Question: What exactly are neutrinos?

Matthew Malek answered on 7 Mar 2014:

I can’t tell you EXACTLY what neutrinos are… because we are still probing their nature, and there’s lots we haven’t figured out yet. However, I can tell you a few things that we do know:

– Neutrinos are fundamental particles — some of the building blocks of nature.

– Neutrinos are the second most abundant type of particle that we know of, second only to the photon (i.e., light). There are about 300 neutrinos per cubic centimeter thoughtout the universe, including about 10,000 neutrinos left over from the Big Bang in your body right now!

– There are four forces in nature: Electromagnetic, Strong, Weak, and Gravity. In particle physics, we ignore gravity, because it is sooooooooo much weaker than the other three (even the Weak force). Particles with electric charge feel the electromagnetic force. Neutrinos have no electric charge. Particles with something called “color charge” feel the Strong force. Neutrinos have no “color charge”. So they only feel the Weak force and, as you may have guessed, interact very weakly. Look at your thumb for a second. In that second, about 50,000,000,000 neutrinos just passed through your thumbnail — moving at very nearly the speed of light — and yet none of them ever noticed you were there. That’s how weakly they interact.

– Neutrinos were first postulated in the 1930s by Wolfgang Pauli to restore energy conservation in nuclear decays. If you add up all the energy after the decay, it is less than what you had at the beginning. This upset people, so Pauli postulated that a new particle — the neutrino — carried away the extra energy. He thought that it could never be detected, though, and wasn’t happy with his creation. He said: “I have done a terrible thing; I have postulated a particle that cannot be detected.” It took a quarter century to prove him wrong, but eventually the first neutrinos (anti-neutrinos, actually) were detected in 1957 by Cowan & Reines. They put a particle detector near a nuclear reactor and finally discovered some. To celebrate, they sent out empty boxes to their friends with a note reading: “This box is guaranteed to contain at least 100 neutrinos”.

That’s a lot, I know… but it is only the start! There is SO much more I could say about neutrinos. Before I ramble on endlessly, perhaps I should ask: Have I answered your question well enough? Is there anything else that you would like to ask about neutrinos? If so, fire away with more questions and I’ll answer them, too. (But not tonight…)