If a neutrino do not interact with anything and can pass through a light year of lead, how humans manage to detect them?


If a neutrino do not interact with anything and can pass through a light year of lead, how humans manage to detect them?

In: 6

Neutrinos interact with matter via the [weak interaction](https://en.wikipedia.org/wiki/Weak_interaction). So while indeed most of them will pass through a detector without interaction, a small fraction will interact. When a neutrino scatters off an atom, it can create a charged particle and/or break apart the nucleus. It’s these things that neutrino detectors detect. Neutrino detectors are large volumes of some kind of material that gives a detectable signal (e.g. a flash of light) in response to ionising radiation. They also need to be very well shielded from other sources of radiation, so that the background noise doesn’t overwhelm the neutrino signal.

Neutrinos interact via two of the four fundamental forces: gravity and the weak force, which is… well, weak.

When a neutrino passes through a detector, there is a *chance* of it being detected, but that chance is low. Thankfully, there are so many neutrinos passing through earth that some will be detected. What we do is wait for an interaction to happen via the weak force (eg transfering some of its momentum to a nearby electron via the weak force (specifically a W boson)), and then we detect the effects on the things that were interacted with (eg in our example, the electron can be accelerated to relativistic speeds, and so if we put it in heavy water then there will be a flash of light from Cherenkov radiation).

Well, the chance of a neutrino interacting with any matter is really really small. Luckily, though:

* The nuclear reactions in the Sun put out a HUGE amount of neutrinos; 100 billion neutrinos pass through your fingernail every second. If we could see in “neutrino” the Sun would shine extremely brightly. We’re basically positioned very close to a huge and constant source of neutrinos.

* The detector facilities are [huge](https://i0.wp.com/eusci.org.uk/wp-content/uploads/2020/05/Kamioka.jpg?fit=786%2C427&ssl=1) and those cameras are sensitive enough to detect the flash of energy from a single interaction.

So basically, the size of the detector is huge, and the amount of neutrinos passing through the Earth is huge, and scientists have built a number of detector facilities. So overall we detect about 30 per day, about one per hour, across all the detector facilities that have been built.

With a lot of difficulty.

Neutrino detectors [tend to be really, really big](https://www.businessinsider.com/super-kamiokande-neutrino-detector-is-unbelievably-beautiful-2018-6).

Usually giant water tanks (some using heavy water), buried deep underground, surrounded by detectors that look for interactions between neutrinos and stuff in the water.

Neutrinos do interact with things, but only via the weak interaction (and gravity, but only in negligible amounts). That detector linked above is the [Super-Kamiokande detector](https://en.wikipedia.org/wiki/Super-Kamiokande) in Japan; a tank containing 55,000 tons of water, 1,000m underground, with 13,000 light detectors around it. Specifically it detects Cherenkov radiation which is a weird thing you can get when neutrinos interact (weakly) with electrons or the nuclei of atoms.

About 10^11 neutrinos pass through each square centimetre of the Earth’s surface each second. The Super-Kamiokande detector (with a surface area of over a thousand square metres) detects about 8 per day.

We detect neutrinos with a lot of difficulty, by having really big detectors, and knowing that huge numbers of neutrinos pass through every second.

The premise of the question is incorrect, neutrinos do interact with other things.

The probability of the interaction is just very low so you need a lot of them to get a few interactions.

Interaction is not a binary always interact or never interact. If you shine visible light on a wooden wall it will not pass through. But instead, use radio waves and X-rays and a large part of them will get through. These are just photons with different wavelengths and they interact with matter in quite different ways.

So radio waves can pass through matter a lot easier than visible light, The same way neutrinos can pass through matter many times easier than radio waves. If the chance of interacting is low enough it can likely pass through a light year of lead. But that does not mean that some neutrons interact with the first meter of the lead, it will only be a minuscule fraction that does that.