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Neutrinos:

Neutrinos are very small electrically-neutral elementary particles. The word 'neutrino' is actually Italian for 'little neutral one.' They're called elementary particles because they cannot be broken down into smaller particles. Sometimes, they're called subatomic particles because of their small size, but they aren't actually found inside atoms.

Neutrinos were first predicted back in 1930 by physicist Wolfgang Pauli to explain what happened in radioactive beta decays. A beta particle is an old-fashioned name for an electron (or its antiparticle, the positron). Thus, radioactive beta decay is when an atomic nucleus emits an energetic electron or positron. From this you might deduce that 'radioactive' just means prone to emitting things such as particles--and you would be correct. So, anyway, Pauli realized radioactive beta decay violated conservation of energy because some of the energy involved seemed to disappear. He hypothesized there must be a hard-to-detect particle there that carried away the seemingly-missing energy. Other physicists liked this idea because it saved one of their favorite laws, namely, conservation of energy.

It wasn't until 1956 that scientists Clyde L. Cowan and Frederick Reines discovered neutrinos by setting up detectors inside a large tank of water near a nuclear reactor. Their experiment worked despite the fact that neutrinos are hard to detect, because the reactor produced thousands and thousands of neutrinos. Since there were so many neutrinos, once in a while, one would smash into one of the many protons in the water, creating a neutron and a positron. They detected the light energy given off when the resulting positron annihilated an electron in the water. So, mission accomplished! This point is worth emphasizing: scientists know neutrinos exist because they have seen evidence of them in experiments.

Why are neutrinos hard to detect? One reason is they're electrically neutral, so they're not affected by electromagnetism. Another way to say this is: they don't interact with, or feel, electromagnetic radiation. Originally, neutrinos were also thought to have zero mass, and if so, they wouldn't be affected by gravity. To put this in more familiar terms: something with zero mass weighs zero pounds.

Practically the only thing neutrinos do feel is a force called the Weak Force. As the name implies, the Weak Force is well, weak. It's not as powerful as electromagnetism and it only operates over very short distances. In fact, it was beta decay that led physicists to hypothesize the existence of this Weak Force; no other forces could explain beta decay. The Weak Force is important in nuclear fusion. Because of the mathematics behind the Weak Force, physicists hypothesized that there were three different kinds, or flavors, of neutrinos, called electron neutrinos, muon neutrinos, and tau neutrinos.

Neutrinos are created in supernovas and a lot were created in the Big Bang (the giant explosion that started our universe). It turns out the greatest local source of neutrinos is our Sun. Neutrinos are produced in the core of the sun via nuclear fusion reactions. The most important of these is when the Weak Force enables two protons interact to form a deuteron (a proton combined with a neutron), an anti-electron called a positron, and a corresponding electron neutrino.

When scientists did experiments to detect these electron neutrinos coming from the Sun, it seemed like a bunch of them were missing. For a while this mystery was called the Solar Neutrino Problem. The great thing about a scientific mystery is it enables us to learn new stuff. The solution to the Solar Neutrino Problem was: the Sun did emit the expected number of electron neutrinos but some of them changed into the other kinds of neutrinos on the way to Earth. This is called neutrino oscillation and it had a further consequence: neutrinos must have a non-zero mass. Therefore, neutrinos do feel the gravity force. This discovery of neutrino oscillations and consequent neutrino mass was awarded the Nobel Prize in Physics in 2015.

You might be wondering why you should care about neutrinos. Fun fact: tens of thousands of neutrinos are passing through your body right now! Do you feel them? Probably not. You don't feel them because neutrinos hardly interact at all with the regular matter that makes up your body.

Neutrinos are still a very active area of research in physics.

For example, see Viewpoint: The Plot Thickens for a Fourth Neutrino from November 26, 2018.

Read about the Solar Neutrino Problem here.