A refugee family in New York
Hugh David Politzer was born in New York City on August 31, 1949. His parents had escaped from Czechoslovakia to England in 1939, the year Nazi Germany occupied the country, and came to the United States after the Second World War. He grew up in New York and attended the Bronx High School of Science, the public school famous for producing scientists, graduating in 1966.
He studied physics at the University of Michigan, earning his bachelor’s degree in 1969, and went on to graduate school at Harvard, where his adviser was Sidney Coleman, one of the most admired theoretical physicists and teachers of his generation.
The puzzle of the strong force
By the early 1970s physicists had a problem. Experiments at the Stanford Linear Accelerator Center had shown that protons and neutrons contain smaller, point-like particles, the quarks. Oddly, when struck very hard by high-energy electrons, the quarks inside a proton behaved almost as if they were free, barely bound to each other at all. Yet no one had ever knocked a single quark loose. Whatever force held quarks together seemed weak at close quarters and unbreakable at a distance.
No known theory behaved that way. In the theory of electricity and light, quantum electrodynamics, a force gets stronger, not weaker, at shorter distances, and many physicists believed that every realistic theory of particles must do the same. Some even offered arguments that this was unavoidable. If so, there would be no way to calculate the strong force reliably, and the strong interaction, which binds every atomic nucleus, would remain beyond the reach of precise theory.
A question of sign
Physicists describe how a force changes with distance by a quantity called the beta function. If it is positive, the force grows stronger at short distances, as in electromagnetism, where the strength of the electric force, measured at CERN’s LEP collider at high energies, was found to be noticeably greater than at low energies. If it is negative, the force weakens at short distances. In the early 1970s the German theorist Kurt Symanzik looked for a theory with a negative beta function but did not find one, and the Dutch physicist Gerard ’t Hooft came very close in the summer of 1972. By then, according to the Nobel committee’s account, physicists had started to despair, and some had even presented evidence that all realistic theories must have a positive beta function.
The calculation that settled the question required carefully adding up the contributions of all the particles that can flicker in and out of existence around a charge. In theories of the type Politzer studied, the force-carrying particles themselves make a contribution with the opposite sign to that of ordinary particles, and if there are not too many kinds of quark, they win. The beta function comes out negative.
Asymptotic freedom
As a graduate student at Harvard, Politzer calculated how the strength of the force changes with distance in a particular class of theories, called non-Abelian gauge theories, in which the particles that carry the force themselves carry the kind of charge they respond to. He found that in such theories the force can grow weaker as particles come closer together. At very short distances, or very high energies, quarks would behave almost like free particles. Physicists call this asymptotic freedom.
His paper, Reliable Perturbative Results for Strong Interactions?, appeared in Physical Review Letters in June 1973, back to back with a paper by David Gross and his graduate student Frank Wilczek at Princeton, who had reached the same result independently. Politzer was 23. The Nobel committee likened the force between quarks to a rubber band: slack when the quarks are close, and pulling harder the further apart they move.
The discovery made a theory of the strong force possible. Applied to quarks carrying a charge physicists call colour, and to the gluons that carry the force between them, it became quantum chromodynamics, or QCD. Asymptotic freedom explained why quarks look free when struck hard in experiments, and it meant that at high energies the strong force could be calculated with the same reliable methods used for electromagnetism. The other side of the coin, that the force grows ever stronger with distance, offered a way to understand why quarks are never seen alone.
Charmonium
Politzer quickly put the new theory to work. With Thomas Appelquist he studied what would happen if there were a heavy fourth quark, called charm, bound to its own antiquark. Because the quarks would be heavy and close together, asymptotic freedom meant their behaviour could be calculated, rather like a hydrogen atom held together by the strong force. Their work played a central role in predicting the existence and properties of charmonium, and when such particles were discovered in November 1974, the agreement helped persuade physicists that QCD was right.
What QCD gives the world
Quantum chromodynamics is one of the three pillars of the Standard Model of particle physics, the theory that describes every known particle and every force except gravity. The Nobel committee said that the 2004 laureates made it possible to complete the Standard Model. QCD describes the force that holds together the protons and neutrons in every atomic nucleus, and so the matter of which we and everything around us are made.
QCD also explains something surprising about ordinary matter. The quarks inside a proton are very light; most of the proton’s mass, and so most of the mass of everything around us, comes from the energy of the strong force that binds the quarks and gluons together. And because the force grows stronger as quarks are pulled apart, a quark knocked out of a proton in a high-energy collision never emerges alone: the energy of the stretched force creates new quarks and antiquarks, which appear in detectors as narrow sprays of particles called jets. The observation of such jets, including evidence for gluons at the German laboratory DESY in 1979, gave some of the most striking confirmations of the theory.
Its predictions have been tested in great detail at particle colliders, especially at CERN in Geneva. Today the calculations made possible by asymptotic freedom are used every day by experimental physicists: to work out what should happen when protons collide at the Large Hadron Collider, to separate rare signals from the enormous background of ordinary collisions, and so to recognise new particles. The discovery of the Higgs boson at CERN in 2012 depended on such calculations. QCD also underpins nuclear and astrophysical research, from the structure of the proton to the matter inside neutron stars.
Fundamental theories like this rarely produce a device of their own. What they give is understanding, the reliable knowledge on which later science and technology are built.
A Caltech professor
After his PhD in 1974, Politzer spent three years as a junior fellow of Harvard’s Society of Fellows and in 1977 moved to the California Institute of Technology, where he became professor of theoretical physics and later the Richard Chace Tolman Professor. In 1986 the American Physical Society awarded him the J. J. Sakurai Prize for theoretical particle physics, and in 2003 he shared the European Physical Society’s High Energy and Particle Physics Prize with Gross and Wilczek. The three shared the Nobel Prize in Physics in 2004 for the discovery of asymptotic freedom in the theory of the strong interaction.
He brings his banjo into his teaching at Caltech: preparing for a class in September 2024, he recorded the lowest octave of a banjo for his class to demonstrate the “missing fundamental”, the way the ear hears a low note even when the sound contains almost none of it.
In 2008 he joined 19 other American physics laureates in urging President George W. Bush to restore cuts to the funding of basic research, and in 2011 he was elected to the American Academy of Arts and Sciences.
Banjos, film and a rock band
Politzer has an unusually playful side for a Nobel laureate. In 1989 he played the physicist Robert Serber in a small role in Fat Man and Little Boy, a film about the Manhattan Project. In the 1980s he was lead singer of a band called Professor Politzer and the Rho Mesons, which released a single titled The Simple Harmonic Oscillator.
He plays the banjo, and in recent years he has turned his physicist’s skills to it, publishing a stream of studies on his Caltech website about how the banjo’s drum-like head, bridge, rim and strings shape its sound, and how resonator guitars produce their distinctive twang, complete with measurements and sound samples. He still posts new work there, and he has used the instrument in his teaching.
Heritage and legacy
Politzer has said little in public about his family background or religion. His parents were refugees who fled Czechoslovakia in 1939 on the eve of the Nazi occupation, and the Jewish Virtual Library includes him among Jewish Nobel laureates in physics.
His legacy is a single, startling idea, found while he was still a graduate student: that the force binding quarks fades away at short distances. That insight turned a puzzle into a precise theory, completed the Standard Model and gave physicists the tools they still use to understand the heart of matter.
Sources & further reading
Press release: The Nobel Prize in Physics 2004, NobelPrize.org · Popular information: The Nobel Prize in Physics 2004, NobelPrize.org · H. David Politzer, Jewish Virtual Library · Banjo Physics, David Politzer, Caltech · Hugh David Politzer, Wikipedia · Researched from the sources above and checked against them. Spotted an error? Tell us.
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