“In any case, the number three fitted perfectly the way quarks occur in nature.” -Murray Gell-Mann
You might think that we know it all, at least as far as knowing-it-all is possible. After all, we know that matter is made up of atoms, which are made up of electrons and nuclei, and the nuclei are made up of protons and neutrons, and then the protons and neutrons are made up of quarks and gluons.
Along with the electrons, the quarks and gluons are — as far as we know — indivisible, which places them among the fundamental particles of the Universe.
Every proton and neutron is an example of a more general type of particle known as a baryon, which is a particle made up of three quarks, as well as the gluons that hold them together. Each quark has two types of charge: an electric charge, just like an electron has, as well as a color charge!
Unlike the electric charge, which is fixed for particles and has its force carried by a chargeless particle (the photon), a quark always carries a color — either red, green or blue — but that color always changes over time, since the gluons that carry the force are also colored!
Last year, I wrote up a brief introduction called The Strong Force For Beginners, which I encourage you to look at if you want more details about how this works. With six quarks and eight different gluons mediating the strong force, the number of different baryons, or stable/quasi-stable combinations of three quarks, are tremendous. The key is that, to exist, the combinations of quarks needs to be completely colorless when taken all together.
But baryons are not the only possibility for satisfying this.
Each quark has a color charge, and each anti-quark has an anti-color charge, but these are not independent! For example:
Anti-red is the same as blue+green, since red+anti-red or red+blue+green both = white.
Anti-blue is the same as red+green, since blue+anti-blue or blue+red+green both = white.
Anti-green is the same as blue+red, since green+anti-green or green+blue+red both = white.
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