David Kaiser in Aeon:
Every fraction of a second, invisible particles called neutrinos whiz past the vans and Winnebagos on Highway 169 headed toward McKinley Park in northeastern Minnesota, just shy of the Canadian border. Having begun their journey at Fermilab, an imposing physics laboratory outside Chicago, some of those speeding neutrinos smack into 5-ton slabs of steel within an underground mine in the town of Soudan, Minnesota (population: 446), sending sparks of charged particles arcing toward sensitive detectors. Quite unlike the camper vans and RVs, the neutrinos complete their journey – 735 kilometres across the Upper Midwest – in less than a 3,000th of a second.
Neutrinos are fundamental to the construction of the Universe. They are tremendously abundant, outnumbering atoms by about a billion to one. They modulate the reactions that cause massive stars to explode as supernovas. Their properties provide clues about the laws governing particle physics. And yet neutrinos are among the most enigmatic particles, largely due to their reticent nature: they have no electric charge and practically no mass, so they interact only extremely weakly with ordinary matter. Some 65 billion of them stream through every square centimetre of your body – an area the size of a thumbnail – every second, without your ever noticing them.
Through elaborate sleuthing, physicists have identified three distinct types of neutrinos, which differ in their subtle interactions with other particles. Stranger still, the neutrinos can ‘oscillate’ between types, shedding one identity and adopting another as they travel through space. That discovery led to a significant expansion of the standard theory of how particles behave.