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The 2004 Nobel Prize in Physics

05.10.2004 -

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The Royal Swedish Academy of Sciences has decided to award the Nobel Prize in Physics for 2004 "for the discovery of asymptotic freedom in the theory of the strong interaction" jointly to

David J. Gross
Kavli Institute for Theoretical Physics, University of California, Santa Barbara, USA,

H. David Politzer
California Institute of Technology (Caltech), Pasadena, USA, and

Frank Wilczek
Massachusetts Institute of Technology (MIT), Cambridge, USA.

A 'colourful' discovery in the world of quarks

What are the smallest building blocks in Nature? How do these particles build up everything we see around us? What forces act in Nature and how do they actually function?

This year's Nobel Prize in Physics deals with these fundamental questions, problems that occupied physicists throughout the 20th century and still challenge both theoreticians and experimentalists working at the major particle accelerators.

David Gross, David Politzer and Frank Wilczek have made an important theoretical discovery concerning the strong force, or the 'colour force' as it is also called. The strong force is the one that is dominant in the atomic nucleus, acting between the quarks inside the proton and the neutron. What this year's Laureates discovered was something that, at first sight, seemed completely contradictory. The interpretation of their mathematical result was that the closer the quarks are to each other, the weaker is the 'colour charge'. When the quarks are really close to each other, the force is so weak that they behave almost as free particles. This phenomenon is called ”asymptotic freedom”. The converse is true when the quarks move apart: the force becomes stronger when the distance increases. This property may be compared to a rubber band. The more the band is stretched, the stronger the force.

This discovery was expressed in 1973 in an elegant mathematical framework that led to a completely new theory, Quantum ChromoDynamics, QCD. This theory was an important contribution to the Standard Model, the theory that describes all physics connected with the electromagnetic force (which acts between charged particles), the weak force (which is important for the sun's energy production) and the strong force (which acts between quarks). With the aid of QCD physicists can at last explain why quarks only behave as free particles at extremely high energies. In the proton and the neutron they always occur in triplets.

Thanks to their discovery, David Gross, David Politzer and Frank Wilczek 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 discovery which is awarded this year's Nobel Prize is of decisive importance for our understanding of how the theory of one of Nature's fundamental forces works, the force that ties together the smallest pieces of matter - the quarks. David Gross, David Politzer and Frank Wilczek have through their theoretical contributions made it possible to complete the Standard Model of Particle Physics, the model that describes the smallest objects in Nature and how they interact. At the same time it constitutes an important step in the endeavour to provide a unified description of all the forces of Nature, regardless of the spatial scale - from the tiniest distances within the atomic nucleus to the vast distances of the universe.


The strong force explained

The strong interaction - often called the colour interaction - is one of Nature’s four basic forces. It acts between the quarks, the constituents that build protons, neutrons and the nuclei. Progress in particle physics or its relevance for our daily life can sometimes appear hard to grasp for anyone without a knowledge of physics. However, when analysing an everyday phenomenon like a coin spinning on a table, its movements are in fact determined by the fundamental forces between the basic building blocks - protons, neutrons, electrons. In fact, about 80% of the coin’s weight is due to movements and processes in the interior of the protons and neutrons - the interaction between quarks. This year’s Nobel Prize is about this interaction, the strong or colour force.

David Gross, David Politzer and Frank Wilczek discovered a property of the strong interaction which explains why quarks may behave almost as free particles only at high energies. The discovery laid the foundation for the theory for the colour interaction (a more complete name is Quantum ChromoDynamics, QCD). The theory has been tested in great detail, in particular during recent years at the European Laboratory for Particle Physics, CERN, in Geneva.


The Standard Model and the four forces of Nature

The first force that must have been evident to humans is gravity. This is the interaction that makes objects fall to the ground but also governs the movements of planets and galaxies. Gravity may seem strong - consider, for example, the large craters formed by comets hitting the earth, or the huge rockets that are required to lift a satellite into space. However, in the microcosmos, among particles like electrons and protons, the force of gravity is extremely weak (fig.1).

The three forces or interactions, as phycisists prefer to call them, that are applicable to the microcosmos are described by the Standard Model. They are the electromagnetic interaction, the weak interaction and the strong interaction. Through the contributions of several earlier Nobel Laureates the Standard Model has a very strong theoretical standing. This is because it is the only mathematical description which takes into account both Einstein’s theory of relativity and quantum mechanics.

The Standard Model describes quarks, leptons and force-carrying particles. Quarks build, for instance, the protons and neutrons of the atomic nucleus. Electrons that form the outer casing for atoms are leptons and, as far as is known, are not constructed from any smaller constituents. The atoms join up to form molecules, the molecules build up structures and in this way the whole universe can finally be described.

Quelle: The Royal Swedish Academy of Science

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