon in experiments (though they all differed to his original thought experiment in some way) and none have been found to break the second thermodynamics law. The reasoning behind the demon not being able to break this law are as follows:
- a demon would create more entropy / disorder trying to segregate the molecules than it could ever eliminate
- the demon would need some method of measuring the speed of the particles, and acquiring that information would cause the demon to expend more energy, increasing the entropy within the demon itself even as the entropy of the gas was lowered, so the system’s disorder would still increase.
- even if the demon used a method of measuring the particles’ speed that was thermodynamically reversible, it would need to either discard or store the data it was gathering. Discarding it would still increase entropy / disorder, and eventually it would run out of space to store all that data so would have to start discarding it anyway. Messy creatures, demons.
- the uncertainty principle would prevent the demon from sorting the molecules, because it wouldn’t be able to find their exact locations in the chambers when it measured their momentums.
(Interesting fact: Daemons (or daimons) in computing [certain processes that run on servers to respond to users] were named after Maxwell’s demon. They hide in software and computer systems, waiting for the right moment to emerge and do something either good or bad, before burrowing away out of sight once more. They are created by hackers, software designers, spies, and scientists, and they have come about from the growth of AI. These daimons can learn and even give themselves tasks. Examples of daimons can include chatbots.)
Laplace’s Demon
In 1814, the French polymath Pierre-Simon Laplace looked at how Isaac Newton predicted how an apple would fall according to the law of gravity and wondered if the same way of thinking could be used to predict everything?
Laplace imagined the idea of a demon – or “intellect,” in his words – that was able to know the position and momentum of every single particle in existence and all the laws of nature. Such an intellect would be able to calculate the entire future of the universe and that nothing would be uncertain.
Physicists believe that no such intellect can have all of this knowledge. For example, according to Einstein’s theory of special relativity, no information can travel faster than light, so although some events can affect the future, we can’t know about them in the present moment. Information about those events just doesn’t have time to reach us.
Even if the intellect could get information from all of the universe, due to the uncertainty principle, that intellect couldn’t possibly know exactly where each particle was and what it was doing. All it would be able to do was guess the probabilities of each particle’s properties, which would involve keeping track of a mind-boggling array of outcomes, and it wouldn’t know which one of those outcomes was going to be the real one. Cue one demon with a migraine.
This demon inspired the British mathematician Charles Babbage’s invention of the early computer, goaded Charles Darwin into thinking about the development of life, and caused Schrödinger to wonder whether something with similar powers could be in charge of cellular order.
Laplace’s demon existed for around 100 years and was “exorcised” at the beginning of the 20th century when quantum mechanics was discovered along with the uncertainty principle.
Loschmidt’s Demon
After steam engines had been invented and we were in the throes of the Industrial Age, Austrian physicist Ludwig Boltzmann wondered how to explain entropy (the process by which everything becomes more disorded and messy over time). He reckoned he could explain it by looking at the tiny parts of big systems, e.g. particles of gas filling a room.
His older colleague Josef Loschmidt, however, doubted his methods and posed a challenge in 1876. He said to imagine the universe frozen in time, then reverse the direction that each particle was travelling in. (This original challenge didn’t involve a demon, but later on one was added that could see and freeze particles.)
At the particle level, reversing their movements – and therefore reversing time – would show nothing awry, but if you zoomed out and looked at larger structures, you would see events and objects reversing in time and undoing themselves! This gives rise to the question of why time only seems to run in one direction for us, if time can easily be reversed on a microscale level?
Later experiments would attempt to reverse time, inspired by Loschmidt’s demon. In the 1950s, Erwin Hahn used radio waves to temporarily push electric dipoles (like the hydrogen atoms in water molecules) into rotating in sync, thereby lowering entropy. This made it look like time was going backwards for the molecules. Once he switched off the radio waves, the molecules fell back into disarray.
Nowadays, we understand entropy doesn’t mean that systems always have to descend into chaos. Some systems even change into things that have more order to them, but only temporarily. (Entropy is the ultimate endpoint for everything, though.)
The universe began in a very ordered state, which gave it only one direction to go in… towards a messier state. There are many more ways to wreck an ordered system than to make it even more organised, making disorder much more likely.
So Loschmidt’s demon could reverse the movement of particles, but it would find it a real bugger.
Nietzsche’s Demon
This was a being imagined by the philosopher Friedrich Nietzsche in his book The Gay Science in 1882. In it, he proposed a being that whispers in your ear one lonely night to tell you that you have to live your life over and over again, “every pain and every joy and every thought and sigh and everything unutterably small or great in your life will have to return to you… all in the same succession and sequence.”
This demon would supposedly force you to take full responsibility for your own life.
Descartes Demon
In Meditations in First Philosophy, the 17th century French scientist and philosopher Rene Descartes imagined a demon that could put you into a convincing fabrication of reality and make you think that it was real life. Could this demon completely fool you, and could it fool you forever?
According to Descartes, we could defeat the demon, because it wouldn’t be able to force a conscientious thinker to state “I do not exist” and truly believe that statement.
This demon inspired the idea behind the film The Matrix.
Bacon’s Demon
Mentioned by the philosopher Francis Bacon, this was the sphinx which haunted the wilderness around the city of Thebes. It had birds’ wings and the claws of a gryphon, and it would ambush travellers before asking them perplexing riddles. Oedipus managed to answer the riddle and kill the sphinx.
Francis Bacon said that Oedipus represented science and the sphinx represented nature, and that though nature could be dangerous, the rational thinking of science could understand it and render it harmless.
Miscellaneous
- Einstein “exorcised” the Filon-Pearson demon that could travel faster than light, teleport, act at a distance, and used a force named “gravitation” (not space-time) to push and pull things.
- There are quantum Maxwell’s Demons, nanoscale demons, and nuclear magnetic resonance demons
- Norbert Wiener, a mathematician, “bred” Laplace’s and Maxwell’s demons to imagine a cybernetic demon that can learn from feedback.
- John Wheeler, a cosmologist, proposed demons that live in black holes, feasting on energy and information and appearing to make entropy vanish.
- The philosopher John Searle came up with a demon that lives inside the brain and eats neural synapses.
Sources:
https://en.wikipedia.org/wiki/Maxwell%27s_demon
How 3 imaginary physics demons tore up the laws of nature
Demons to think with: the rebels you mustn’t ignore
https://aeon.co/essays/why-physics-today-stands-on-the-wings-of-angels-and-demons
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