When I was offered a five-year postdoctoral fellowship in astrophysics at the Institute for Advanced Study at Princeton in 1988, I did not know how the Sun shines. This was particularly embarrassing given that the person who offered me the job, John Bahcall, dedicated most of his career to developing the so-called Standard Solar Model. John pioneered the calculation of the expected flux of weakly-interacting particles called neutrinos, from the core of the bright nuclear reactor in our sky.
Following a letter request in the early 1960s from the experimental physicist Ray Davis, John predicted that about a hundred billion solar neutrinos cross our thumbnail every second. Most of them are produced by the proton-proton chain reaction, which fuses protons to helium and beryllium. The leakage of neutrinos from the Sun is enabled by their exceptionally weak interaction with matter. As a result, John realized that they can uniquely probe the interior of the Sun, which is otherwise opaque to light.
John’s rigorous calculation inspired Ray to design an experiment to detect these elusive neutrinos based on an idea by the physicist Bruno Pontecorvo to capture neutrinos from a reactor using chlorine. Ray realized that the proposed experiment was not feasible with reactor neutrinos. But solar neutrinos could potentially convert enough chlorine atoms in a large tank to argon atoms. The argon atoms are radioactive and could be counted in small quantities.
Neutrinos have three flavors for their weak interactions with matter: electron, muon and tau. A massive enough detector can detect their rare interactions statistically.
The flux of electron neutrinos was heroically measured in 100,000 gallons of chlorine cleaning fluid at the Homestake experiment constructed by Ray. But surprisingly, this giant detector harvested only about a third of the electron neutrino flux predicted by John.
The response from the scientific community was that either John or Ray made a mistake. In a NOVA PBS interview, John recalled:
“There was a famous meeting at Caltech, just a few physicists — Dick Feynman, Murray Gell-Mann, Willie Fowler, Bob Christie, and a couple of others — in a small meeting room, where Ray presented his results and I presented my calculations of what he should have measured. There was some discussion of it afterwards, and it was pretty inconclusive. There was a discrepancy; it looked like one of us was wrong.
I was very visibly depressed, I guess, and Dick Feynman asked me after the meeting if I would like to go for a walk. We just went for a walk, and he talked to me about inconsequential things, personal things, which was very unusual for him, to spend his time in quite idle conversation; it never happened to me in the many years that I knew him that he did that before or afterwards. And only toward the end of the walk, which lasted over an hour, he told me, “Look, I saw that after this talk you were depressed, and I just wanted to tell you that I don’t think you have any reason to be depressed. We’ve heard what you did, and nobody’s found anything wrong with your calculations. I don’t know why Davis’s result doesn’t agree with your calculations, but you shouldn’t be discouraged, because maybe you’ve done something important, we don’t know. I don’t know what the explanation is, but you shouldn’t feel discouraged.””
Indeed, no errors were found for decades. The discrepancy between theory and experiment became known as the solar neutrino problem. During my five years at Princeton, John used to take me for a walk every time I faced challenges. In retrospect, it was Feynman’s way of “paying it forward.” I do the same with some of my students or postdocs.
The deficit in electron neutrinos was eventually explained in 1985 by Stanislav Mikheyev and Alexei Smirnov who realized, based on earlier work in 1978–1979 by Lincoln Wolfenstein, that the gradual decline in matter density from the solar core outwards can resonantly enhance the transformation of electron neutrinos to the muon and tau flavors. This became known as the MSW effect, named after the first letters of the last names of these physicists.
In subsequent years, other experiments such as Kamiokande, SAGE, GALLEX and Super Kamiokande, confirmed the early results from the Homestake experiment. Eventually, the Sudbury Neutrino Observatory (SNO) experiment in Ontario, Canada employed the first detector capable of detecting neutrino oscillations. In 2001, SNO found that indeed neutrino mixing explains the deficit of detected electron neutrinos in Davis’ detector.
John felt vindicated when the SNO result was announced and was quoted in The New York Times saying, “I feel like dancing, I’m so happy.” On PBS NOVA he noted: “For three decades people had been pointing at this guy and saying this is the guy who wrongly calculated the flux of neutrinos from the sun, and suddenly that wasn’t so. It was like a person who had been sentenced for some heinous crime, and then a DNA test is made and it’s found that he isn’t guilty. That’s exactly the way I felt.”
In 2002, Ray Davis received the Physics Nobel Prize for his pioneering experimental work. But as argued in an editorial of Nature Magazine (after John and Ray passed away in 2005 and 2006 respectively), John should have received the Nobel Prize together with Ray. In 2015, the Physics Nobel Prize was jointly awarded to the director of SNO, Art McDonald, and to Takaaki Kajita who discovered oscillations of neutrinos produced by cosmic-rays impacting the Earth’s atmosphere using Super-Kamiokande.
I was reminded of this history by the solar eclipse in North America on April 8, 2024, forty years after my first lunch meeting with John. As I arrived for a one-day visit at Princeton, I met Freeman Dyson who said: “Do you know John Bahcall?” I said: “No”, and so Freeman called John who fortunately was around. John invited me to lunch and the rest is history. If John would have been traveling that day, I would not be practicing theoretical astrophysics.
On eclipse day, I realized that even though sunlight is blocked by the Moon, solar neutrinos traverse the Moon unhindered. If we could only see neutrinos, we would realize that they cannot be eclipsed by the Moon.
Similarly, nothing can eclipse John’s legacy for evidence-based science. Not his early doubters, nor the Nobel committee.
ABOUT THE AUTHOR
Avi Loeb is the head of the Galileo Project, founding director of Harvard University’s — Black Hole Initiative, director of the Institute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics, and the former chair of the astronomy department at Harvard University (2011–2020). He is a former member of the President’s Council of Advisors on Science and Technology and a former chair of the Board on Physics and Astronomy of the National Academies. He is the bestselling author of “Extraterrestrial: The First Sign of Intelligent Life Beyond Earth” and a co-author of the textbook “Life in the Cosmos”, both published in 2021. His new book, titled “Interstellar”, was published in August 2023.
