Life's Little Quarks

Few scientists can claim that they have helped to unravel the most fundamental mysteries of the universe. But three such researchers were honored with this year's Nobel Prize in Physics for developing a mathematical model of the strong nuclear force. Their prize-winning work was a major step in the quest to develop a comprehensive theory of physics--a so-called theory of everything.

David J. Gross, 63, of the University of California, Santa Barbara; H. David Politzer, 55, of California Institute of Technology; and Frank A. Wilczek, 53, of Massachusetts Institute of Technology shared the nearly $1.4 million prize for "the discovery of asymptotic freedom in the theory of the strong interaction," the Nobel Foundation announced on Oct. 5.

The strong interaction--also known as the strong nuclear or color force--is one of the four fundamental forces of nature. It acts between quarks, the tiny components of protons and neutrons, and also provides the force binding atomic nuclei together. The other fundamental forces are electromagnetism, which acts between charged particles; the weak force, responsible for ß-decay and other phenomena; and gravity.

"This was the attempt, really, pretty much spanning the 20th century, but in mature form starting in the 1930s, to understand what atomic nuclei were made of and what held them together," Wilczek tells C&EN. "It was realized at that time that one had a fundamentally new force--not just the electromagnetism and gravitation that people had been familiar with for centuries.

"Experiments had revealed that protons and neutrons were very complicated objects, and real simplicity only emerged at the next [smaller] level, and that's where we come in," he says. "What we found is that very, very special forms of interactions could lead to things getting simpler at high energies. And so by following out that idea consistently, we were led to a unique theory of what protons and neutrons and strongly interacting particles in general are made out of and how they interact. That's what's now called QCD, or quantum chromodynamics," Wilczek says.

By 1973, the year in which Politzer and the Gross-Wilczek team independently published their models, scientists had already developed a successful quantum theory of electromagnetism. They had also discovered that electromagnetism and the weak force arise from a common origin.

Together, those theories and QCD form the Standard Model of particle physics, which provides a powerful mathematical description of electromagnetism, the weak force, and the strong nuclear force. It is the most comprehensive theory of physics to date and has proven quite successful at accurately modeling most quantum phenomena. As chemists continue to work at increasingly smaller scales, the model provides a fundamental framework for understanding--and a theoretical tool for predicting--new quantum effects.

The Standard Model, however, is still incomplete. And scientists have met with only limited success at incorporating gravity, the fourth and final force, into the model to create a "theory of everything" that provides a basic understanding of all physical phenomena.

Gross, Politzer, and Wilczek's model explains why quarks cannot be isolated in a lab by invoking the peculiar concept of asymptotic freedom: Essentially, the strong nuclear force grows in strength as the distance between interacting quarks increases and only loosely binds quarks when they are in close proximity.

This behavior is often likened to balls held together by a rubber band. When the balls are close together, the band exerts little force, corresponding to the situation in protons and neutrons where quarks behave essentially as noninteracting "free" particles. Start to pull the balls apart, though, and the band will exert increasing resistance in the opposite direction.

In fact, under normal conditions, the energy required to separate quarks far exceeds the energy needed to create new quark-antiquark pairs. Instead of the original quark being ejected, the new quarks and antiquarks recombine into new subatomic particles.

Because the strong force dominates many of the high-energy collisions that take place in particle accelerators, QCD is the primary tool for modeling those interactions. "The other thing that I guess was unanticipated was that it turns out to be a special gift that things are simple at high energies," Wilczek says. It's made it "possible to understand conditions in the very early universe, like within the Big Bang, where things were much hotter, and therefore at higher energies and much denser. Instead of things getting messy and complicated, now we realize it's actually simple, and so it's been possible to push back the frontiers of the early universe."

About his own role, though, Wilczek remains modest. "The problem of the strong interaction was a--if not the--central problem of physics in many ways starting in the 1930s, once people figured out atoms," he says. "So many, many people contributed."