A Q&A with physicist David Politzer on 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 student at Harvard in 1973, he made a stunning discovery that would forever reshape the field of particle physics. He was thinking about a physics problem and decided to do a long, careful calculation to understand it better. When he was finished, he realized that the formula he had 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 along with gravity, the weak force and electromagnetism – works differently from the others. The strong force is what holds together the smallest known pieces of matter, quarks, inside the nuclei of atoms. But rather than getting weaker as quarks move away from each other, as other forces do, the strong force remains very strong.
This phenomenon can be compared to pulling on a rope with “quantum, relativistic mechanical mojo,” as Politzer puts it. Inside atoms, these quantum chains connect quarks together. Any attempt to pull on the string between the quarks only produces more string. If you pull hard enough, the string breaks and turns into more quarks. “But the strings are very flexible when the quarks are close together,” says Politzer. This flexibility means that quarks act as if they were 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 2004 Nobel Prize in Physics along with David Gross and Frank Wilczek, who made the same discovery independently. This breakthrough 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. Essentially, Politzer’s discovery equipped QCD with working equations that could be used to calculate how particles interact. Gell-Mann, who coined the term “quarks” based on a line from James Joyce’s novel Finnegans alarm clockwon the Nobel Prize in Physics in 1969 for his suggestion that quarks are the building blocks of matter.
“Politzer’s work has changed particle physics more than any other work in the last 50 years,” says Mark Wise, the John A. McCone Professor of High Energy Physics at Caltech and Politzer’s colleague. “This has allowed physicists to quantitatively understand many processes that were incomprehensible before 1973. This includes processes that address questions of physics outside of the strong interactions themselves, for example the discovery of the Higgs boson at the Large Collider. hadrons.”
We caught up with Politzer to learn more about the roots of his far-reaching discovery.
Were you interested in physics from a young age?
My older brother, six years older than me, 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, taking the subway from Manhattan with friends, an hour each way. One summer, near the end of high school, I wanted to apprentice with a banjo maker. I had just built a banjo. So I wrote to a banjo maker in Colorado, but he thought the folk 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 I did 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 a student?
In the mid-1960s, Murray Gell-Mann invented the “eightfold pathway,” in which three types of quarks combine in different ways to form strongly interacting particles called hadrons (which include protons and neutrons). Some days he thought quarks were just mathematical fictions for seeing patterns, and other days he thought they were a physical reality. That was the theoretical side of things. Experiments were also performed that did not match the theories. One of the best-known experiments took place at SLAC (a federally funded particle accelerator operated by Stanford University) in 1968 and produced puzzling results, known as the Gee Whiz conspiracy , because every time someone saw the plot, all they could say was “damn it”.
In this experiment, electrons reached high speeds and bounced off a stationary target. The electrons came out as if they hit something very hard and with a lot of inertia inside the protons. Of course, we now know that the electrons hit the quarks and the process generated new particles. Richard Feynman (who, before Politzer, was the Richard Chace Tolman Professor of Theoretical Physics at Caltech) had his own theory about what was happening and didn’t believe Gell-Mann quarks had anything to do with it. The two would make fun of each other about it.
Had you done any research on quarks yourself at that time?
Earlier, as a freshman, I worked on a famous oyster vaporization experiment. It is totally true. We knew it must be difficult to extract a quark from the nucleus by itself, because we had never seen it happen and it hasn’t happened until now. High-energy cosmic rays come from the sky and hit the ocean. What happens if they release quarks from atoms? We were looking for evidence of fractional 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 focus biologically. There was a barrel of oysters, and they were spraying them too. We passed the steam between charged plates and tried to concentrate the split charge. Well, we’ve never seen a quark. But there was a reason we ate a lot of oysters.
How did you become involved in the strong force problem?
I haven’t started working on this problem. In graduate school at Harvard, a friend of mine and I drove to New York in my car to attend a conference. We talked all the way about physics. I thought of a question related to his project with our professor, Sidney Coleman (PhD ’62). I later asked Coleman about it and he said, “That’s really interesting. Do you mind if I work on this with you?” We never got far, but I learned a lot. A calculation I tried for this project didn’t help, but it turned out great for the strong force question.
At that time, there was something called the Weinberg-Salam model, which described the weak force and how it relates to electromagnetism. This model is what we call a non-abelian gauge theory. It’s a lot like electromagnetism, except it has several different types of charges instead of just one electric charge, and they add up in a funny way. Physicists wanted to apply the same type of theory to the strong force, but did not know how to put it into equations. Meanwhile, in 1971, a Dutch graduate student named Gerard ‘t Hooft (formerly a Sherman Fairchild Distinguished Scholar at Caltech in 1981) had done the calculations and made them work. At first, no one paid much attention to it. Another professor of mine at Harvard, Shelly Glashow, gave me a copy of the paper and said, “This guy is either a genius or a weirdo.” Gerard ‘t Hooft’s solution was very peculiar and virtually impossible to follow, but his mathematics had solved the problems of infinities in the Weinberg-Salam model. He made the equations kosher.
Regardless, it was this mathematical framework that I turned to at one point in my own research on a problem unrelated to the strong force. The first thing I did was a simple but tedious calculation related to non-abelian gauge theories. Nowadays, calculus is a homework for physics students, but back then it took a few days by hand on paper. I quickly realized that the results meant that something called the beta function for the strong force had a minus sign. This essentially means that the effects of the strong force, unlike those of other forces, diminish as quarks get closer together. This is asymptotic freedom. I realized this would make the Gee Whiz plot work. I did the calculations over and over again and always got the same answer.
Did people immediately believe your result?
I sent a draft of the document to my advisor, Sidney Coleman, and he didn’t believe it. By the way, I nominated Sidney for the Caltech Distinguished Alumni Award because he was an excellent professor with influence throughout the particle physics community, and he won. In any case, it is thanks to him that the title of the article, “Reliable disruptive results for strong interactions? has a question mark, which I regret today years later, because I knew the calculation was correct.
Murray Gell-Mann immediately understood what calculus meant: that his QCD theory was not hypothetical. This meant that the possibility of making precise calculations within the framework of this theory immediately opened up. Feynman was skeptical, and it took him a few years to realize that some experiments he thought contradicted QCD were actually consistent. He had to wait for lessons from higher energy collisions. Everything comes together at higher energies.
What were the broader implications of your calculation?
When I entered graduate school, particle physics was a disaster. There was a lot of experimental and theoretical stuff going on in the field that was interesting, provocative, exciting, and contradictory. By the time I finished graduate school, there was a standard model that worked, accurate predictions that could be made, and experiments that could be done. As my colleague Mark Wise said, the state of particle physics changed completely after the mystery of the strong force was finally solved.
What is your favorite part of research, both in fundamental physics and in your more recent studies of stringed instruments?
For me, I like to understand how something works. That’s great. Now, whether other people already know it doesn’t change how it feels to discover it for yourself. They might tell you, and you don’t understand them, and that happened to me. But there is a joy in discovering for yourself.