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10/05/2004 07:00:00

David Politzer Wins Nobel Prize in Physics

PASADENA, Calif.--Hugh David Politzer has won the 2004 Nobel Prize in physics for work he began as a graduate student on how the elementary particles known as quarks are bound together to form the protons and neutrons of atomic nuclei. The announcement was made today by the Royal Swedish Academy of Sciences.

Politzer, a professor of theoretical physics at the California Institute of Technology, shares the prize with David Gross and Frank Wilczek. The key discovery celebrated by today's prize was made in 1973, when Politzer, a Harvard University graduate student at the time, and two physicists working independently from Politzer at Princeton University--Gross and his graduate student Wilczek--theorized that quarks actually become bound more tightly the farther they get from each other.

This discovery has been known for 31 years as "asymptotic freedom," and is often described by physics professors to their students with the analogy of a rubber band increasing in tightness as it is pulled apart. Asymptotic freedom established quantum chromodynamics (QCD) as the correct theory of the strong force, one of the four fundamental forces of nature.

Caltech president David Baltimore, himself a Nobel laureate, said he was pleased that another Caltech faculty member has joined the list of the Institute's Nobel recipients. "It's wonderful that David was acknowledged for something that was so far back in his career," Baltimore said. "It shows what young people can do if they think differently."

Politzer joined the Caltech faculty as a visiting associate in 1975, the year after finishing his Harvard Ph.D. in physics and three years after publishing his work on asymptotic freedom. He earned tenure in 1976, became a full professor in 1979, and served as head of the physics department (executive officer, in Caltech parlance) from 1986 to 1988.

A native of New York City, Politzer earned his bachelor's degree from the University of Michigan in 1969. The paper that inaugurated his Nobel Prize-winning work, titled "Reliable Perturbative Results for Strong Interactions?" appeared in the journal Physical Review Letters in 1973 and was Politzer's first published article. Politzer's initial foray into the public limelight came in 1989, when he was recruited to play physicist Robert Serber in the movie Fat Man and Little Boy, which recounted the story of the Manhattan Project and starred Paul Newman as the hard-driving project leader Gen. Leslie Groves. The director of the film, Roland Joffe, had been recruiting career physicists to play some of the roles, and he settled on Politzer, whose academic specialty was quite similar to that of the man he would play.

Politzer, who did not even own a television, later told a reporter from Caltech's internal publication On Campus that he had been reluctant to take the part, but had relented after Joffe convinced him that the "role would not require too much in the way of time or talent." During his two weeks on the set, Politzer warmed up to the project and began discussing nuclear defense policy with Paul Newman, with whom he shared a memorable dinner of spaghetti and salad--the latter dressed with "Newman's Own," of course.

Today's award brings to 31 the total number of prizes won by 30 Caltech faculty and alumni through the years (Linus Pauling won awards in both chemistry and peace). Founded in 1891, Caltech is located on a 124-acre campus in Pasadena. The Institute also manages the nearby Jet Propulsion Laboratory and operates eight other off-campus astronomical, seismological, and marine biology facilities. Caltech has an enrollment of some 2,000 students, more than half of whom are in graduate studies, and a faculty of about 280 professorial members and 65 research members, and some 560 postdoctoral scholars. Caltech employs a staff of more than 2,600 on campus and 5,100 at JPL.

U.S. News &World Report consistently ranks Caltech's undergraduate and graduate programs as being among the nation's best. The average SAT score of members of recent incoming freshman classes has consistently been at 1500.

Four Crafoord Prizes have been awarded to faculty members and alumni. Forty-seven Caltech faculty members and alumni have received the National Medal of Science; and nine alumni (two of whom are also trustees; and one is also a faculty member), two additional trustees, and one additional faculty member have won the National Medal of Technology. Since 1958, 14 faculty members have received the annual California Scientist of the Year award. On the Caltech faculty there are 78 fellows of the American Academy of Arts and Sciences; and on the faculty and Board of Trustees, 71 members of the National Academy of Sciences and 43 members of the National Academy of Engineering.

Caltech has more than 21,000 alumni.

 

The following information was written by Caltech's MacArthur Professor of Theoretical Physics John Preskill, a colleague of Politzer's. Preskill prepared the text upon learning that Politzer had won the Nobel Prize, and sent it from England this morning:

Of the four fundamental forces--the others besides the strong nuclear force are electromagnetism, the weak nuclear force (responsible for the decay of radioactive nuclei), and gravitation--the strong force was by far the most poorly understood in the early 1970s. It had been suggested in 1964 by Caltech physicist Murray Gell-Mann that protons and neutrons contain more elementary objects, which he called quarks.

Yet isolated quarks are never seen, indicating that the quarks are permanently bound together by powerful nuclear forces. Meanwhile, studies of high-energy collisions between electrons and protons performed at the Stanford Linear Accelerator Center (SLAC) had probed the internal structure of the proton, and Caltech's Richard Feynman had suggested in 1969 that the results of these experiments could be explained if quarks inside a proton are nearly free, not subject to any force. Feynman's suggestion, together with the observation that quarks are unable to escape from nuclear particles, posed a deep puzzle: how could nuclear forces be both strong enough to account for the permanent confinement of quarks and weak enough to account for the SLAC experiments?

The discovery of asymptotic freedom provided a highly satisfying resolution of this puzzle. The calculations of Gross, Wilczek, and Politzer showed that in quantum chromodynamics (QCD), quarks are held together strongly when separated by a distance comparable to the size of a proton, explaining quark confinement. Yet for the smaller separations explored in the high-energy SLAC experiments, the attraction is weaker, supporting Feynman's proposal.

Before this development, many physicists had anticipated that understanding the strong nuclear force would require revolutionary new concepts. But surprisingly, QCD has a remarkable mathematical similarity to quantum electrodynamics (QED), the theory that successfully explains electromagnetic phenomena. In QED the force between two electrically charged particles is mediated by the exchange of a photon (a particle of light) between the two particles; in QCD, the quarks carry a different kind of charge, called "color," and the force between two colored particles is mediated by the exchange of a "gluon" between the particles.

The crucial difference between the two theories is that while the photons of QED carry no charge of their own, the gluons of QCD are themselves colored particles. A quark is surrounded by a sea of "virtual" gluons that arise due to quantum fluctuations, and the color of the virtual gluons enhances the quark's own color. A probe coming closer and closer to the quark is influenced less and less by the virtual gluons, so that the effective color charge of the quark seems to weaken; this is asymptotic freedom.

Gross, Wilczek, and Politzer used pencil and paper to perform their breakthrough calculation. In 1973, the methods they needed were newly developed and fraught with subtleties. Today, the calculation is routinely assigned to physics graduate students as a homework exercise.

QCD predicts that the strength of the force between quarks changes with distance in a particular calculable way that has been well confirmed in experiments studying high-energy collisions of elementary particles. The theory makes other detailed predictions, such as the masses of various strongly interacting nuclear particles, which can be extracted only through large-scale numerical computations performed using supercomputers; these too are in satisfying agreement with experiment.

Because QCD, the theory of the strong nuclear force, turned out to be so similar to QED and to the theory of the weak nuclear force, it became possible after the discovery of asymptotic freedom to conceive of unified theories that incorporate all three forces into a common framework. Such theories have been proposed, but still await experimental confirmation. A further challenge, being pursued by many physicists today, is to achieve an even broader unification theory that encompasses the gravitational force as well.