A Q&A with physicist David Politzer on solving the mystery of the strong force more than 50 years ago
When David Politzer, Caltech’s Richard Chace Tolman Professor of Theoretical Physics, was a fourth-year student at Harvard in 1973, he made an astonishing discovery that would forever reshape the field of particle physics. He had been thinking about a physics problem and decided to perform a long, painstaking calculation to understand it better. By the time he 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 shown that the strong force – one of the four fundamental forces of nature in addition to gravity, the weak force and electromagnetism – works differently from the others. The strong force is what holds the smallest known pieces of matter, quarks, together in the atomic nuclei. But instead of getting weaker as the quarks move further apart, as is the case with the other forces, the strong force remains very strong.
This phenomenon can be compared to pulling a string with a “quantum mechanical and relativistic force,” as Politzer says. Within atoms, these quantum strings connect the quarks together. Every attempt to pull the string between the quarks only produces more string. If you pull hard enough, the string will snap and even more quarks will change. “But the strings are very weak when the quarks are close together,” says Politzer. This floppiness means that the quarks behave as if they are free when they are very close together. 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 independently made the same discovery. The breakthrough had major implications for quantum chromodynamics (QCD), a theory proposed in 1972 by the late Caltech professor Murray Gell-Mann to describe the strong force. Essentially, Politzer’s discovery provided QCD with working equations that can be used to calculate how particles interact. Gell-Mann, who coined the term ‘quarks’ after a line from James Joyce’s novel Finnegan’s Wakewon the Nobel Prize in Physics in 1969 for suggesting that quarks are fundamental building blocks of matter.
“Politzer’s work has changed particle physics more than any other work in the past fifty years,” said Mark Wise, Caltech John A. McCone Professor of High Energy Physics and colleague of Politzer. “It enabled physicists to quantitatively understand many processes that were incomprehensible before 1973. This includes processes that affect physics questions beyond the strong interactions themselves, for example the discovery of the Higgs boson at the Large Hadron Collider.”
We spoke with Politzer to learn more about the roots of his far-reaching discovery.
Were you interested in physics from a young age?
My big brother, six years older, went to Bronx Science, a high school in New York City, and then to MIT. He did real physics and good experiments. He made the point that cool guys do physics, and I caught the bug from him. I also went to Bronx Science and rode the subway an hour from Manhattan with friends. One summer, toward 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 company and never wrote me back. I ended up going to 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’, in which three types of quarks combine in different ways to create the strongly interacting particles known as hadrons. [which include protons and neutrons]. Some days he thought the 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 the matter. Experiments were also done that did not agree with 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 baffling 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 speeds and reflected off some solid target. The electrons came back as if they hit something very hard and with a lot of inertia into the protons. Of course, we now know that the electrons hit quarks and that 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 going on and didn’t believe Gell-Mann’s quarks had anything to do with it. The two would make fun of each other about it.
Had you been doing any research into quarks yourself around this time?
Previously, as a freshman, I worked on an experiment famous for vaporizing oysters. This is completely true. We knew it must be difficult to get a quark out of the nucleus ourselves, because we had never seen it done before and still haven’t to this day. High-energy cosmic rays come from the sky and hit the ocean. What happens when they release quarks from the atoms? We were looking for evidence for the fractional charges of quarks. 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 become biologically concentrated. There was a barrel of oysters, and we evaporated those too. We passed the vapor between charged plates and attempted to concentrate the fractional charge. Well, we’ve never seen a quark. But there was a reason why we ate a lot of oysters.
How did you become involved in the strong power problem?
I didn’t start working on that problem. In high school at Harvard, a friend of mine and I were traveling in my car to New York City to go to a conference. We talked about physics all the way. I was thinking of a question related to his project with our professor, Sidney Coleman [PhD ’62]. Later I asked Coleman about it, and he said, “That’s really interesting. Do you mind if I work on that with you?’ We never got far, but I learned a lot. One calculation I tried for this project didn’t help, but it turned out to be amazing for the strong power demand.
Around this time there was something called the Weinberg-Salam model, which described the weak force and how it intertwines with electromagnetism. This model is what we call a non-Abelian gauge theory. It’s a lot like electromagnetism, except it has different types of charges instead of one electric charge, and they add up in a funny way. Physicists wanted to apply the same kind of theory to the strong force, but were unsure how to convert it into equations. Meanwhile, in 1971, a Dutch student named Gerard ‘t Hooft [formerly the Sherman Fairchild Distinguished Scholar at Caltech in 1981] had done the calculations and made them work. Initially, no one paid much attention to this. Another professor of mine at Harvard, Shelly Glasshow, gave me a copy of the article and said, “This guy is either a genius or crazy.” Gerard ‘t Hooft’s solution was very idiosyncratic and almost impossible to follow, but his mathematics had solved the problems with infinities in the Weinberg-Salam model. He made the comparisons kosher.
Anyway, this mathematical framework is what I turned to at one point in my own research into a problem unrelated to the strong force. The first thing I did was a simple but tedious calculation involving non-Abelian gauge theories. Nowadays arithmetic is homework for physics students, but back then it took a few days to do it by hand on paper. I quickly realized that the results meant that something called the beta function, because the strong force has a minus sign. This essentially means that the effects of the strong force, unlike those of the other forces, become smaller as the quarks get closer together. This is asymptotic freedom. I realized this would make the Gee Whiz plot work. I ran 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 a great teacher with influence throughout the particle physics community, and he won. Anyway, thanks to him the title of the article is, “Reliable disruptive 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. It meant that the possibility of performing accurate calculations within this theory immediately opened up. Feynman was skeptical, and it took him a few years to realize that some experiments he thought contradicted QCD actually agreed. He had to wait for the lessons of higher energy collisions. Everything comes together at higher energies.
What were the broader implications of your calculation?
When I went to graduate school, particle physics was a mess. There were a lot of experimental and theoretical things in the field that were interesting, provocative, exciting and mutually contradictory. By the time I finished graduate school, there was a standard model that worked, accurate predictions that could be made, and experiments you could do. 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 doing research, both in fundamental physics and in your more recent studies of stringed instruments?
For me, I enjoy figuring out how something works. That is amazing. Whether other people already know doesn’t change how it feels to find out for yourself. Maybe they tell you, but you don’t understand them, and that happened to me. But it is a pleasure to find out for yourself.