Three Americans won the Nobel Prize in Physics yesterday for describing the force that shapes the structure of atomic nuclei, a key to understanding the so-called Standard Model that describes how nature's tiniest particles interact.

The Royal Swedish Academy of Sciences announced that theoretical physicists David J. Gross, Frank Wilczek and H. David Politzer will share $1.36 million for explaining that quarks, the particles that make up protons and neutrons, bind more closely together as they are pulled apart.

This rubber-band effect, known as "asymptotic freedom," makes it impossible to isolate single quarks, and it provides a framework to describe the nuclear "strong force," one of nature's four basic forces along with gravity, electromagnetism and the "weak force" that causes radioactive decay.

"This comes as no surprise," said Sylvester J. Gates, director of the University of Maryland's Center for String and Particle Theory. "I was in South Africa with Gross last year, and the local paper described David as a 'likely future recipient of the Nobel Prize.' "

Gross, 63, director of the Kavli Institute for Theoretical Physics at the University of California at Santa Barbara, said in a telephone interview that he was "asleep when the Academy called" at 2:30 a.m. Pacific time.

The Massachusetts Institute of Technology's Wilczek, 53, however, acknowledged during a telephone news conference that "I'd be lying if I said it came as a shock -- the theory is very important, and the data that proves it have been known for 20 years. I had a sleepless night, and I'm pleased that it's over with."

Gross was a young assistant professor at Princeton University and Wilczek was his first graduate student when they began working on quarks and nuclei in the 1960s. Politzer was working independently on the same questions as a Harvard University graduate student. Politzer, now 55 and at the California Institute of Technology, did not speak to reporters yesterday.

Physicists began exploring atomic nuclei in the 1930s and soon realized that gravity and electromagnetism were not strong enough forces to hold nuclei together or shape them. The first attempts to find the forces at work were "rather crude," Wilczek explained, consisting of "throwing particles together to see what came out."

This process, remembered by physicists as "the zoo," got "completely out of hand in the 1960s," Wilczek continued. Scientists knew about quarks, the basic building blocks of protons and neutrons, but "there was no real understanding of what a quark was, and individual quarks were never discovered."

Late in the 1960s, researchers found that quarks inside protons and neutrons were behaving almost as free particles not locked in a tight nuclear embrace. "How could they be freely moving and not get out?" Gross said. "It was crazy."

But in back-to-back papers in Physical Review Letters in 1973, Gross and Wilczek, and Politzer separately, modeled this curious behavior mathematically -- showing that the bonds holding quarks together strengthened as the particles drew apart. "They were permanently bound inside the nucleus," Gross said. For this reason, it was impossible to extract a single quark.

The researchers explained this counterintuitive behavior -- gravity and electromagnetism, by contrast, diminish with distance -- by suggesting that gluons, the particles that carry the "strong force" between the quarks, interact with quarks and with each other to create "asymptotic freedom."

Wilczek noted that he and his colleagues did their original work with pencil and paper, "and the original calculations filled notebooks." Today, he added, the basic framework "is an exercise in quantum field theory classes, but it is still difficult to do in less than a single page."

The theory came to be called "quantum chromodynamics," so named because different quarks have different electric charges and different characteristics, which give them unique signatures that physicists call "colors."

Once the three physicists explained these puzzling properties, their colleagues needed to demonstrate them: "One of my colleagues at the time said, 'David, this is a great thing you've come up with, and a great theory, but it will never be proven,' " Gross said.

But after a decade, quantum chromodynamics remained unscathed: "These gentlemen saw something no one else had seen," Gates said in a telephone interview. "The property of strengthening the bonds as quarks move apart is absolutely critical. Without it you couldn't form protons and neutrons, and they wouldn't bind together."

Quantum chromodynamics also helped to explain the Standard Model, which describes the interactions of the three basic forces of particle physics -- the strong force, weak force and electromagnetism. Scientists are still trying to link the fourth force, gravity, to the Standard Model to achieve a "theory of everything."