freyar
Extradimensional Explorer
Can you summarize of our present level of understanding of what's occuring in the nucleus of moderate-sized atom?
There's some useful background information if you search around upthread a bit (or maybe in another thread in the Misc. Geek Talk forum --- these threads have gotten a bit jumbled in my memory). Basically, protons and neutrons (the particles that make up an atomic nucleus) are made up of 3 quarks each held together by virtual gluon particles. Gluons mediate the strong nuclear force in much the same way that photons mediate the electromagnetic force. The analogy ends there, though; the strong force is so strong that a great deal of the structure of protons and neutrons is determined by the virtual gluons (and also virtual quarks) inside them. In the rest of atoms, the virtual photons don't really show up separately from the fact that there is an electric force binding the atoms together.
The difficulty is that the strong force is strong enough that quarks and gluons can't really exist freely in the nucleus. They're stuck in the protons and neutrons. What holds the protons and neutrons together is a remnant of the strong force. The analogy here is the van der Waals force between atoms; atoms are electrically neutral, but, if you put two of them close together, their electrons arrange themselves so the atoms attract each other. That's the short answer. If you want a little more information: The mathematical description is much more complicated for the strong force, though, and it turns out that different approximations to the full equations work better for different sized nuclei (I'm not really an expert on this in detail, but I have heard about it). For medium-sized nuclei, the main way to think about it is that protons and neutrons are a bit "sticky" when they bump into each other, and this "stickiness" helps hold the nucleus together. Another part of this left-over nuclear force for these nuclei is due to virtual pions, which are unstable particles made up of one quark and one anti-quark. The best description is different (and somewhat harder to explain) for really small or large-ish nuclei. This is a notoriously difficult subject, by the way.