FactReactorYour Body Is A Cosmic Ray Detector
Watch on YouTubeSomething 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
Clarification
The beat there was no room for, and it is the correction a physicist would leave anyway: saying "its clock runs slow" is only half the picture. From where you are standing, yes - the muon's clock is crawling and that is why it survives the trip. But from where the muon is standing nothing about its life is stretched at all. It gets the ordinary 2.2 microseconds. What changes in its frame is the distance: the fifteen kilometres of atmosphere contract to a few hundred metres, and the fall is simply short. Same particle, same arrival, two ways of writing it down, and neither one is more true than the other.