All the Demons

Demon is derived from an ancient Greek word, originally from the Proto-Indo-European *dai-, meaning to divide or to distribute. This implied that a demon doled out fortune or fate, something between a god and a human.

This is the meaning Pierre-Simon Laplace had in mind when he described a demon that knew two things about the universe. First, it knew the position and direction of movement of every particle there was. Second, it knew all the laws of physics.

Laplace’s demon was useful as a thought experiment because it could do things regular human minds just couldn’t do. For one thing, if you could reconstruct the past with complete accuracy, so you could basically create a completely accurate film of any past event. Humans can’t do this, but Laplace’s imaginary demon could.

Perhaps more eerily, the demon could also see the future with absolute certainty. Profound questions about the nature of free will in an apparently deterministic universe are very much at the forefront of physics today, and Laplace was tackling them with his demon two centuries ago, writing around 1814.

Think about it for a moment: if you know everything that’s going to happen, what will happen if you try to deviate from the cosmic script? The TV show Devs does a fantastic job of showcasing Laplace’s demon in the 21st century. I don’t want to spoil the show in case you want to watch it, but if this sort of thing interests you, it’s well worth a watch.

Demons like this were all the rage during the Victorian era. James Clerk Maxwell devised the most famous of all of these thought experiments involving a demon, when he invoked a little magical being to do the dirty (impossible) work. Here’s how I described what’s now called Maxwell’s Demon:

You start with a box full of a gas. It’s not really too important what kind of gas is in there, but what matters is that there are lots of atoms in there, flying around all over the box. It seems to be a frenzy of random activity upon first glance.

After you observe the box carefully for a few minutes, though, you begin to notice that some of the atoms are flying around really quickly, while others are moving very slowly by comparison. This gives you an idea.

You’ll make a little wall right there in the middle of this box, dividing the right side from the left. Then, you’ll place a tiny door there that can be opened or closed at just the right time. This little portal can allow one atom at a time to slip through the wall, over to the other side of the box.

The idea you have is to let only the cold atoms to enter over on the right side, while only the hotter ones stay on the left side. After a while, you would start to see stragglers—fast particles on the right or slowpokes on the left. Each time you get the chance, when one of those particles is about to hit your wall in the middle, you just open that tiny portal.

Only, Maxwell knew that you couldn’t really do this, so he invoked a tiny supernatural creature he called his demon to do his dirty work. His demon would make sure that the right side had only cold particles, and the left side had only hot particles. One way to interpret this was that you could just capture heat from anywhere in the universe, isolating into a small area and using it whenever you needed to.

Maxwell’s and Laplace’s Demons set the stage nicely for one more really big thought experiment involving an imaginary entity. Ludwig Boltzmann was a prominent Austrian physicist based in Vienna, a hotbed for bold academic thought where dozens of great minds were drawn together by an exciting atmosphere of innovation.

Boltzmann picked up where Maxwell left off. He knew that Maxwell’s Demon had to use intelligence to separate those cold and hot parts, and he knew that this meant there was no such thing as a free lunch. In other words, Maxwell’s Demon couldn’t be real because it cost energy to identify particles in the first place.

However, Boltzmann came up with something really clever to make a point. He recognized that the 2nd Law of Thermodynamics states that entropy of a system increases over time, but based on incredibly large statistics, it’s also possible for a local region to become more organized all on its own.

As long as the total system’s entropy increases, the 2nd law is not violated. A small space can spontaneously organize itself, in a manner of speaking.

This is where the idea of a Boltzmann brain comes to be. The idea is that, in a universe with a kajillion-jillion possibilities, everything that can happen will happen. That’s a bit of an exaggeration if you don’t assume the universe is infinite, but if you have enough raw ingredients, you are bound to end up with the right combination if you roll the dice enough times.

The implications are profound. Are you, dear reader, a Boltzmann brain?

Or, are the laws of physics deterministic, like Laplace thought, with no way possible to deviate from the prescribed course? Are there other sorts of demons we should be discussing today?