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December 2004 (Volume 13, Number 11)
Three American physicists have been awarded the 2004 Nobel Prize in Physics. David J. Gross (University of California, Santa Barbara), David Politzer (Caltech) and Frank Wilczek (MIT) were honored "for the discovery of asymptotic freedom in the theory of the strong interaction." Published in 1973, their discoveries led to the development of the theory of quantum chromodynamics (QCD), a companion to quantum electrodynamics (QED), the crown jewel of modern physics, which describes the interactions of the electromagnetic force with matter.
QCD describes the strong force, also known as the color interaction, which holds together the quarks that make up the various constituents of the atom (protons and neutrons). The existence of quarks had been known since the 1960s, but scientists discovered a couple of strange features. First, quarks have electric charges that are a fraction of the proton's — -1/3 or +2/3-something scientists have yet to explain. Second, in addition to its electrical charge, each quark also has a special quantized property called the color charge; quarks can carry the color charges red, blue or green.
And finally, it is not possible to produce free quarks; they are fundamentally confined. They can, however, sometimes appear to be free particles, or grains inside a proton—an effect witnessed in several scattering experiments between electrons and protons.
What Gross, Politzer and Wilczek discovered initially appeared to be contradictory: for some reason, the closer quarks come to each other, the weaker the color charge between them. When quarks are very close to each other, the force is so weak that they behave much like free particles.
This behavior is called asymptotic freedom. When quarks move apart, the force remains essentially constant as the distance between them increases, thereby preventing quarks from escaping as free particles.
This happens because the force carrying particles (gluons) interact not only with quarks, but also with each other. Asymptotic freedom made it possible to calculate the small distance interaction for quarks and gluons, and to compare it with experimental data obtained by colliding the particles at very high energies.
The three physicists devised an elegant mathematical framework to express this discovery, leading to the development of QCD, an integral aspect of the Standard Model of particle physics, which describes all physics connected with the electromagnetic force, the weak force, and the strong force. The theory has since been rigorously tested experimentally, most recently at CERN in Switzerland. Thanks to QCD, physicists are able to explain why quarks only behave as free particles at extremely high energies. In the proton and neutron,they always occur in triplets.
Perhaps the most significant implication of QCD asymptotic freedom is that it opens up the possibility of a Grand Unified Theory describing all of the forces of Nature.
The model must also incorporate the recent discovery that neutrinos have non- zero mass, which may in turn lead to better understanding of the nature of the dark matter that seems to dominate in our universe.
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