Facts About Quarks


Physicists once believed that atoms were the smallest particles of matter -- until they discovered that atoms were actually electrically charged systems consisting of protons, neutrons and electrons. Probing deeper, they found even protons and neutrons to be composed of smaller ones. They chose a whimsical name for the particles that form protons and neutrons -- which are types of hadrons. They called them quarks, and these fundamental building blocks of matter come in different colors and flavors.

The Microscopic Scale of Matter

  • Atoms are microscopic, but they are anything but solid. If you drew a model of an atom that depicted protons with a 1-centimeter radius, the space between the nucleus and the electrons would equal 15 football fields. In this scale model, electrons and quarks would have a diameter of less than a human hair. They are so tiny that scientists aren't sure whether they have any extension at all -- they may actually be points. In the standard model of the universe, all matter is composed of quarks, leptons -- of which electrons are an example -- and the carrier particles that quarks exchange.

Quarks Come in Flavors

  • Particle physicists tend toward fancy in the nomenclature they use for fundamental particles. The word "quark" comes from a passage in "Finnegans Wake," by James Joyce: "Three quarks for Muster Mark ..." Physicists have discovered six types of quarks, which they often conceive as a single type of particle with six "flavors." These flavors form three pairs: up and down; charm and strange and top and bottom. Up and down quarks form the bulk of the matter in the observable universe; they are the components of all protons and neutrons.

How Quarks Combine

  • Quarks have a fractional charge, and they can combine in such a way as to produce a particle with an integer charge or none at all. An up quark has a charge of 2/3, and the charge of a down quark is minus 1/3. When two up quarks combine with a down quark, they produce a proton, which has a positive charge of 1. When two down quarks combine with an up quark, they produce a neutron, which has no net charge. These particles are special classes of hadrons called baryons. Mesons, the other type of hadrons, can be formed from one up quark and one down anti-quark or the reverse. They can have a positive or negative charge of 1, as well as 0.

Quarks of a Different Color

  • Physicists explain how quarks stay together by assigning them a color charge and postulating distinct particles, called gluons, that they are constantly exchanging. This exchange produces the strong force, which is powerful enough to overcome the electromagnetic force that would otherwise force the quarks -- and the particles they produce -- apart. Physicists assign quarks three colors and three anti-colors, although these colors have nothing to do with the visible spectrum. Quarks and anti-quarks can combine in nine different ways according to their color charge, and each gluon has a combination of two colors. Contrary to expectation, however, there are only eight different types of gluons -- not nine.

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