strong nuclear force

Even more interesting, however, is that the colour force gets stronger with distance. If one were to try and extend particles interacting through QCD, he or she would find that the force between said particles would increase to infinity.

This poses a problem for those scientists who want to observe quarks, which interact through a colour charge. The quarks cannot actually be separated. The solution to this is to observe the quarks when they are very close together. Thus the force between them shall be weak. Using this priniciple, scientists were able to rattle protons and neutrons to observe their constituent quarks.

The development of QED in the first half of the twentieth century showed that electrons were bound to atoms by the electromagnetic force alone, it remained to show how the particles left over, atomic nuclei, were held together.

All nuclear particles were positive or neutral, and much closer together than the electrons, which meant a different (and much stronger) force had to be responsible. And thus was born the "nuclear force".

Later, it was shown that beta decay operated by a different mechanism from that holding together atomic nuclei (an isolated neutron decays into a proton). Furthermore, the interaction that controlled beta decay was shown to be much weaker than the one holding atomic nuclei together, so the former was named the "weak nuclear force" and the latter the "strong nuclear force".

The strong nuclear force, then, consisted of interactions between certain types of particles, named "hadrons". The vast majority of hadrons were the "nucleons", that is, protons and neutrons in atomic nuclei, interacting by exchanging pi mesons.

However, particle accelerators kept turning up more and more exotic hadrons, and there wasn't a very good theory to explain them.

Then, in the 1960's came Murray Gell-Mann and his theory of "quarks". This wasn't widely accepted at first, but eventually, scientists began to see that it was the only theory that explained the zoo of particles they had discovered.

Quark theory eventually developed into its current version, QCD, with quarks being inextricably bound together by the color force.

However, notice that QCD doesn't directly explain the "strong nuclear force", meaning the mechanism that holds nucleons together in atomic nuclei: For hadrons to interact, they can't exchange naked bits of color, because naked bits of color aren't allowed to exist. They have to exchange color singlets: mesons or glueballs.

In the end, atomic nuclei exist as a side effect of the color force:

• A hadron is perturbed away from another one by quantum fluctuations we can't measure. This builds up "stress" in the color field, that is, potential energy.
• Sometimes these perturbations cancel each other out.
• Rarely, perturbations or external events exceed the binding force. Then we say the nucleus is undergoing some process of radioactive decay.
• Often, however, the system builds up enough potential energy to create a pi meson, but not enough to break away:
1. A pion is created.
2. The creation of the pion converts the potential energy to matter and mechanical energy.
3. The pion travels from one nucleon and is consumed by the other one.
4. The system's potential energy being reduced, the two nucleons move closer together.

If you think about the mechanism I just described a minute, you will realize that the "strong nuclear force" is the QCD analogue of the electromagnetic "Van der Waals" forces that hold certain molecules together (such as the layers of graphite).