Right now, several particles a minute are passing through your head at close to the speed of light. They are muons, made when cosmic rays from beyond the solar system smash into the upper atmosphere, and they have been arriving your whole life. You have never seen one.

A cloud chamber makes them visible — not the particle itself, but its wake. The trick is to create a layer of air supersaturated with alcohol vapour: air holding more vapour than it should be able to, poised to condense but with nothing to condense onto. When a charged particle rips through, it strips electrons from the molecules along its path and leaves a thin trail of ions. Those ions are exactly the seeds the vapour was waiting for, and a line of droplets condenses along the track.

You are seeing a contrail, microseconds old, left by something you cannot see and that has already gone.

The tracks come in kinds, and you can tell them apart. Alphas are short, fat and straight — heavy and highly ionising, they stop fast. Betas are thin and wandering, because a light electron gets knocked about. Muons are long, straight and thin, crossing the whole chamber without deviating, because they are heavy, fast, and barely interact.

C.T.R. Wilson built the first one in 1911 and won the 1927 Nobel Prize for it. The always-on version you will build — a warm alcohol reservoir over a plate chilled with dry ice — is Alexander Langsdorf's diffusion chamber of 1936.

Einda

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The Cloud Chamber