Tuesday, October 05, 2004
Nobel Prize in Physics for asymptotic freedom
The 2004 Nobel Prize in Physics has been awarded to David Gross of the Kavli Institute for Theoretical Physics at UC Santa Barbara, David Politzer of Caltech, and Frank Wilczek of MIT, for their work on asymptotic freedom in quantum chromodynamics (QCD). It was predicted by Peter Woit and many others; the most obvious response is "it's about time."
QCD is the theory of the strong interactions, in which quarks possess a certain "color" (purely metaphorical, of course) and are bound together in protons and neutrons by massless particles called gluons. It's extremely similar to how protons and electrons possess a quantity called "charge" and are bound together in an atom by photons. But there is also a crucial difference -- you can pull an electron apart from an atom (and thank goodness, since TV and other necessities would otherwise be impossible), but you can't pull quarks out of protons and neutrons. The basic reason why is asymptotic freedom -- the remarkable quality that the QCD force gets weaker at higher energies (short distances) and stronger at low energies (large distances). In the early '70s physicists were struggling to understand new data on the structure of protons and neutrons from "deep inelastic scattering" experiments at the Stanford Linear Accelerator (SLAC) and elsewhere, in which high-energy electrons were fired at these heavier nuclear particles. It all snapped into place once Gross, Politzer and Wilczek discovered asymptotic freedom (through some heroic calculations) and immediately applied it to make sense of the data -- the quarks were becoming free (non-interacting) asymptotically (as the energies were increased). Bjorken and others had discussed the possibility of asymptotic freedom, but it was Gross, Politzer and Wilczek who actually demonstrated that QCD (a non-abelian Yang-Mills theory, to be specific) would have that property. These days QCD is a phenomenally successful theory, and forms a crucial part of the Standard Model of particle physics. This is a Nobel Prize that is long overdue and well deserved.
Update: Let me add a few notes to the hurried description given above. First, both Wilczek and Politzer were graduate students at the time this research was done; Gross was an assistant professor at Princeton and Wilczek's advisor, while Politzer was advised by Sidney Coleman at Harvard. Great ideas in theoretical physics are often (always?) originated by the young. Second, both Gross and Wilczek have at different times been kind enough to write letters of recommendation for me or support me in other ways, for which I'm sincerely grateful. I recall clearly my senior year as an undergraduate at Villanova, wandering around Jadwin hall at Princeton and knocking on David Gross's door unannounced, and how he chatted with me about what it was really like to be a theoretical physicist. At the time I had no clue who he was; now I know. Finally, Frank Wilczek was an undergraduate here at Chicago, getting his degree in math in 1970. You may ask, does that mean the UofC will count him as yet another Chicago Nobelist? Don't be silly; of course we will. That makes 29 Nobel Prizes in the physical sciences, and counting.