# How do we know that trillions of neutrinos pass through our body if we almost cannot detect them in the first place

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I’ve seen many videos on the topic, but none seem to answer the question, How do we know there are trillions of neutrinos passing through our body when we have a really hard time detecting them. Why do we say there are so many of them when we cannot prove it?

In: Physics

Physicists have the premise that nothing just appears out of nowhere or vanishes without a trace. Much like balancing a chemical formula, the interactions of tiny particles must balance when something decays. In the case of beta decay a particle was theorized would need to be emitted, the neutrino, but due to the properties it must have it would be very difficult to detect.

We know beta decay is happening in the sun in huge quantities so if this particle was being emitted we would expect huge numbers of them to be passing through us, and only very rarely interacting in ways we can detect.

It turns out that we can build detectors to find those very rare interactions. Since we expect them to be extremely rare even if there are huge numbers of them around, even our small number of observed interactions is enough to prove their existence and their high volume. If there weren’t trillions passing through us then not only would beta decay in the sun be unexplained, we also wouldn’t know why the neutrinos that we did detect were inexplicably interacting far more frequently than expected.

Usually when we look for subatomic particles, we already know what we’re looking for before we check. No one built a particle detector, switched it on, and suddenly found neutrinos. We predicted the existence of neutrinos based on our other physics models, and then we built detectors after the fact to see if we were right.

We looked at the sun, and noted that some particular reactions that are going on inside should be making neutrinos. Given the size of the sun, we can estimate how many are made. Given how far Earth is away, and how big Earth is, we can estimate how many reach us. And if we design a detector that can spot them, we can estimate how many the detector *should* spot. We then build that detector, switch it on, collect data, and see what we find. If we get the number we expected, it means our original hypotheses were likely correct. If not, it means we have to revise our hypotheses until they can explain what we’re actually seeing.