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Freeing Up the Strong Force
5 October 2004 (All day)
It might be fun to blow things up, but if you want to win a Nobel Prize in physics, perhaps it's wiser to keep things--especially equations--from blowing up. That's what this year's winners did to win the plaudits of their colleagues.
The three new laureates, Frank Wilczek, David Gross, and H. David Politzer, discovered a property of the strong force--the force that glues quarks to each other--known as "asymptotic freedom." Not only did the idea explain some baffling experimental results in particle colliders, it also showed how to keep the equations that describe the strong force from blowing up--producing meaningless infinities in certain situations.
Particle physics is swimming with scenarios in which the equations that describe the behavior of a particle seem to explode. For example, the plain-vanilla equations of the Standard Model say, strictly speaking, that the charge of the bare electron is infinite; as you approach the electron, the measured charge increases without bound. Researchers have developed mathematical tools to keep their theories from being derailed by this type of infinity.
In the early 1970s, physicists studying the strong force were beating their heads against a similar problem. The infinity-coping techniques developed for the electric force (and for the weak force, which is responsible for phenomena such as nuclear decay) didn't work for the strong force. In 1973, Politzer, currently at the California Institute of Technology, and, separately, Wilczek, at the Massachusetts Institute of Technology, and Gross, at the Kavli Institute for Theoretical Physics, realized the problem: Unlike the other forces, the strong force is weaker at close range.
Stick a particle right next to a quark and it wouldn't feel the strong force at all; it would be "asymptotically free" from the strong force, and quarks forced into close proximity would behave more or less like hard particles rather than a sticky clump. This is precisely what experimentalists had found a few years earlier at the Stanford Linear Accelerator Center by scattering electrons off of protons. Turning around the logic of asymptotic freedom explains why quarks are never found roaming free from each other: At large distances and low temperatures, the strong force is too powerful to overcome.
"They made the discovery and saw the significance of it," says Niels Kjaer Nielsen, a physicist at the University of Southern Denmark in Odense. "[The prize] is fully deserved." And although Wilczek admits that he expected the prize at some point, today's early-morning phone call from the Royal Academy caught him off guard. In fact, he was right out of the shower, dripping wet and naked. "It was a bit uncomfortable talking to one Swede after another," in that condition, he says.