The prize went to Frank Wilczek, an alumnus of the University of Chicago and now a professor at the Massachusetts Institute of Technology; David Politzer of the California Institute of Technology; and David Gross of the University of California, Santa Barbara.

''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,'' the Nobel jury said.

The nuclei of atoms are made up of particles called protons and neutrons, which, in turn, are made up of quarks.

The theory of the strong force, which was developed in 1973, defied the prevailing notions of how the fundamental forces of nature work. Unlike gravity, for example, which gets stronger as objects come closer together, the strong force gets weaker.

The most startling aspect of their theory postulates that the force between quarks becomes stronger the farther apart they are pulled, like a stretched rubberband.

In other words, one quark cannot be completely pulled apart from another quark, which explained one of the biggest mysteries of physics - why scientists had not been able to see individual quarks in their high-powered atom smashers.

''The forces between the quarks becomes weaker and weaker as the quarks get closer to each other,'' said Robert Wald, chairman of the University of Chicago's physics department. ''Conversely they get stronger and stronger as the quarks get pulled away from each other, which accounts for why we don't ever see free quarks by themselves.''

The strong force makes the world we know possible, from the atoms in our bodies to those in distant galaxies.

''If you didn't have the nucleus, you wouldn't have atoms, you wouldn't have anything,'' Wald said.

The theory of how the strong force works also set physicists on the road to unifying three of the four forces of nature - the weak and strong forces and the electromagnetic force. Gravity, so far, remains outside their attempts at unification.

Wilczek was a 21-year-old graduate student of Gross' when the two made their discovery of the strong force, independently of Politzer, who was working along the same lines.

Although Wilczek majored in mathematics at the University of Chicago, he took an advanced physics course from Peter Freund, whom he credits for directing his interest into physics.

''He [Wilczek] sat in the back of the class and during each session he would ask one brilliant question which took me at least 15 minutes to answer,'' said Freund, now professor emeritus. ''He impressed me with his extremely powerful intuition.''

On Monday, U.S. researchers Richard Axel and Linda Buck won the Nobel Prize in medicine for their research explaining how mammals smell.

The chemistry prize will be announced today, the literature prize Thursday and the peace prize Friday. The economics prize will be announced Oct. 11.