“Quark” is a word that people automatically associate with science. It’s memorable because it’s unusual–not to mention fun. (Q. What sound does a physicist’s duck make? A. Quark, quark.) But how many people really know what a quark is?
Not many, and since I was one who didn’t, I decided to write a column on the subject. (Yes, I’m afraid it’s true; this column is just an excuse for me to improve my education. But you’re welcome to come along for the ride.)
One of the grand quests of science has been the search for fundamental particles–the smallest possible bits of matter. The ancient Greek philosopher Democritus made a good start about 400 B.C. by suggesting that all matter is made of minute particles, indivisible and indestructible, which he called atoms, from the Greek word meaning “that which cannot be divided.”
That’s pretty well where the theory stood until 1805, when English chemist John Dalton suggested that not only is everything made of atoms, every element’s atoms are unique. He pointed out that when a compound decomposes, the elements of which it is made are released unchanged.
Dalton saw atoms as similar to unbreakable billiard balls, but in 1897, British physicist Joseph John Thomson discovered the electron, a tiny fragment of atomic matter with a negative electrical charge. Thomson decided the atom wasn’t a billiard ball at all; it was more like a pudding of positively charged matter with electrons stuck in it like plums.
Then along came Ernest Rutherford, another English physicist (who, by the way, lived and worked in Montreal for several years). After extensive experimentation, Rutherford decided in 1911 that atoms must contain a dense central nucleus. Since his experiments showed that radiation could knock out hydrogen nuclei from a variety of other nuclei, he concluded that the hydrogen nucleus was the basic component of all nuclei, and he called it a “proton,” after the Greek for “first.”
Finally, in 1932, James Chadwick discovered the neutron, which has the same mass as a proton but no electrical charge, and it seemed the search for fundamental particles was over. Atoms, far from being indivisible, consisted mostly of empty space, with tiny, negatively charged electrons whirling around a larger, very dense nucleus of positively charged protons and uncharged neutrons.
It was a nice simple model, and it lasted for years. Most of us learned something like it in school. Unfortunately …
In the 1960s, new particle accelerators–machines that smash particles together to find out what they’re made of–revealed more kinds of particles than anyone had dreamed possible: more than 80. Electrons, protons and neutrons weren’t enough. Who could possibly make sense of the emerging complexity?
Two Caltech theorists, Murray Gell-Mann and George Zweig, that’s who. Independently, they suggested that the new “elementary” particles weren’t elementary at all–they were actually just different arrangements of a small number of truly elementary particles.
Zweig wanted to call these new particles “aces,” but Gell-Mann elected to call them “quarks,” from a line in James Joyce’s novel Finnegan’s Wake: “Three quarks for Muster Mark!” “Quarks” stuck.
Both theorists agreed there were only three “flavours” of quarks. (Why “flavours”? Well, by coincidence, quark is also the German word for “cottage cheese”…) Anyhow, in order to produce “normal” particles like protons and neutrons, you need just two flavours of quark, called “up” and “down.” The up quark has a charge of +2/3 and the down quark has a charge of -1/3. A proton consists of two ups and one down, whose charges add up to +1, while a neutron has two downs and an up, whose charges cancel each other out.
The third flavour was called “strange,” because it explained some of the other strange particles scientists were seeing.
What happened to electrons among all these ups and downs and stranges? Well, electrons aren’t made up of quarks–they’re a different kind of fundamental particle called a lepton. Leptons either have a charge of -1, like the electron, or 0, like the electron neutrino. For every quark there is a corresponding lepton; the electron corresponds to the up quark and the electron neutrino to the down quark. These four particles comprise the “first family” of fundamental particles.
The strange quark belongs to the “second family,” which soon grew larger as scientists continued to discover unexpected particles. Strange, with its -1/3 charge, was joined by “charm,” with a +2/3 charge, and matched by the second-family leptons–the muon and the muon neutrino.
Beginning to see a pattern here? So were physicists. Soon there was also a third and, it’s believed, final family, featuring the bottom quark, also called “beauty,” charge -1/3, the top quark, also called “truth,” charge +2/3, and, on the lepton side, the tau and the tau neutrino.
The particles in the second and third families are only seen in high-energy collisions, so really the model for normal matter–you and me and the cat–isn’t much more complicated than it was; we’re still made up mostly of three particles: electrons, up quarks and down quarks. The complete collection of six leptons and six quarks, which is called, somewhat unimaginatively, “the standard model,” is very popular with scientists because it works, it’s simple, it’s symmetrical, and it makes neat-looking wall charts.
It does have one little problem, though: so far, all the particles in it have been found except the top quark–“truth.” The search for “truth” is, therefore, the current Holy Grail of particle physicists. (And it must be great to be a physicist involved in the search. Picture the conversation at a cocktail party: “What do you do for a living?” “Oh, I’m devoting my time these days to the search for truth.”)
Physicists are reasonably confident truth exists–the standard model has been so successful in so many other ways–but it’s just possible it doesn’t , in which case…
Well, let’s just say there’d be a lot of very depressed physicists out there.
Not to mention a lot of useless wall charts.