Why do quarks change color?

Color games in the quantum world

The tools of the trade of modern nuclear physicists are no gray theory: The strong force acts between "colored" quarks, which physicists imagine through the exchange of "colored" gluons. This then results in composite objects such as protons, atomic nuclei and finally nuclear physicists again.

Four forces are enough for the physicist to describe the diversity of our world: Gravity keeps us with our feet on earth. The electromagnetic force ensures that electrons move around atomic nuclei and through power lines. The weak force lets atomic nuclei fall apart, which thanks to the strong force exist in the first place.

The strong force ensures that objects such as protons, neutrons and atomic nuclei are formed from quarks and gluons. Its strength is unsurpassed, it is a hundred times greater than that of the electromagnetic force. Therefore it easily overcomes the electromagnetic repulsion of quarks with the same electrical charge. And positively charged protons also stick together to form atomic nuclei because of them.

The "color" property

Quarks and anti-quarks

To describe the strong interaction, the American Murray Gell-Mann and the German physicist Harald Fritzsch came up with a set of rules called Quantum Chromodynamics (QCD) in 1972. The name means something like quantum color theory of motion. Because all strongly interacting particles have had a "color" since 1965. This is not to be taken literally. Because quarks are of course not really colored. Color came into play in 1964 when physicists had to give the quarks they had just invented another property in order to better tell them apart. Since then there have been quarks in red, green, and blue. Anti-red (cyan), anti-green (magenta) and anti-blue (yellow) are available for the antiparticles of quarks.


According to quantum chromodynamics, colored quarks hold on to the exchange of gluons to glue: glue) together. Just as the electric force works between electric charges, in the case of a strong force the colored charges are on the train. Gluons are also color-charged; they are even two-colored, have one color and one anti-color. According to quantum chromodynamics, a red quark can now turn blue by sending a red-anti-blue gluon to a blue quark, which then turns red. The two particles exchanged color and interacted strongly with each other. The change in movement that also occurs during this process ensures that the quarks do not drift apart, but stick together. In 1979 the existence of the gluons at DESY in Hamburg was confirmed.

The standard model knows eight gluons

The Starke Kraft is not only at the top when it comes to strength, but also when it comes to the number of force particles. The electromagnetic force is described by the exchange of a type of photon. The weak interaction occurs through the exchange of three different particles: the W-plus, the W-minus and the Z-zero. Eight different gluons are at work in the strong force.

This multitude comes about because you have to consider all color-anti-color combinations. With three colors and three anticolors one would think that there are nine gluons. But because the combinations red-anti-red, green-anti-green and blue-anti-blue result in “white”, they do not play a role in quantum chromodynamics.

Bound quarks

One of the most mysterious properties of the Strong Force is that quarks and gluons cannot exist as completely free isolated particles in nature; rather, they always appear in groups: as compounds with other quarks, antiquarks, and gluons. They then form quark triads like protons and neutrons or short-lived quark-antiquark compounds that are called mesons.

This phenomenon is scientifically called confinement. It is due to the fact that the strong force becomes stronger and stronger the further the two quarks are separated from each other. If you still drive two bound quarks further apart, you will put so much energy into the strong connection between these particles that new quarks will effortlessly arise from this energy if the bond breaks. In this way, you get two quark groups from one quark group.

Strong power - short range

If the strong force is so strong, why do we not feel it anymore in everyday life? Why don't we get stuck to a chair that is also made of quarks? How come we can let go of someone we just shook hands with? The solution to the riddle lies in the short range of the strong force: It only works over distances that correspond to around a hundred thousandth of the size of an atomic nucleus. She does not feel what lies beyond.

Where does the crowd come from?

Without Quantum Chromodynamics (QCD), not only would the atomic nuclei break apart inside you, they would also be pretty light. Because only a few percent of the mass of ordinary matter is due to the quarks inside the atomic nuclei of the atoms of the molecules of the cells of the organs of your body. Since electrons are much lighter, they don't make the roast fat either. All the rest is created by the QCD interaction. Gluons are massless, but they have an energy which, according to the theory of relativity, makes them difficult.

Physicists have largely succeeded in explaining how mass is generated via the QCD interactions. This makes it possible to predict how the mass of hadrons will change if they are generated in nuclei or in an accelerator during a nucleus-nucleus collision. Initial experiments seem to support these ideas, but only improved, currently ongoing and future planned experiments will be able to confirm or refute these theoretical predictions beyond any doubt.