A Q&A with physicist David Politzer about solving the mystery of the strong force more than 50 years ago
When David Politzer, the Richard Chace Tolman Professor of Theoretical Physics at Caltech, was a fourth-year graduate student at Harvard in 1973, he made a startling discovery that would forever reshape the field of particle physics. He had thought about a physical problem and decided to do a long and laborious calculation to understand it better. By the time he was done, he realized that the formula he derived had profound implications for another puzzling question: How does the strong force bind the nuclei of atoms together?
Politzer’s calculations had revealed that the strong force—one of the four fundamental forces of nature besides gravity, the weak force, and electromagnetism—acts differently than the others. The strong force is what holds the smallest known particles of matter, quarks, together inside the nuclei of atoms. But instead of weakening as quarks move further apart, as is the case with other forces, the strong force remains very strong.
This phenomenon can be compared to the pulling of a string that has “quantum mechanical and relativistic mojo,” as Politzer puts it. Inside atoms, these quantum strings bind quarks together. Any attempt to pull the string between the quarks just makes more string. If one pulls hard enough, the string snaps and turns into more quarks. “But strings are very unstable when quarks are close together,” says Politzer. This frustration means that quarks act as if they are free when they are very close to each other. In technical terms, this phenomenon is called asymptotic freedom.
For the discovery of asymptotic freedom, Politzer won the Nobel Prize in Physics in 2004 together with David Gross and Frank Wilczek, who made the same discovery independently. The discovery had major implications for quantum chromodynamics (QCD), a theory proposed by the late Caltech professor Murray Gell-Mann in 1972 to describe the strong force. Basically, Politzer’s discovery gave QCD working equations that could be used to calculate how particles interact. Gell-Mann, who famously coined the term “quark” after a line from James Joyce’s novel Finnegans wakewon the Nobel Prize in Physics in 1969 for suggesting that quarks are the basic building blocks of matter.
“Politzer’s work changed particle physics more than any other work in the last 50 years,” says Mark Wise, Caltech’s John A. McCone Professor of High Energy Physics and colleague of Politzer. “It enabled physicists to understand, quantitatively, many processes that before 1973 were incomprehensible. This includes processes related to matters of physics outside the strong interactions themselves, for example, the discovery of the Higgs boson at the Large Hadron Collider .”
We sat down with Politzer to learn more about the roots of his far-reaching discovery.
Were you interested in physics at a young age?
My older brother, six years older, went to Bronx Science, a magnet high school in New York, and then to MIT. He did real physics and good experiments. He made it clear that cool guys do physics, and I caught the bug from him. I also went to Bronx Science, riding the subway from Manhattan with friends an hour each way. One summer, near the end of high school, I wanted to become an apprentice to a bathroom maker. I had just built a bathroom. So I wrote to a banjo maker in Colorado, but he realized the popular thing was over, sold his business, and never got back to me. I ended up going to college at the University of Michigan and loved it. I got as many B’s as A’s in physics and math. But I loved working in research labs.
What was known about the strong force and quarks when you were in graduate school?
In the mid-1960s, Murray Gell-Mann invented the “Eightfold Way,” where three types of quarks combine in different ways to create strongly interacting particles known as hadrons. [which include protons and neutrons]. Some days, he thought quarks were just mathematical fictions that allowed you to see patterns, and other days, he thought they were a physical reality. That was the theoretical side of things. Experiments were also being done that did not match the theories. One of the most famous experiments took place at SLAC [a federally funded particle accelerator operated by Stanford University] in 1968 and produced confusing results that became known as the Gee Whiz plot because whenever anyone saw the plot, all they could say was “gee whiz.”
In this experiment, electrons were accelerated to high speed and bounced off a fixed target of some kind. The electrons bounced out as if they hit something very hard and with a lot of inertia inside the protons. Of course, we now know that electrons were hitting quarks and the process was generating new particles. Richard Feynman [who, before Politzer, was the Richard Chace Tolman Professor of Theoretical Physics at Caltech] had his own theory of what was going on and didn’t believe Gell-Mann quarks had anything to do with it. Both would make fun of each other for this.
Had you done any research on quarks yourself at this time?
Previously, as a freshman, I worked on a famous oyster steaming experiment. This is completely true. We knew that it must be difficult to eject a quark from the nucleus itself because we had never seen it happen and we haven’t seen it to this day. High-energy cosmic rays come from the heavens and strike the ocean. What happens if they release quarks from atoms? We were looking for evidence of partial quark charges. The idea was that wherever the quark ends up, it will have a net charge. So maybe it’s in the seawater, maybe it’s in the salt, maybe it’s in the algae. Things are concentrated biologically. There was a barrel of oysters, and we were going to steam them, too. We passed the steam between the charged plates and were trying to concentrate the partial charge. Well, we’ve never seen a quark. But there was a reason why we ate so many oysters.
How did you get involved in the strong force problem?
I have not started working on this problem. In graduate school at Harvard, a friend and I traveled to New York City in my car to attend a conference. We talked all the way about physics. I thought of a question about his project with our professor, Sidney Coleman [PhD ’62]. Later I asked Coleman about it, and he said, “That’s really cool. Do you mind if I work on it with you?” We never got very far, but I learned a lot. A calculation I tried for this project didn’t help, but it turned out to be great for the strong force question.
Around this time, there was something called the Weinberg–Salam Model, which described the weak force and how it is intertwined with electromagnetism. This model is what we call a non-abelian gauge theory. It’s a lot like electromagnetism, except there are several different types of charge instead of one electric charge, and they add up in a funny way. Physicists wanted to apply the same theory to the strong force, but weren’t sure how to fit it into the equations. Meanwhile, in 1971, a Dutch student named Gerard ‘t Hooft [formerly the Sherman Fairchild Distinguished Scholar at Caltech in 1981] he had done the math and made it work. At first, no one paid much attention to this. Another of my professors at Harvard, Shelly Glashow, gave me a copy of the letter and said, “This guy is either a genius or crazy.” Gerard ‘t Hooft’s solution was highly idiosyncratic and practically impossible to follow, but his mathematics had fixed the problems with infinities in the Weinberg–Salam Model. He made the equations kosher.
However, this mathematical framework is what I turned to at one point in my research on a non-hard-force problem. The first thing I did was a straightforward but tedious calculation about non-abelian gauge theories. These days, calculus is homework for physics students, but back then it took days on paper. I quickly realized that the results meant something called a beta function because the strong force has a minus sign. This means, in essence, that the effects of the strong force, unlike those of other forces, become smaller as the quarks get closer to each other. This is asymptotic freedom. I figured that would make the Gee Whiz plot work. I did the math time and time again and kept getting the same answer.
Did people immediately believe your result?
I sent a draft of the letter to my advisor, Sidney Coleman, and he didn’t believe it. Incidentally, I nominated Sidney for the Caltech Distinguished Alumnus Award because he was a great teacher with an impact on the entire particle physics community, and he won. However, because of it, the title of the paper, “Reliable perturbative results for strong interactions?” there is a question mark, which I now regret years later, because I knew the calculation was correct.
Murray Gell-Mann knew immediately what the calculation meant – that his QCD theory was not hypothetical. This meant that the possibility of making precise calculations within this theory opened up immediately. Feynman was skeptical, and it took him several years to realize that some experiments he thought contradicted QCD actually agreed. He had to wait for lessons from higher energy collisions. Everything merges into higher energies.
What were the broader implications of your calculation?
When I entered graduate school, particle physics was a mess. There were many experimental and theoretical things in this field that were interesting, provocative, exciting, mutually contradictory. By the time I finished graduate school, there was a standard model that worked, accurate predictions you could make, and experiments you could do. As my colleague Mark Wise said, the state of particle physics changed completely once the mystery of the strong force was finally solved.
What is your favorite part of doing research, both in fundamental physics and in your more recent studies of stringed instruments?
For me, I like to understand how something works. This is wonderful. Now, if other people already know it, it doesn’t change how you feel about figuring it out yourself. They can tell you, and you don’t understand them, and that has happened to me. But there is a joy in figuring it out for yourself.
