The Hunting of the Quark

DURING THE EARLY 1970s I WAS FORTUNATE to become deeply involved, as an MIT grad student and postdoc, in a series of high-energy physics experiments at Stanford University that led to the discovery of quarks — exceedingly tiny bits of matter inside the protons and neutrons of atomic nuclei [1]. Looking back now half a century later, I can see that this unparalleled experience entailed more than just being a contributor to a major physics breakthrough. It was where I learned about evidence, and how difficult it can be to prove something beyond a reasonable doubt.            

Evidence is rarely black and white. It usually comes in a variety of different shades of gray. Occasionally scientists stumble across a “smoking gun” that quiets the skeptics and convinces almost everyone. But that is the exception rather than the rule. And I learned to be skeptical of evidence, at least at first glance. Measurements can be wrong, “raw” data need to be corrected, and interpretations often depend on the interpreter. Good evidence is that which has withstood the reasoned scrutiny of skeptics trying their best to dispute it and has convinced a diverse community of its validity [2]. As in a court of law, evidence usually has to be argued.

THE IDEA OF QUARKS had been independently espoused in 1963–64 by Caltech theorists Murray Gell-Mann and George Zweig. Drawing upon a quote from James Joyce’s Finnegans Wake, “Three quarks for Muster Mark,” Gell-Mann dubbed them quarks because there had to be three different varieties, which he called “up,” “down,” and “strange.” Zweig named them “aces” in part because he thought there would be four of them. In both theories, they were oddball bits of matter, sporting electric charges either 1/3 or 2/3 of that on a proton or electron. Which should have made them easy to spot in a particle detector, but eager experimenters failed to find anything like that in multiple experiments. By 1966 most particle physicists — including Gell-Mann, but not Zweig — had pretty much given up on the search. “So we must face the likelihood that the quarks are not real,” Gell-Mann admitted in a lecture that May. They might show up in mathematical equations but not in particle detectors. 

But the following year a team of physicists led by Jerry Friedman and Henry Kendall of MIT and Dick Taylor of Stanford began firing high-energy electrons from the new Stanford Linear Accelerator at targets of liquid hydrogen, which contained gazillions of protons. They were not intentionally searching for quarks, just trying to probe the proton’s innards and determine how matter was spread about inside. But they experienced a big surprise. Electrons were ricocheting off protons at large angles far more often than the dominant physics theories of the day could readily accommodate. They were behaving as if you had fired a rifle bullet into a snowball and it came back and hit you in the face! At a gut level, it seemed as though electrons were striking something tiny and hard — or “point-like” — deep in the heart of protons.            

Artist’s conception of what happens when a high-energy electron strikes a parton inside a proton.

Perhaps the quarks? That was indeed deemed possible, but there were lots of competing interpretations being promoted by other theorists. Gell-Mann remained dubious at first, firmly insisting that quarks could have only a “mathematical” existence. To him, they could not be tangible objects of human experience. But his Caltech colleague Richard Feynman took them for real, dubbing the possible proton constituents “partons” — and Stanford theorist James “BJ” Bjorken began calling them “quark partons.” Still, there were other prominent theorists who remained skeptical of point-like proton inhabitants, instead trying to explain the electron-scattering data within the confines of their own pet theories.

SO FRIEDMAN, KENDALL, TAYLOR AND COMPANY returned to the electron shooting range for two additional experiments in 1970, this time inserting deuterium (heavy hydrogen) as well as hydrogen targets into the electron beam and measuring the scattering rates much more precisely. This is where I joined the team, along with another MIT grad student, Arie Bodek. We landed at San Francisco Airport in May 1970 and headed for Stanford just as college campuses across the United States were erupting in protest after President Nixon’s Cambodia invasion. Windows were being shattered in Cambridge as we left and in downtown Palo Alto as we arrived. And four Kent State students had just died.

The two massive detectors used to detect energetic electrons that had rebounded from hydrogen and deuterium targets (stationed to the left of photo). Bodek and Riordan’s experiment occurred on the foreground detector.

By comparing the rates of electron scattering from hydrogen and deuterium (which also contains neutrons), we could get a better idea whether or not we were striking quarks. If so, electron scattering from neutrons should occur less frequently than from protons, as indeed was occurring. In fact, the rate of scattering from neutrons seemed at first to be plummeting toward zero in his analysis! Which threw some ambulance-chasing theorists into a frenzy of theorizing to try to account for this unheralded behavior — until Arie found an error in the computer program he had developed to extract the neutron results from the hydrogen and deuterium data. Once he corrected that, his results began behaving themselves. The neutron-to-proton ratio N/P remained greater than 1/4, as Bjorken and Feynman had staunchly insisted must be the case if the quark-parton theory was to hold true. Zero was out of the question for them, completely impossible. To me as a close-in observer, the resolution of this apparent quandary in their favor strongly suggested that we were indeed striking real, physical, fractionally charged quarks in our experiments.

FOR MY OWN RESEARCH, I examined how the rate of electron scattering varied with the angle at which they rebounded from the targets. By doing so with the much more accurate data we had measured, I could extract a ratio R that gave information about the intrinsic spins of the putative partons. If they happened to be quarks, they had to have “spin-1/2” in physics jargon. By early 1973 that indeed seemed to be the case — or rather, it gave the best fit to my R measurements. The results were still fairly ambiguous, however, and other possible parton-spin values such as spin-0 could accommodate the R data almost as well. Shades of gray.

I was aided in this analysis by none other than Feynman himself, who in a letter to Kendall suggested a way to analyze the R data that had not previously occurred to me. Thus he was among the first to receive a copy of my MIT PhD dissertation, in May 1973. “Dear Dr. Riordan,” began his June 4 letter of reply, the very first one I received addressing me that way. “Thank you very much for your detailed description of the behavior of R. I have no questions as the results were so completely described.”

His framed letter hangs prominently on my office wall, as it has for decades.

This exchange illustrates the role of personal authority in certifying evidence. For as a newly minted, wet-behind-the-ears PhD physicist, I was still pretty uncertain about the accuracy and significance of my tentative conclusion that the partons had spin-1/2. But after it had been endorsed by a Nobel laureate of Feynman’s stature, it took on a life of its own and soon began circulating rapidly within the world physics community, which quickly accepted it as true, however shaky my results seemed to me.  So that summer, I didn’t have to hit the lecture circuit and argue my case before audiences of skeptical physicists. Instead, I began a much-needed vacation, traveling around Europe, thinking that I could always stop by at the big international high-energy physics conference that year in Bonn, West Germany, to see how my results were embraced  — or not embraced — by this global scientific community.

But I skipped that late August conference, due in part to the high cost of staying there a few days. In September, however, I visited the CERN laboratory in Geneva on my way back to Amsterdam and a return flight to New York. In the CERN cafeteria, an MIT theorist who had advised me on my R analysis that spring noticed me walking by with my tray loaded with good food. “Where were you in Bonn?” he called out loud enough for others to overhear. “Your name was mentioned more than Feynman’s!”            

Looking back now on that 1973 summer vacation, I guess I could have acted differently, and stopped by the conference after all. Had I done so, I might have become a tenured experimental physicist rather than an underpaid science writer!

BY THE SUMMER OF 1973, then, the evidence for quarks had become extremely strong. In our electron-scattering experiments, we seemed to be striking point-like particles within protons and neutrons. And those “partons” were acting much as expected for quarks, revealing fractional charges and spin-1/2 behavior. Evidence for quarks was turning up elsewhere, too — for example, in neutrino-nucleon and proton-proton scattering experiments at CERN. Such corroboration by physicists making completely independent measurements with different probes reinforced the quark-parton theory. It’s a lot like what happens in a court of law, where witness testimony about a purported felony is corroborated by physical evidence obtained at the crime scene.

But there was one gnawing problem: although we seemed to be hitting them hard, the quarks apparently never came speeding out of the struck protons and neutrons into the cold light of day. Other physicists, at Stanford and elsewhere, positioned detectors on the opposite side of the targets from the electron detectors, hoping to observe fractionally charged particles zooming out whenever an electron ricocheted away. But for some unknown reason, that never happened! It was a big paradox.

Although we seemed to be hitting them hard, the quarks never came speeding out of the protons and neutrons into the cold light of day.

Richard Feynman was among the first to recognize that we were brushing up against a paradox here. “Oh yes, yes, if you suppose all partons are quarks, and you cannot produce real quarks, you’re in the face of a possible paradox,” he speculated in December 1971, when the cloud of confusion about Arie’s N/P ratio was still in the air. “It would be very pleasant!”

By this he meant that such a paradox suggests we were encountering really new physics. And that radically new physics also showed its face at the 1973 Bonn conference. A theory dubbed “quantum chromodynamics” (or QCD) suggested that the force confining two quarks inside the protons and neutrons (as well as in other subatomic particles called “mesons”) could become stronger and stronger as they separate — not weaker and weaker, as happens with gravity and electromagnetism. It’s like the force a rubber band exerts as you yank on its ends. That could account for why the quarks never emerged despite being hit so hard. And it also could explain why they seemed to be drifting freely about deep inside.

This was another reason my dissertation had been lionized at Bonn. For if true, QCD required that small deviations from point-like parton behavior — called “scaling violations” in physicist jargon — had to occur. And those deviations were beginning to appear in our much more accurate electron-scattering data, although it wasn’t entirely convincing yet. So Arie and I returned to the data analysis, including from a third experiment he had led in 1972. We thought we had made a convincing case that we were indeed observing scaling violations, but our new results received only a lukewarm reception by the skeptical high-energy physics community. We had convinced ourselves and our team members, but not many outsiders. Part of the problem was the fact that we were the ones who had first discovered this scaling behavior; so how could we now turn about face and say it was being violated? Thus it eventually fell to other experimental groups, especially at CERN, to prove beyond doubt that scaling violations were occurring, strongly supporting the QCD theory.           

The final, and truly convincing, piece of evidence for quarks came in late 1974 from other kinds of high-energy physics experiments at Stanford and Brookhaven National Laboratory on Long Island. A surprising new family of heavy mesons turned up — weighing in at over three times the proton mass — that could only be explained by the existence of a fourth, heavy quark dubbed the “charm” quark, much as Zweig (and later Bjorken) had suggested a decade earlier. These stunning discoveries were what I have dubbed “the slap in the face” that finally forced all but a few skeptical physicists to take these quarks for real. If anything, this was the smoking-gun evidence for quarks.

The quark-discovery team gathers in Stockholm for the 1990 Nobel Prize, awarded to Dick Taylor, Jerry Friedman and Henry Kendall (front row, left to right). Author Michael Riordan (with glasses and mustache) stands in the back row, just to the left of James Bjorken. Arie Bodek is at far left.

QUARKS ARE TAKEN FOR GRANTED TODAY, half a century later — as if little more problematical than pebbles or stones, thought to be fundamental objects at the heart of matter. A total of six quarks are now known to exist, and that seems to be it. For all physicists know, they have no size whatsoever. Many subsequent theories and experiments have tried to discern a quark substructure, without success. So we may in fact have finally reached the innermost layer of the cosmic onion. 

Or maybe not. We don’t really know for sure, one way or the other. Physics rarely provides ultimate answers. Much more often, it suggests new questions.

In 1987 I recounted this stimulating intellectual journey in far more detail in my book, The Hunting of the Quark [1]. It portrayed the journey as an epistemological voyage of discovery, addressing such philosophical questions as “What is real?” and “How can we know what we claim to know?” 

Decades later, I still don’t have the answers. But I can offer knowledgeable conclusions about the scientific process as we experienced it during the early 1970s. It largely came down to the respect for empirical evidence that has been ingrained deeply in Western thought since at least the eighteenth-century Enlightenment — and to some extent before that. No matter how beautiful a theoretical idea may be, its validity as a possible description of Nature rests on the strength of the experimental evidence that can be found for it. Which usually has to be argued before a diverse community of skeptical scientists [2].

No matter how beautiful a theoretical idea, its validity rests on the strength of the experimental evidence that can be found for it.

And I often find myself applying these stringent standards for establishing truth to life in general, especially in politics. As scientists say, “Extraordinary claims must be supported by extraordinary evidence.” But we languish today in a troublesome political environment in which respect for evidence is no longer an essential part of the process of establishing the truth. Extraordinary claims are made by a U.S. president without a shred of evidence to back them up — while his acolytes accept and promote these specious claims blindly, without skepticism.

This is dangerous. We are in grave danger of abandoning the Enlightenment principles on which our nation was founded over two centuries ago and returning to something like the papal absolutism that dominated a century earlier. But nobody has an infallible insight on what is true.  Only by a return to the respect for empirical evidence and reasoned, skeptical argument can we hope to attain genuine truth and avoid the “alternative facts” that have recently begun polluting public discourse.

Top photograph: The Stanford Linear Accelerator Center as it appeared during the late 1960s and early 1970s. The breakthrough electron-scattering experiments occurred in the largest concrete building at lower left.


1. Michael Riordan, The Hunting of the Quark: A True Story of Modern Physics. (New York: Simon & Schuster, 1987). Revised and updated edition published in electronic edition by Plunkett Lake Press in 2018. The original paperback edition is available in the Orcas Island Public Library.

2. Harvard University science historian (and 2015 Orcas Currents lecturer) Naomi Oreskes made a similar point in her book, Why Trust Science? (Princeton, NJ: Princeton University Press, 2019). An excellent summary can be found in her Time magazine essay, “Science Isn’t Always Perfect — But We Should Still Trust It.”

A somewhat technical lecture by Michael Riordan about his contributions to the quark discovery can be found on YouTube.

Author Profile

Orcas Island physicist and writer Michael Riordan is author of the award-winning 1987 book The Hunting of the Quark and coauthor of The Solar Home Book (1977), The Shadows of Creation (1991), Crystal Fire (1997) and Tunnel Visions (2015). His articles and essays have appeared in the New York Times, Seattle Times, Scientific American, and many other publications. He serves as Editor of Orcas Currents.

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