Physics.

Three physicists who developed a theory to explain the strong interaction that holds together atomic nuclei--one of the four basic forces in the universe--have won the 2004 Nobel Prize in Physics.

David J. Gross of the University of California, Santa Barbara, H. David Politzer of the California Institute of Technology in Pasadena, and Frank Wilczek of the Massachusetts institute of Technology (MIT) will share the $1.36 million prize.

Early last century, scientists discovered that atoms--once thought to be the smallest building blocks of matter--actually are made of protons, neutrons, and electrons. Atom-smashing experiments in the late 1960s confirmed what theorists had begun to suspect earlier that decade: that protons and neutrons are themselves made of smaller components dubbed quarks. However, none of those high-energy experiments ever produced an isolated quark. The strong force apparently always withstood the high-energy violence of the experiments, and the quarks presumably remained confined within protons and neutrons, which measure only about [10.sup.-15] centimeters across.

In 1973, when Gross and Wilczek were at Princeton University and Politzer was at Harvard University, the throe researchers independently discovered a property of the strong interaction that they called "asymptotic freedom." According to this phenomenon, the force of attraction between quarks actually gets weaker when the quarks are close together. Somewhat like the stretching in a rubber band being pulled, the force of attraction gets dramatically stronger as the distance between quarks increases--a result that explains why quarks are never found in isolation.

Before the researchers came up with the concept of asymptotic freedom, any relationship among the plethora of new particles observed during atom-smashing experiments remained hidden. The concept "cleared away the fog" surrounding the strong interaction, comments Edward Witten, a physicist at Princeton University. For the first time, scientists could predict what types of subatomic particles would result from high-energy collisions, he notes.

Experiments and current theories suggest that quarks existed in isolation only in the first 0.1 second or so after the Big Bang, says Wilczek. That's how long it took the young universe to cool down to about 10 trillion[degrees]C, a point at which quarks glommed in triplets to cream the neutrons and protons that make up atomic nuclei.

The understanding of the strong interaction between quarks that stems from Gross, Politzer, and Wilczek's Nobel-winning work is "one of the great cornerstones of our understanding of modern physics," says Marc A. Kastner, head of the physics department at MIT.

"This is a Nobel that's been overdue," says Alfred Mueller, a physicist at Columbia University.

Although the fundamental theory behind the strong interaction is well supported by data, it's also very Complicated. Physicists are still trying to figure out how all the subatomic particles that result from high-energy" collisions are created, Mueller notes.

Because announcements of the Nobel prizes often occur around midday in Sweden, American recipients receive notification that they've won in the exceptionally early hours of the morning sometimes at inopportune moments. For Wilczek, the phone call came at 5:12 a.m., when he was in the middle of a shower. Dripping wet, sans towel, he ended up chatting with a series of people from the Nobel committee. "It was quite a long call," he notes.