The strongest of the subatomic forces is the aptly named strong nuclear force. In the realm in which it operates, it is about 100 times stronger than the next-strongest force (electromagnetism). But it isn’t just its strength that distinguishes it from the other forces. It has other properties that differ from, for instance, the features of a magnet. The force between two magnets extends over a long distance and becomes stronger as the magnets are brought closer to one another. In contrast, the strong nuclear force is a lot more like glue. If you have two marbles made sticky by some kind of adhesive, they will cling together when they are made to touch each other. However, once the two marbles are separated by even a very small distance, they no longer feel any attractive force at all.
In homage to the force’s commonalities with how glue behaves, the force-carrying particle for the strong force is called the gluon. Gluons are responsible for binding protons and neutrons together inside the nucleus of an atom. This is crucial for building atoms, but this nuclear binding is actually a side effect of what the gluon really does—hold together the quarks that make up protons and neutrons. In high-energy physics experiments, it is the quark-quark binding that is of the greatest interest.
The distance over which the nuclear force is active is about 1 femtometer (10-15 or one quadrillionth of a meter). To give an idea of just how mind-bogglingly small that is, if a proton were as thick as a sheet of paper, by comparison you’d be so big that, if you stood on the Earth, your head would touch the Sun.
In the last Nutshell, we were introduced to the photon, the quantum of the electromagnetic force. Because the photon is electrically neutral—that is, it has no electric charge—photons don’t interact with each other. In contrast, every gluon has a strong nuclear charge. Thus gluons interact not only with quarks, but also with other gluons. This gluon self-interaction property is one of the reasons that the strong force acts like glue instead of magnets.
The charge of the nuclear strong force is known as color. In a subatomic context, three colors—red, blue and green—are carried by the three quarks in a proton, resulting in a simple color scheme. In contrast, the force-carrying gluons have a rather complex color palette, one with a mix of both color (the charge carried by quarks) and anticolor (the charge carried by antiquarks). In total, there are eight different color combinations that gluons can carry. (If you’re wondering why three colors and three anticolors combine to make eight gluons and not nine, the answer can be found here.)
Gluons were discovered at the German laboratory DESY in the late 1970s. They play a key role in many of the studies performed at the Tevatron and the LHC.
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