A Q&A with physicist David Politzer about 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 graduate student at Harvard in 1973, he made an astounding discovery that would forever reshape the field of particle physics. He had been mulling over a physics problem and decided to do a lengthy, painstaking calculation to understand it better. By the time he was finished, 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 in addition to gravity, the weak force, and electromagnetism—operates differently than the others. The strong force is what holds the smallest known pieces of matter, quarks, together inside the nuclei of atoms. But rather than grow weaker as the quarks move farther apart from each other, as is the case with the other forces, the strong force remains very strong.
This phenomenon can be compared to pulling on a string that has "quantum mechanical and relativistic mojo," as Politzer says. Inside atoms, these quantum strings tie the quarks together. Any attempt to pull on the string between the quarks just makes more string. If one pulls hard enough, the string snaps and turns into more quarks. "But the strings are very floppy when the quarks are close together," Politzer says. This floppiness means that the quarks act like they are free when very close together. In technical terms, this phenomenon is called asymptotic freedom.
For the discovery of asymptotic freedom, Politzer won the 2004 Nobel Prize in Physics together with David Gross and Frank Wilczek, who made the same discovery independently. The 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 bestowed QCD with working equations that could be used to calculate how particles interact. Gell-Mann, who famously coined the term "quarks" after a line from James Joyce's novel Finnegans Wake, won the 1969 Nobel Prize in Physics for the suggestion that quarks are 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 Politzer's colleague. "It enabled physicists to understand, quantitatively, many processes that before 1973 were incomprehensible. This includes processes that bear on physics issues outside of 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 big brother, six years older, went to Bronx Science, a New York City magnet high school, and then 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 one hour each way. One summer, near the end of high school, I wanted to apprentice myself to a banjo maker. I had just built a banjo. So, I wrote to a banjo maker in Colorado, but he figured the folk thing was over, sold his business, and never wrote me back. I ended up going to college instead 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 at the time you were in graduate school?
In the mid 1960s, Murray Gell-Mann invented the "Eightfold Way," where three kinds of quarks combine in different ways to make the strongly interacting particles known as hadrons [which include protons and neutrons]. Some days, he thought the quarks were only mathematical fictions that enabled you to see patterns, and on other days, he thought they were a physical reality. That was the theory side of things. Experiments were also being done that didn't match the theories. One of the most well-known experiments took place at SLAC [a federally funded particle accelerator operated by Stanford University] in 1968 and produced perplexing results that came to be known as the Gee Whiz plot because whenever anybody saw the plot, all they could say was "gee whiz."
In this experiment, electrons were ramped up to high speeds and bounced off a fixed target of some kind. The electrons came back out as if they hit something very hard and with a lot of inertia inside the protons. Of course, we now know the 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 for what was going on and did not believe that Gell-Mann's quarks had anything to do with it. The two of them would make fun of each other about it.
Had you done any research yourself on quarks around this time?
Earlier, as a freshman, I worked with an experiment famous for vaporizing oysters. This is totally true. We knew it must be hard to get a quark out of the nucleus by itself, because we'd never seen it happen and haven't to this day. High-energy cosmic rays come from the skies and hit the ocean. What happens if they release quarks from the atoms? We were looking for evidence of 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 get biologically concentrated. There was a barrel of oysters, and we'd vaporize them too. We passed the vapor between charged plates and were trying to concentrate the fractional charge. Well, we never saw a quark. But there was a reason we ate a lot of oysters.
How did you get involved in the strong force problem?
I didn't start out working on that problem. In grad school at Harvard, a buddy of mine and I traveled to New York City in my car to go to a conference. We talked the whole 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. Mind if I work on that with you?" We never got far, but I learned a lot. One calculation I attempted for this project didn't help, but it turned out to be stupendous 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 entwined with electromagnetism. This model is what we call a non-Abelian gauge theory. It's a lot like electromagnetism except that it has several different kinds of charge instead of one electric charge, and they add in a funny way. Physicists wanted to apply the same sort of theory for the strong force but weren't sure how to put it into equations. Meanwhile, in 1971, a Dutch grad student named Gerard 't Hooft [formerly the Sherman Fairchild Distinguished Scholar at Caltech in 1981] had done the calculations and made them work. At first, nobody paid much attention to this. Another one of my professors at Harvard, Shelly Glashow, gave me a copy of the paper and said, "This guy's either a genius or nuts." Gerard 't Hooft's solution was very idiosyncratic and virtually impossible to follow, but his math had fixed the problems with infinities in the Weinberg–Salam Model. He made the equations kosher.
Anyway, this mathematical framework is what I turned to at some point in my own research on a problem not related to the strong force. The first thing I did was a straightforward but tedious calculation related to non-Abelian gauge theories. These days, the calculation is homework for physics students, but back then it took a few days by hand on paper. I soon realized the results meant something called the beta function for the strong force has a minus sign. This means, in essence, that 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 did the calculations again and again and kept getting the same answer.
Did people believe your result right away?
I sent a draft of the paper to my advisor, Sidney Coleman, and he didn't believe it. Incidentally, 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, because of him, the title of the paper, "Reliable Perturbative Results for Strong Interactions?" has a question mark, which I now regret years later because I knew the calculation was correct.
Murray Gell-Mann knew right away what the calculation meant—that his QCD theory was not hypothetical. It meant that the possibility for doing precise calculations within this theory opened up right away. Feynman was skeptical, and it took him a couple of years to realize that some experiments that he thought contradicted QCD actually agreed. He needed to wait for the 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 mess. There were a lot of experimental and theoretical things in the field that were interesting, provocative, exciting, mutually contradictory. By the time I finished graduate school, there was a standard model that worked, precise predictions that could be made, and experiments that you could do. Like my colleague Mark Wise said, the state of particle physics completely changed 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 string instrument studies?
For me, I love figuring out how something works. That's great. Now, whether other people already know it, that doesn't change how it feels to figure it out for yourself. They might tell you, and you don't understand them, and that's happened to me. But there's a joy in figuring it out for yourself.
Written by Whitney Clavin