The Strong Interaction

Ernest Rutherford’s nuclear model of the atom, consisting of a tiny massive nucleus surrounded by a cloud of electrons. With the discovery of the neutron in 1932 by James Chadwick it became clear that the nucleus is really formed of protons and neutrons. So a natural question would be, given the electromagnetic repulsion between the positively charged protons, what holds the nucleus together? A worthy guess would be the gravitational force, which is always attractive and which acts on all particles that have mass. This can’t be it, however, since it’s so much weaker than the electromagnetic interaction.

There must be some other type of interaction that manifests itself on the very small scale of atomic nuclei. And, given its dominance at this scale over electrostatic repulsion, it must be pretty darn strong. That’s why physicists call this one the strong interaction (or sometimes the strong nuclear force). Not a terribly imaginative name, but it has stuck.

If gravity only works between particles with mass, and the electromagnetic interaction acts only between charged particles, what types of particles are interacting via the strong force? To explain this, we need another property that plays a role similar to electric charge or mass when it comes to the strong force. But before we can even do that, we need to take a much closer look at protons and neutrons.

In 1964, two physicists independently came up with the ideas that eventually came to be known as the quark model. They were the theorists Murray Gell-Mann and George Zweig. The problem they had was that particle accelerators kept improving, and were able to accelerate their beams to higher and higher energies. Experimenters using these machines kept finding more and more subatomic particles. There were so many of them, with all different masses and electric charges and intrinsic spins, that calling them “elementary particles” was getting to be a joke.

At first, Zweig and Gell-Mann (along with others) were trying to find a classification scheme just to keep all the particles straight. But they soon realized that they could account for most of the new particle’s properties if they were made of only four varieties of more fundamental particles, which Gell-Mann called “quarks.” The varieties or kinds of quarks were called “flavors.” (We should also mention that the antimatter partners or antiquarks were also required to account for all the new particles’ properties.)

QUANTUM LEAP

Although Murray Gell-Mann assigned the name “quark” to the constituents of protons and neutrons, we can actually thank Irish author James Joyce for its proper spelling. Gell-Mann drew this from the author’s classic, Finnegans Wake, and its line “three quarks for Muster Mark!”

The quark model immediately predicted that the old familiar protons and neutrons were not actually fundamental particles, but were themselves each made of three quarks in two flavors. Only a few more years elapsed before the big accelerator at Stanford University saw evidence of structure within protons. Sure enough, the pointlike particles they observed had the right characteristics, and now the quark model is the accepted model for all particles that are subject to the strong interaction.

The property quarks possess that makes them feel the strong force has come to be called “color” charge. Unlike mass, which comes in only one variety (positive), or electric charge, which comes in two (positive and negative), this color charge comes in three varieties–colorfully named red, blue, and green. Quarks of like color repel (e.g., red-red), while quarks with different color attract (e.g., red-green or red-blue).

Moreover, quarks always fuse together to form larger, composite particles whose overall color is neutral (e.g., equal parts red + blue + green). Protons and neutrons are each formed as the composite of three quarks of different colors, and are thus color neutral. The attractive interaction that holds a nucleus together is some sort of residual leftover from the fundamental attraction between quarks. Exactly how this happens is an area of ongoing research.

ATOM TRAP

The use of “color” for the interactive property of the strong force has nothing to do with color in the everyday sense. Quarks are not actually green, blue, or red. We know they can’t be since quarks are many, many times smaller than the wavelength of green, blue, or red light.

At the composite level, the strong force is the same between two protons as between two neutrons or between one proton and one neutron. We call this property of the strong force “charge independence.” And what about the electrons that surround the nucleus? Since electrons are not quarks themselves and have no color charge, they are blissfully unaware of the strong interaction. It has no effect on them. (Incidentally, this makes the electrons a very sensitive probe to study nuclei because its interaction with the nucleus is due almost exclusively to the weaker electromagnetic interaction.) The quarks also have electric charge, so beams of electrons can be used to learn where exactly quarks reside, and what they are doing, inside protons or neutrons.

The range of this strong force must be very short; otherwise it would not only lead to interactions within nuclei but also between nuclei in neighboring atoms. In fact, the strength of this interaction falls rapidly to zero at separations of about one femtometer (10-15 m).


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