How to Understand Quarks

For most of the twentieth century, physicists thought the protons and neutrons inside the atomic nucleus were elementary particles: that is, they couldn’t be decomposed into smaller particles, although it was known that the neutron could decay into a proton, an electron and a neutrino. The advent of quark physics in the 1960s showed that protons and neutrons are in fact made up of yet smaller particles called quarks. Read on to learn how to understand quarks.

Instructions

Difficulty: Easy

1. Every proton and neutron is composed of three quarks. A proton is composed of two “up” quarks carrying fractional charges of +2/3 and one “down” quark with a fractional charge of -1/3. A neutron, by contrast, is made up of two down quarks and one up quark--which accounts for its zero charge. These fractional charges are an artifact of the olden days of physics, when scientists thought the +1 charge of a proton was as small as you could get. Two quarks can also combine to create a particle called a “meson.”
2. Quarks interact via the strong force. As befitting its name, the strong force is what keeps protons and neutrons packed together in the atomic nucleus. Unlike the electromagnetic and gravitational forces, the strong force actually becomes stronger as two quarks attempt to separate from each other--which is why it’s impossible to find a free quark in nature.
3. Quarks have a property known as “color.” Like its charge, the “color” of a quark is a kind of bookkeeping system used by physicists to determine its properties—it has nothing to do with actual colors. Quarks have “red,” “green” or “blue” color charges, which guarantee that no two quarks occupy the same place at the same time—which would violate a law known as the Pauli Exclusion Principle.
4. Quarks come in three “generations.” Like electrons and neutrinos, each quark has two counterparts at higher energies, which normally don’t occur in everyday matter but do pop up in particle accelerators. The vast majority of matter is made of up and down quarks, but there are also more exotic “strange” and “charm” quarks in the next generation, and “bottom” and “top” quarks in the generation above that.
5. Quarks owe their name to James Joyce. By now, all this talk of colored, charged, and strange quarks grouping in threes and bouncing “up” and “down” may have your head spinning. In fact, one of the first physicists to investigate these particles, Murray Gell-Mann, dubbed them “quarks” after a typically strange sentence in James Joyce’s novel “Finnegans Wake”: “Three quarks for Muster Mark!”