Strong theory: Nobel winners cast light on the gluons that hold nuclear particles together. Credit: ARSCIMED/SPL

While the Nobel prize for medicine has been awarded for work on smell, this year's physics Nobel stems from research on colour.

But it is not exactly colour as we know it. The prize has been scooped for work on ‘colour interaction’: the poetic way in which physicists refer to the strong nuclear force, which binds together the fundamental particles called quarks that make up protons and neutrons in an atomic nucleus.

The prize is shared by David Gross of the University of California, Santa Barbara, David Politzer of the California Institute of Technology and Frank Wilczek of the Massachusetts Institute of Technology for their work illuminating the bizarre nature of this force — and making it calculable, at least some of the time.

“It's fantastic news,” says particle physicist Jeff Forshaw of the University of Manchester, UK. “Until they came along, we were really struggling to understand the strong force.”

Of the four basic forces in nature — gravity, electromagnetism, and the weak and strong nuclear forces — the strong force is particularly counter-intuitive, with the highly unusual property of getting stronger as the quarks get further apart.

This feature, comparable to the behaviour of a rubber band as it is stretched, explains why quarks never appear on their own, but always in combinations. Protons and neutrons each contain a triplet of quarks.

In 1973, Gross, with his graduate student Wilczek, and Politzer published two papers back to back in which they outlined the way the strong force operated (D. J. Gross and F. Wilczek Phys. Rev. Lett. 30, 1343; 1973 and H. D. Politzer Phys. Rev. Lett. 30, 1346; 1973). It had been worked out by then that quarks have a quantized property called ‘colour charge’, which is analogous to electrical charge but comes in three ‘colours’: red, blue and green. The theory that attempted to explain the colour interaction was called quantum chromodynamics (QCD).

Before these two papers, no one knew how to solve the equations of QCD theory. The problem was the sheer complexity of the interaction. Just as electrical forces between particles are mediated by photons passing between them, so the strong force is mediated by particles called gluons. But whereas photons have no electrical charge, gluons do have a colour charge, so they interact with each other as well as with quarks. This meant that “the theory was so strongly interacting that it was incalculable”, says Forshaw.

Gross, Wilczek and Politzer got round this problem by calculating that the strength of the colour force gets weaker as the particles' energies increase — or equivalently, as the particles get closer together. At sufficiently high energy, the quarks act as though they are ‘free’. This became known as ‘asymptotic freedom’.

Their work showed particle physicists that if they wanted to understand the strong force they needed to probe it at high energies, where the weakening of the interaction made QCD a testable theory. Studies of high-energy collisions between particles, in particular at the DESY accelerator in Hamburg and the Large Electron-Positron collider at CERN in Geneva, have now verified in great detail the predictions of asymptotic freedom.

Not all aspects of the strong interaction are understood, however. For instance, there is still no way to use QCD to explain some basic properties of quarks at the relatively modest energies characteristic of everyday matter.

But it is precisely the understandable, high-energy region of QCD that researchers are looking at to find a ‘theory of everything’. Modifications of QCD at high energies, for example, should be able to describe the Higgs boson, the particle thought to be responsible for giving matter mass.

The Nobel Prize in Chemistry was announced after Nature went to press. For coverage see → http://www.nature.com/news