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By Root 925 0

deuteron. One of the wonderfully convincing features of a successful scientific theory is its ability to predict something that has never before been seen. The subsequent observation of that “something” in an experiment provides compelling evidence that we have understood something real about the workings of the universe. Taking the point a little further, the more predictions a theory can make, then the more impressed we should be if future experiments vindicate the theory. Conversely, if experiments do not find the thing that is predicted, then the theory cannot be right and it needs to be ditched. There is no room for debate in this kind of intellectual pursuit: Experiment is the final arbiter. Dirac’s moment of glory came just a few years later when Carl Anderson made the first direct observations of positrons using cosmic rays. For their efforts, Dirac shared the 1933 Nobel Prize and Anderson the 1936 prize. Esoteric though the positron might appear to be, its existence is today used routinely in hospitals all over the world. PET scanners (short for “positron emission tomography”) exploit positrons to allow doctors to construct three-dimensional maps of the body. It is not likely that Dirac had medical imaging applications in mind when he was wrestling with the idea of antimatter. Once again it seems that understanding the inner workings of the universe turns out to be useful.

There is one other particle that is presumed to exist, but it would be to rush things to mention it just yet. It is represented by the Greek symbol φ (pronounced “phi”) and it is lurking on the third and fourth lines of the master equation. Apart from this “other particle,” all of the quarks, charged leptons, and neutrinos (and their antimatter partners) have been seen in experiments. Not with human eyes, of course, but most recently with particle detectors, akin to high-resolution cameras that can take a snapshot of the elementary particles as they fleetingly come into existence. Very often, spotting one of them has won a Nobel Prize. The last to be discovered was the tau neutrino in the year 2000. This ghostly cousin of the electron neutrinos that stream out of the sun as a result of the fusion process completed the twelve known particles of matter.

The lightest of the quarks are called “up” and “down,” and protons and neutrons are built out of them. Protons are made mainly of two up quarks and one down, while neutrons are made from two downs and one up. Everyday matter is made of atoms, and atoms consist of a nuclear core, made from protons and neutrons, surrounded at a relatively large distance by some electrons. As a result, up and down quarks, along with the electrons, are the predominant particles in everyday matter. By the way, the names of the particles have absolutely no technical significance at all. The word “quark” was taken from Finnegan’s Wake, a novel by Irish novelist James Joyce, by American physicist Murray Gell-Mann. Gell-Mann needed three quarks to explain the then known particles, and a little passage from Joyce seemed appropriate:Three quarks for Muster Mark!

Sure he has not got much of a bark

And sure any he has it’s all beside the mark.

Gell-Mann has since written that he originally intended the word to be pronounced “qwork,” and in fact had the sound in his mind before he came across the Finnegan’s Wake quotation. Since “quark” in this rhyme is clearly intended to rhyme with “Mark” and “bark,” this proved somewhat problematic. Gell-Mann therefore decided to argue that the word may mean “quart,” as in a measure of drink, rather than the more usual “cry of a gull,” thereby allowing him to keep his original pronunciation. Perhaps we will never really know how to pronounce it. The discovery of three more quarks, culminating in the top quark in 1995, has served to render the etymology even more inappropriate, and perhaps should serve as a lesson for future physicists who wish to seek obscure literary references to name their discoveries.

Despite his naming tribulations, Gell-Mann was proved correct in his hypothesis