Bolstered by the successes of using quantum field theory to describe electrodynamics and the weak interaction, theorists immediately tried to come up with a quantum field theory for the strong nuclear force. Unfortunately, however, the sailing wasn’t quite as smooth this time around.
We originally named the strong force “strong” because it had to overcome the significant repulsion between positively charged protons packed tightly in atomic nuclei. The electromagnetic force that pushes positive charges away from each other gets stronger as the charges get closer together. And the nucleus is so extremely small that this repulsive force is really huge. The neutrons play a role, though since they have no electric charge they can help hold things together only if there is a different, strongly attractive force between protons and neutrons.
The complexity arises because the neutrons and protons are not elementary themselves, but composite particles made up of the more fundamental quarks. After realizing this, theorists turned their attention to whatever force holds quarks together to form protons and neutrons in the first place, hoping that solving this fundamental issue would lead to an understanding of the more visible nuclear force.
That turned out to be a big job, and took a couple of decades of hard work. Experimentalists were handicapped in their efforts to get clear data by the fact that no isolated quarks could be found. It turned out to be impossible to accelerate a beam of pure quarks in order to directly measure their interactions.
A lot of back and forth between theory and experiment eventually led to a field theory for quarks that we call quantum chromodynamics. The name was chosen to reflect the fact that the essential quality of quarks that allows them to participate in the strong interaction was called color charge (or simply color), in analogy with electric charge in QED.
The exchange boson hurled around between interacting quarks was called the gluon (g). Again, the very short range over which the strong interaction works suggested that the gluons would be massive particles. Due to the fact that there were three varieties of color charge, there had to be nine types of gluons to cover all possible interactions between quarks and antiquarks. (There actually turn out to be eight unique gluons due to a mathematical redundancy.)
The first theoretical task was to explain the fact that quarks are never spotted in isolation, something we now call quark confinement. Something about this interaction prevented quarks from ever being separated from color-neutral bound states. The solution was to postulate a force that behaved very differently from all other known forces.
Instead of getting weaker with increased distance, the QCD force would have to get stronger as the separation between quarks increased. Then an infinite amount of energy would be required to separate any bound quarks, which could never happen. What does happen is that when you try to separate quarks by adding energy, you end up creating more quarks through the process of pair production. These newborns always assemble themselves in groups that have no net color.
DEFINITION
The gluon is the carrier of the strong force between quarks. Gluons come in eight colors, are electrically neutral, and have one unit of intrinsic spin.
Confinement is the strict requirement that quarks only exist in color neutral bound states. Neither quarks nor gluons can exist outside of such states, which we observe as particles.
This made for a theory that looked very different from QED. It was still a quantum field theory, just a little weirder. Since the quarks can never be separated, you can’t talk about the range of the force in the same way as before. Just like photons, the gluons turn out to have zero mass. The big difference between them is that the gluons themselves have color charge (photons don’t have any electric charge). This means that gluons not only mediate the strong interaction, but are also influenced by it. Gluons can exchange gluons with other gluons, and that makes the theoretical treatment pretty messy.
By now the details have been pretty well worked out, and we have a complex but successful theory to explain how quarks assemble into all of the observed hadrons. But what about the original objective, explaining the attraction between hadrons themselves, like between the proton and neutron? None of these objects have net color charge, so at first glance it appears that QCD would not apply to them.
If you think about it, though, we’ve already encountered a similar situation. We know that atoms are formed by the electromagnetic attraction between electrons and protons. We also know, though, that atoms are sometimes attracted to one another, resulting in the formation of molecules. We discussed how molecules are formed though the sharing of valence electrons.
Physicists believe that the attraction between two neutrons, or a neutron and a proton, is somehow like this. While the attraction that holds protons and neutrons in a nucleus is relatively strong (compared to gravity and the electromagnetic force), it is weak compared to the force that keeps quarks forever confined inside neutrons and protons. Some nonuniformity in the distribution of quarks and gluons inside protons and neutrons must lead to the observed attraction, but it is not yet known exactly how this happens. It remains an area of active research in nuclear physics.
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