If a neutrino do not interact with anything and can pass through a light year of lead, how humans manage to detect them?
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A lot of comments here on modern neutrino experiments which are great, but it sounds like you are interested in the very first experiment to do so. It won its creators a Noble Prize and was called “Project Poltergeist”. The name was somewhat a joke as seeing a neutrino would be like seeing a ghost.
The experiment was completed in 1956 by physicists Cowan and Reines. Reins had worked on the Manhanttan project and knew that an atom bomb would create absolutely huge amounts of neutrinos. As other commentors have said, Neutrinos do interact, it’s just incredibly rare. So if you are going to detect one you need a butload of neutrinos.
So Reines designed a method for detection. If you have a massive tank of water near a source of neutrinos there is a small chance the neutrino will interact with the hydrogen atoms creating a neutron and positrons. The positron would then interact with electrons in the water and annihilate each other and release 2 gamma rays. There was already a method to detect Gamma rays.
There was only one problem with this experiment. How do you build a water tank and gamma ray detector that can survive being 100m away from a nuclear explosion?
Turns out you can’t. Reines sat on this idea for many years (Cowan helped improve the design by adding Cadium Cholride to the water but thats beyond ELI5). Eventually another colleague let him know that these new devices called “Nuclear reactors” may create the butload of neutrinos needed to dectect.
So Reines and Cowan set up their dector at the Savannah River Site where there were 5 reactors running. Their detector worked, they won a Noble Prize and the rest is history…
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