Quark quandaries

Every now and then I think it's a good idea to dip into a basic aspect of physics that may not have been in the school curriculum. Take, for instance, the quark. I don't refer to the low fat cheese sometimes given this name but the particle at the heart of every atom in your body (and everywhere else for that matter).

Proton structure

Once upon a time we talked about the basic particles in the nucleus in the middle of the atom being protons and neutrons. They haven't gone away, but they are no longer considered fundamental particles. Each is made up of three smaller particles – quarks. There’s a whole mess of quarks distinguished by characteristics known as flavors (no, really). The different flavors are charm, strangeness, top/bottom and up/down. (Even the more prosaic names can sound a bit odd with antimatter versions. One is the ‘anti-bottom quark.’) The proton is two ups and one down; the neutron two downs and one up.

Up quarks have a 2/3 charge and down quarks -1/3, resulting in a positive charge of 1 for the proton and no charge at all for the neutron. We aren’t used to nature coming up with quantities in thirds. But bear in mind the unit of charge is arbitrary. We really ought to say that up and down quarks have charges of 2 and -1 respectively – so a proton has a charge of 3 units – but because protons and electrons were the simplest particles known when the units were established we are stuck with thirds.

No one has ever seen a quark, nor broken a proton or neutron into its components. It is particularly difficult to do so, because the force that holds the quarks together gets stronger as they move further apart. As this is the case, it’s difficult to understand how quarks were ever dreamed up. The reason we believe that quarks exist owes its origins to a different type of physics that emerged in the early days of quantum theory.

As quantum theory was developed, two different approaches emerged. One had clear parallels in the real world. The second, matrix mechanics, was purely mathematical. It was by building on purely mathematical concepts, until they closely predicted what was seen in the real world, that the quark emerged. The existence of quarks themselves has since been indicated by experiments that show three constituents in a proton – and by the very short-lived production of otherwise unknown particles made up of combinations of different quarks. It’s possible things will go horribly wrong, and quarks will turn up not to exist – but it’s unlikely.

Although “quark” is usually pronounced to rhyme with bark, when American physicist Murray Gell-Mann came up with the name he wanted it to rhyme with dork. Gell-Mann says he used the “kwork” sound first without thinking about how to spell it, before coming across a line in James Joyce’s Finnegans Wake, which reads “three quarks for Muster Mark!” The way quarks come in threes made this line and the spelling very apt, but Gell-Mann wanted to keep his original pronunciation (Joyce clearly intended it to rhyme with mark).

Given all the fuss about the Higgs boson lately, there are some interesting observations to be made about the mass of quarks. Almost all the mass of atoms - and hence of you - comes from protons and neutrons. But the vast majority of has nothing to do with the Higgs field. Around 99 percent of the mass of those particles comes not from the intrinsic mass of quarks but from the energy coming from their movement and that of the gluon particles that hold them together. Thanks to Einstein we know energy and mass are equivalent, and though gluons are massless, the energy of the whole vibrant gluon/quark mix inside the protons and neutrons is experienced as mass. Bizarre or what?

Image from Wikipedia