At Mom’s suggestion, I’m going to try putting together a post or two on some of the more obvious “what…” and “why…” questions about IceCube. I say “try” because I’m no expert on the physics involved. There’s lots of hand waving going on here…

What is a neutrino?

There’s plenty more information on wikipedia, but the short version is that a neutrino is another sub-atomic particle. There are gobs of neutrinos all through the universe, but we don’t notice them normally, since they don’t usually interact with normal matter.

To explain that in a bit more detail:

The sub-atomic world is a bit more complicated than the protons, neutrons, electrons, and photons that we learn about in grade school. Protons and neutrons, for instance, are made up of three quarks each, but electrons and photons are not.

Quarks, electrons, photons, and several other things are fundamental particles – meaning that, as far as we know, they aren’t made of smaller parts. Neutrinos are fundamental particles, and there are three kinds of them.

One way that the fundamental particles differ from each other is in the degrees in which they interact with the four fundamental forces: gravitational, electromagnetic, strong nuclear, and weak nuclear. Everything we experience day-to-day comes from the gravitational and electromagnetic forces.

Although gravity is a fundamental force, and very important – it keeps our ice cream from floating away – we don’t worry about it too much here since it doesn’t have much effect at the small scales we’re talking about. To think about this, consider that things are pulled down by gravity, but normal size things don’t get pulled towards each other. This is because you need a very large scale mass (say, Earth) to produce much gravitational force.

Our senses like touch, sound, and sight are results of electrons pushing and pulling electrically charged particles through the electromagnetic force. Chemistry is all about electrons moving around, being attracted and repelled – another example of the electromagnetic force. Compared to gravity, the electromagnetic force is quite strong at smaller scales. Think of how strongly a pair of magnets are pulled together, compared to their weight, when they are close together.

The strong and weak nuclear forces are a bit harder to think about, since they are mainly important at much smaller scales than we normally deal with. The strong nuclear force is what holds protons and neutrons together in the nucleus of an atom. It’s also the strong nuclear force that holds the quarks together to make up individual protons and neutrons. Strong nuclear force doesn’t affect electrons or neutrinos. If the word “atomic” makes you think of something powerful, that power comes from the strong force.

The weak nuclear force acts on even smaller scales than the strong force – one thousandth the diameter of a neutron, as compared to the strong force interacting over the diameter of a medium size nucleus, which would contain some neutrons. The weak nuclear force is interesting to us mainly because it does act on electrons and neutrinos.

Neutrinos have a very low mass, so they are affected by gravity. They don’t have an electrical charge though, which means they don’t interact through the electromagnetic force. We just learned that neutrinos don’t interact through the strong nuclear force, but do interact through the incredibly short-range weak nuclear force.

So, what all this means is that neutrinos basically don’t interact with normal matter. But, when they do interact with normal matter, they do it through the weak nuclear force.