PARIS, Oct 5: What are the smallest building blocks of matter? How do they function? And what forces hold them together? For the past century, physicists striving to build a conceptual architecture to explain the entire Universe, from the tiny spaces within the atomic nucleus to the vast spaces between galaxies, have been pursuing the ever tinier.

That path has led to the quark, to the unveiling of an exotic bestiary of sub-atomic particles and to the 2004 Nobel prize for three Americans, David Gross, David Politzer and Frank Wilczek.

Discovered 40 years ago - its name reputedly comes from a nonsense word coined by James Joyce in his novel "Finnegan's Wake" ("Three quarks for Muster Mark") - the quark makes up protons and neutrons, which in turn comprise the nucleus of atoms.

The trio's big contribution was a discovery into one of the fundamental forces of nature - the "strong force," an unusual bond that keeps quarks together. "[The three] have brought physics one step closer to fulfilling a grand dream, to formulate a unified theory comprising gravity as well - a theory for everything," the Nobel citation said on Tuesday.

The discovery, known as asymptotic freedom, "was totally unexpected," Gross recalled in 1991, 18 years after their breakthrough. "Like an atheist who has just received a message from a burning bush, I became an immediate true believer."

Physicists had been puzzled for years by a quark paradox. Every time they did their maths, they found that at high energies, whenever quarks are close to each other the force that binds them strangely slackens and quarks move almost as if they were free particles. But this cannot in fact happen, as quarks are only found in batches of two or three, and are never single and unattached. Conversely, when quarks move apart, the force becomes stronger, as if the bond is a rubber band - the more it stretches, the stronger the force.

Two studies published in June 1973 in the US journal Physical Review Letters, one by Gross and Wilczek and the other by Politzer, found the answer to this apparent anomaly.

They theorised that sub-atomic force carriers called gluons (from the word "glue") have a unique property, in that they interact with quarks and also with each other. That phenomenon disrupts the quarks' "colour charge" - the kind and strength of electric charge it carries - and weakens interaction between them.

However, the interaction strength increases with increasing distance, and this explains why one quark is wedded to two or three others and cannot be removed from an atomic nucleus.

Asymptotic freedom led to the formulation of the Quantum Chromo Dynamics (QCD) theory, a new and major contribution to the Standard Model of particle physics. Further work suggests there are six types ("flavours") of quark, defined according to their mass and electric charge.

They are called Up, Down, Strangeness, Charm, Bottom (or Beauty), Top (or Truth). There may seem little practical outlet to quark research, but discoveries in this field have the capacity for seismic changes in our perception of the cosmos.

The "strong force" is considered to be one of the four fundamental forces of nature. The three others are gravity, the force that binds our Solar System together and keeps us on Earth; electromagnetic interaction, the electrical charge that binds atomic sub-particles together; and the weak force, which like the strong force also takes place in the atomic nucleus but instead is responsible for its radioactivity, not for its quark bonding.

The Standard Model is not perfect, however. Some physicists believe there is a still smaller sub-atomic particle to be found, called the Higgs Boson, which helps to explain enigmas about mass. -AFP