A muon is a fundamental particle. It is exactly like an electron, but heavier. About 207 times heavier. It carries the same negative charge, has the same spin, and behaves the same way in an electric or magnetic field. The only meaningful difference is mass, and one consequence of that extra mass: a muon is unstable, and it decays in about 2.2 microseconds.
That short lifetime is why most people have never heard of muons, even though billions of them are passing through them right now. They do not exist in ordinary matter. You cannot mine them, store them, or order them from McMaster-Carr. Every muon you ever encounter was made within the last few milliseconds in Earth's upper atmosphere by a cosmic ray collision.
Where muons come from
Out in space, particles from the sun, from supernovae, and from sources we still cannot identify travel at energies that dwarf anything we can produce in a laboratory. A single high-energy cosmic ray proton can carry more kinetic energy than a fast-pitched baseball, compressed into a single particle.
When one of these primary cosmic rays hits the top of Earth's atmosphere, it slams into a nitrogen or oxygen nucleus. The collision shatters the nucleus and produces a spray of secondary particles, many of which are pions, a heavier cousin of the muon. Pions are even more unstable than muons. They decay almost immediately, mostly into muons plus a neutrino. So every muon raining down on the planet started as a pion, which started as a piece of a nucleus, which got hit by something old and fast from deep space.
The shower spreads out as it travels down. A single primary cosmic ray of high enough energy can produce billions of secondary particles, including thousands of muons, splashing across kilometers of ground. At sea level, the muon flux is roughly 1 per square centimeter per minute. Hold up your palm; a few muons pass through it every second.
The relativistic puzzle
Here is the part that should not work. A muon at rest decays in 2.2 microseconds. Even traveling at the speed of light, a particle with that lifetime can only cover about 660 meters before it disintegrates. The atmosphere is far thicker than that. By any classical calculation, no muon should ever reach the ground.
And yet they do. Trillions of them, every day. The reason is special relativity. From the muon's own frame of reference, it lives its full 2.2 microseconds before decaying. But from our frame of reference, watching it streak down through the atmosphere at close to the speed of light, time runs slower for it. Its clock is dilated. To us, the muon lives long enough to traverse tens of kilometers of air. To the muon, the distance is contracted into something it can comfortably cross before decaying.
This is not theoretical hand-waving. Muons are one of the cleanest, most direct experimental confirmations of relativistic time dilation. They are walking proof that Einstein was right.
Why anyone would build a device around them
Muons have two properties that make them unusually interesting for hardware. First, they are penetrating. Most other secondary cosmic ray particles get absorbed by the air or by the first centimeter of any solid material. Muons walk through stone, lead, and people without slowing much. Second, their arrival is genuinely random. You cannot predict when the next muon will reach a particular square centimeter of ground. The arrival time depends on conditions in interstellar space billions of years ago, which is about as far outside any computer's control as it is possible to get.
Those two properties are exactly what you want if you are trying to build a true hardware random number generator. A muon arrival is a coin flip with the rest of the universe. More on why muons make excellent randomness sources here.
How to detect a muon at home
Muons ionize air. When one passes through a sealed glass tube full of low-pressure gas with a high voltage across it, the ionization triggers a tiny avalanche of charge that you can pick off as an electrical pulse. The classic instrument for this is the Geiger-Müller tube, the clicking detector that defined every Cold War film. A J305 GM tube, the kind I use in Muon Sortes, is a small glass cylinder about the size of a finger, biased at around 410V.
The problem is that a single Geiger tube does not just count muons. It clicks for any ionizing event: cosmic ray muons, but also alpha particles, beta particles, gamma photons, and ordinary terrestrial background radiation from concrete, granite, bananas, and the user. To pick out muons specifically, you need coincidence: two tubes stacked vertically, looking for events that fire both at almost the same instant. Most background radiation is too weak to cross both tubes. A muon is penetrating enough to do it routinely. The coincidence trick gets its own writeup.
Why I built a clock around this
Because every random number a coincidence-detector RNG produces is sourced from a fresh particle born high in the atmosphere when something old and fast hit Earth. The decision is drawn from a sample with no terrestrial cause. The clock keeps time. The sky keeps the verdict. Reserve a numbered unit here.