Should colour be loosed on the world …

Adrian Heathcote, University of Sydney

Timothy Paul Smith Hidden Worlds: Hunting for Quarks in Ordinary Matter, New Jersey, Princeton University Press, 2003 (pp. 192). ISBN 0-69112-241-5 (paperback) RRP $43.95.

When I was twelve, two school friends and I had a chemistry club. On Saturday mornings, we would sit out of the way of prying parental eyes to share out a litre of concentrated nitric or sulphuric acid—prizes we had carried home from the city’s chemical suppliers in flasks resting on the handlebars of our pushbikes. Then we would conduct chemistry experiments in the recesses of our separate garages, with equipment that we often made ourselves.

None of us were really interested in making bombs—though of course bombs were made. Rather, in the quiet, pastoral suburbs of Adelaide, we turned the unused back portions of our families’ sheds into spy-holes through which to ‘see’ something of the structure of invisible atoms. Or so we liked to imagine. But after a while the resolution of our experimentation seemed too low. Reality was more than just electrons; there were other particles as well, particles with bizarre names and even more bizarre properties.

By thirteen my interest had moved on to these elementary particles. I could not do much more with my new interest than make lists of the particles and their characteristics in an exercise book. Nor could I find up-to-date information. I was reduced to hanging around a city bookstore to look through the two shelves of books they had dedicated to Science. It was not just that the bookstore was poor—though it was—but also that there simply were no books published on this topic other than university textbooks.

At the time the particle zoo was absurdly overstocked, and my exercise book was full of heavy particles that scientists had found in cosmic ray collisions. The particles were all named after letters from the Greek alphabet. (Why Greek? I wondered; so I read The Odyssey to look for a connection, and failed.) There was talk at the time of new particles that no one could see, called ‘quarks’. Everything, all the particles in my exercise book, were made up of these new particles that had strange non-Greek names. How, I wondered, is it that we can’t see them if we can see the particles that they make up? It seemed like a cheat of some kind. (But what kind of cheat, I wouldn’t discover for a very long time.)

In the 1960s the particle zoo was absurdly overstocked with heavy particles found in cosmic ray collisions.

I felt that I was finally closing in on my quarry: all of reality was quarks! Wonderful! But what are quarks? Here my interest met a brick wall. No one—no teacher, no fellow student—knew anything about them. Quarks were not in books in the library or in the bookstores. Knowledge about them existed, but it was inaccessible from suburban Adelaide.

What I needed was a book like Hidden Worlds: Hunting for Quarks in Ordinary Matter.

Timothy Paul Smith is a young scientist who tells, and tells well, the story of the experimental detection of quarks inside the nucleus of atoms. But he also tells the story of the theory of quarks, and it is the theory that is fascinating.

In 1961 Murray Gell-Man must have been looking at a list of known particles very like the one I had compiled in my exercise book and despaired of its profusion and lack of elegance. Abundance of this kind is not always a sign of richness; it can also be a sign of poverty—poverty of theory and lack of classificatory austerity. What was needed was something like a theory of chemistry, a reduction to a set of simple rules of the various interactions and decays of the particles.

Gell-Man proposed a classification scheme that he called the ‘Eightfold Way’—named in homage to Taoist doctrine (and unwittingly unleashing the physics-verifies-Eastern-mysticism demon that is still with us today). Under this scheme, particles—or a certain subset of them, the strongly interacting particles (those governed by the strong force)—were organised into clusters. The most important cluster, the one that contains the proton and the neutron, was called the Baryon Octet—because the protons are regarded as ‘heavy’ particles, called, collectively, baryons.

The classification scheme predicted the existence of a new particle, called the Ω(the Omega minus). In 1964 it was observed in the Brookhaven National Laboratory in a cloud chamber image that has since become famous (and is reproduced in Smith’s book). Almost simultaneously, Gell-Man and George Zweig independently proposed the existence of fractionally charged particles, which Gell-Man whimsically called ‘quarks’. (Zweig, however, was a young post-doctoral student at the time and his contribution was for a long time overlooked.) The name, as is now well-known—though I certainly didn’t know it when I was thirteen—comes from James Joyce’s Finnegan’s Wake: ‘Three quarks for Muster Mark!’ This makes a strange kind of sense: quark nomenclature comes after classical nomenclature, just as Finnegan’s Wake comes after Joyce’s ‘Greek’ book. (However, I recall once reading a physicist lament the introduction of this ‘baby talk’ into physics.)

Quark nomenclature comes after classical nomenclature, just as Finnegan’s Wake comes after Joyce’s ‘Greek’ book.

Quarks were meant to explain nucleons—protons and neutrons—as well as other strongly interacting particles. Much fuss was made at the time of the fractional electric charge, but in retrospect this seems unastonishing. All it really means is that there is a more fundamental unit than we thought. But the other properties of quarks are a different story. What, for example, is quark ‘colour’?

But at this point we need to back up and remember the basics. There are six ‘flavours’ of quark: up, down, strange, charm, bottom and top, (u, d, s, c, b, and t, for short). Each quark has its anti-quark, so there are also anti-strange and anti-bottom (etcetera) quarks. These are ‘anti-’ in the sense that they have opposite electric charge. But quarks also have another property that distinguishes them: they have ‘colour’. Of course this is not colour in the ordinary sense of the word, it is the name for a strange, new property. Interestingly, however, it behaves remarkably like ordinary colour in that quarks combine into particles that must be ‘colourless’—just as white light is composed of the spectrum of coloured light combined together. The three quark colours are red, green, and blue—but, since this is a little bit like charge, there is also anti-red, anti-green, and anti-blue. Red and anti-red makes a colourless particle, as do three quarks that are red, green and blue.

The way quarks combine to make ‘colourless’ larger particles has two interesting consequences. In quantum field theory, forces ‘hook’ onto certain properties, and are always mediated by the exchange of particles—called virtual particles. Thus the electromagnetic force, for example, hooks onto the property of charge and is mediated by the exchange of photons (particles of light). The strong force hooks onto the property of colour and the exchange particles are called gluons. Gluons bind the quarks together very tightly—in fact the force binds unlike any other force, for the force actually gets stronger as the quarks get farther apart. When they are close together the quarks have what is called ‘asymptotic freedom’—that is, the closer the particles are, the more they behave as though there is no force acting on them at all. But try to move them apart and the force rapidly grows very strong. This means that quarks can never be observed as single, independent, particles. They are always bound.

But it also means that the property of colour is never something we have to contend with on a macroscopic scale. It is locked up in the particles of the nucleus, the protons and neutrons, and similar particles, and doesn’t escape those confines: colour is thus invisible to us.

Some physicists and philosophers treat ‘the quark hypothesis’ as an unverifiable theory.

Some physicists and philosophers have taken all this as a reason to treat ‘the quark hypothesis’ as an unverifiable theory; they speak of our not being able to ‘see’ quarks, and therefore of our not being in a position to know that they exist. But though it is always good to be reminded that scientific knowledge—like all knowledge—is fallible, it is not clear that quarks present a special problem. We ‘see’ quarks by bouncing intense beams of electrons off a nucleon: this is enough to reveal the inner structure—three quarks swimming around freely. The intensity of the beams can even be increased, as Timothy Paul Smith notes, to show the presence of the gluon cloud around the quarks. But this procedure is not different to the way we view the nucleons themselves, or the atom, or even molecules and cells: we scatter particles from the object—whether they be electrons, gamma rays, or light—and collect the results. Indeed, that is how we see trees and buildings—our eyes collect scattered light, which our brains interpret. So the only thing special about quarks is that we can’t physically isolate one—and that doesn’t seem like an important difference (we can’t isolate stars in a distant galaxy either).

Fortunately, the kind of constructivist empiricism that gave rise to this worry no longer holds much sway in philosophy—and so philosophers rarely quibble now about quarks and gluons.

That just leaves us confronting the mystery: have we discovered what reality is made of? I mentioned before that colour is a new property, but in fact understanding what kind of property it is, was exactly the problem that I couldn’t find an answer to when I was thirteen. What is this ‘colour’? I asked; what is isospin? (I thought I understood ‘ordinary’ spin. I didn’t.) What is strangeness? But no matter who I asked, no one knew. But these questions touch on one of the oldest philosophical problems we know of: what is a property? Trying to understand how properties relate to objects is the problem that baffled Plato and Aristotle, and that motivated much of Greek thought about the nature of mathematics. It completely dominated philosophical debate in the Middle Ages, and recurred again in the early modern philosophy that accompanied the scientific revolution of the 17th century. And it has been given new vigour and bite by quantum theory.

This is because in quantum theory, what property we see is only probabilistically determined. ‘Spin’, for example, is much weirder and less able to be visualised than its comfortable name suggests—one certainly can’t just imagine a spinning top. Colour and strangeness are even further removed from anything we are familiar with. In a sense we understand the concepts only ‘through a glass darkly’—where the glass is mathematics, and the light playing on the other side is the abstruse experiments that Timothy Paul Smith so well describes. Indeed there is always something a little bit false about those television programmes and books that present quantum objects as globs of matter or wiggling strings. These descriptions are far too classical in nature.

Thus detecting quarks is a different matter to understanding them. Why, for example, are the top and bottom quarks so rare? No one seems to know. (They were once called the ‘truth’ and ‘beauty’ quarks, perhaps in honour of their rarity. But the names did not really fit. After all, Keats might not have been right in thinking them identical, but they are also surely not polar opposites.) One thing that is known is that the top quark is very massive—100,000 times more massive than the lightest quark—and quite unstable—but why does being massive lead to instability? (In fact the top quark was only observed in 1995 at Fermilab in proton–antiproton collisions.)

Detecting quarks is a different matter to understanding them.

Mass is, of course, an enigma in its own right. The search is still on for that Greta Garbo of elementary particles: the Higgs boson, the particle that is meant to give mass to all other elementary particles. But it is so massive itself that it is unclear whether it has ever been observed. (Smith does not mention the Higgs boson in his account, which is a rather puzzling omission.) Still, it is comforting to know that there are still deep mysteries that have yet to be solved—if only because it holds out the promise of more light being shed on the whole.

In fact the mass of the top quark is intimately bound up with the question of the existence of the Higgs boson. Recent estimates put the mass of the top quark at between 174 and 178 Giga electron volts and this puts constraints on what the mass of the Higgs boson could possibly be. But actually seeing a Higgs boson still seems a long way off—recently (in March 2005) the physicist Tommaso Dorigo predicted that they might just get some evidence of it in 2007! If it exists. And if it does not exist much will be thrown up into the air.

What about my own youthful enthusiasm for quarks? In the middle of the time when I could get no more information on them, I cycled to a distant shop in the middle of a torrential downpour to get a copy of Scientific American (much rarer then than they are now). I was interested in the lead article on radiation pictures from the sun, but inside there was a piece by a famous—though I didn’t know it at the time—logician/philosopher by the name of Alfred Tarski, on an ancient problem: the Liar Paradox. (This same copy of Scientific American sits on my desk as I write this—dated June 1969.)

Here was another trail to follow. The same enigmatic quarry; but a different day, a different method.

Adrian Heathcote is a philosopher at the University of Sydney. His research interests include the philosophy of physics, the philosophy of mathematics, and epistemology.