Your Body Is A Cosmic Ray Detector

Something went through your head while you read that. It was made about fifteen kilometres up, when a cosmic ray from somewhere in the galaxy hit an air molecule and burst into a shower of debris.

What reaches the ground is one splinter of that shower: a muon. About one of them crosses a hand-sized patch of you every second, and it goes through skin, skull and bone without being slowed enough to notice.

Here is the part nobody uses. A muon lives about 2.2 microseconds. Even at nearly light speed that buys it only a few hundred metres, and the fall from fifteen kilometres takes roughly fifty microseconds - more than twenty of its lifetimes. The naive sum says it should be long gone before it gets anywhere near you.

It arrives anyway, because at that speed its clock runs slow. That is special relativity, and it is not something demonstrated once in a lab with an accelerator. It is being demonstrated through your skull about once a second, and has been every second you have been alive.

You are not watching a relativity experiment. You are standing in one.

Sources:
https://pdg.lbl.gov/2022/reviews/rpp2022-rev-cosmic-rays.pdf
https://pdg.lbl.gov/2023/listings/rpp2023-list-muon.pdf
https://en.wikipedia.org/wiki/Muon
https://hyperphysics.gsu.edu/hbase/Particles/muonatm.html