Physics + MathPhysics & Math

# Hunting Solar Neutrinos

For almost four decades, astrophysicist John Bahcall and chemist Raymond Davis Jr. struggled to solve a nagging and important mystery in particle physics: How many neutrinos does the sun produce, and how many reach the Earth? The answer would shed light on how stars like our sun produce energy. In this interview Bahcall, who died in August 2005 at age 70, reflects on his career-long and eventually successful quest.

Nova

### NOVA: This all got started when Ray Davis sent you a letter in the early 1960s asking if you could calculate the rate of neutrino production in the sun. What was the motivation for making such a calculation?

John Bahcall: When I got this letter from Ray, the consensus view among scientists who thought about it at all was that stars like our sun shine by burning hydrogen into helium and converting the small amount of extra mass into a lot of extra energy. But that was not a quantitatively tested theory; it was supported by a lot of circumstantial evidence [but] no precise quantitative evidence. For me and for Ray I think it was a great challenge to see if we could see directly into the interior of a star, deep inside where the temperature and the densities are the highest. That's where the nuclear cauldron is; that's where the nuclei are burnt and the energy is created.

So for us it was both an excitement like climbing to the top of the mountain and really being able to see the whole view clearly. But also it was the challenge of being able to test quantitatively this consensual theory, this theory which everybody thought was right but hadn't been quantitatively tested.

### How did Ray Davis plan to detect solar neutrinos?

Well, [physicist] Bruno Pontecorvo, when he was working in Canada, suggested that one might be able to capture neutrinos from a reactor using chlorine. The neutrinos from the reactor would convert some of the chlorine atoms to argon atoms, which are radioactive and could be counted in small quantities in small counters. One could be sensitive to even something as weakly interacting as a neutrino if one had a huge vat of chlorine.

That was the basic idea that Ray had in mind, but he'd found that he couldn't detect them from a reactor. That was interpreted correctly as saying that a reactor produces antineutrinos, not neutrinos. Antineutrinos will not convert chlorine into argon; you need neutrinos like those produced in the sun to convert chlorine to argon.

Now, the fact that neutrinos are very difficult to detect means that to get one neutrino per day you need a tank the size of an Olympic-size swimming pool filled with a huge vat of cleaning fluid containing chlorine. That very fact is what makes it possible to look right into the center of the sun with the neutrinos in the same way that your doctor can look inside your body with ultrasound or X-rays and make a diagnosis of how your body is working. We wanted to do the same thing with neutrinos: use neutrinos to look right inside the sun [and] see what the nuclear reactions are doing in the very interior.

### So you can see things with neutrinos you can't see with light?

Well, light, as we all know, doesn't penetrate anything. If I put my hand in front of my face, you can't see my face; the light won't go through my hand. It doesn't penetrate any appreciable amount of material. Neutrinos can go through unimaginable amounts of material without being affected. [There is] less than a percent chance that anything would ever happen to them as they passed through the sun, certainly through the Earth.

### So what happened when you started working on this problem?

When I got to Caltech and began the calculations of how many neutrinos there should be from the sun, I realized the problem was immensely more complicated than I had recognized early on, because there were many different reactions competing with each other. The rates of the individual reactions were not well known. All of that had to be determined at the same time. We had to determine precisely the chemical composition and temperature and density and pressure in the sun to high accuracy.

The net result of all those calculations was that we believed that the sun should be emitting a huge number of neutrinos all the time. I can illustrate that. [Your] thumbnail's about roughly a square centimeter. Every second, about a hundred billion of these solar neutrinos according to our calculations would be passing through your thumbnail every second of every day of every year of your life, and you never notice it. They really don't do much, and that's the reason why Ray needed a huge tank of chlorine in order to detect what I said should be one neutrino event per day (and which turned out to be less than that, much less, a third of a neutrino event per day).

### So despite the extremely small number of expected events, Ray Davis was confident he could detect them?

Yes. I told Ray how many argon atoms should be produced in his tank per day, and Ray told me he was sure that he could do that. He invited me back to Brookhaven [National Laboratory] in order to try to sell the experiment. We had to present our ideas to the director of the laboratory, and Ray told me, "Never mind your enthusiasm for your understanding of how the sun shines. Never mind the models that you've made of how many neutrinos come from the sun. Talk to the director only about the nuclear physics, because he will say that that's very interesting and new nuclear physics and very clever nuclear physics. He won't be interested at all in the astronomy."

"He did it with ease and simplicity and elegance and beauty."

I was a young person and very enthusiastic and full of calculations that we'd done about the sun, and I argued with Ray. But Ray said "John, I know the director of Brookhaven. He often says that no astrophysicist can calculate anything with sufficient precision to be of interest to any particle physicist." He said, "Trust me, forget about your models of the sun. Our only chance of selling the experiment is talking about your nuclear physics, and I'll talk about how to do the experiment."

So I deferred to him. I talked about the nuclear physics, [and] the director was very enthusiastic about that. Eventually his wife did an experiment to test those ideas, and Ray described how he could do the experiment. About three weeks later the director approved the experiment with no proposal ever having been written, and very shortly afterwards Ray began to look for a mine where he could do the experiment.

### Why a mine?

Ray had to find some place far underground to do his experiment, because, as we've already discussed, neutrino interactions are extremely rare, they almost don't happen, so you want to go someplace where nothing else can interfere with your experiment. On the surface of the Earth many things can happen. Particular particles from outer space called cosmic rays can cause events in your tank that could be confused with neutrino interactions. Deep underground, none of these confusing events from cosmic rays or other activities on the surface of the Earth occur.

### So what exactly was involved in counting these precious few neutrino events?

It was necessary to find an efficient way of getting the few atoms out of this huge tank every couple of months. Then it was necessary to count them and not confuse the counting of these few atoms with anything else that was happening. All of that was beyond the current range of technology at the time that Ray started it, but Ray was absolutely confident that he could do it. He's just an enormously modest, quiet, unassuming person, and I assumed that it was as he described it—just plumbing.

But I saw it later through the eyes of other experimentalists, and I think it was somewhat miraculous what he did. I didn't realize it at the time, because he was the only person I talked to about it, and for him it was going to be just a matter of plumbing, plumbing on a very big scale and using chemistry that was well understood. But he did it with precision and care and attention to detail and insight into what were all the important processes that I think probably was well beyond the abilities of anybody else.

Then, of course, when his experimental results came out and they were in conflict with our calculations, he and I would be invited to give theory and experimental talks everywhere. And it was only many years later that I realized that other people were very skeptical of Ray's ability to do what he was absolutely confident of and which, as it turned out, he was absolutely right about. He did it with ease and simplicity and elegance and beauty.

### Yes, from the people we've talked to Ray seems to command not just enormous respect but also affection.

Many people know what a great scientist Ray is; it's really quite extraordinary what he's done scientifically. But I, and I think most of the people who know him well, admire him much more for his character than for his scientific abilities, extraordinary as they are. Ray is the kind of person who treats everyone the same—with dignity and respect, with politeness and attention, with generosity and support.

In all the years I have known Ray he has never differentiated between the janitor that came into the office to clean the trash can and the most distinguished professor who came in to ask him a question. He treats everybody with the same friendliness, the same courtesy, the same gentleness, and the same attentiveness.

### But as you said, there was at the time considerable skepticism in the scientific community about your claims.

When you think about it, it's almost unbelievable what we were doing, and I'm glad we didn't think about it too carefully when we were doing it, because I made calculations on a sheet of paper and with the computer about how many neutrinos were produced at high temperatures with nuclear reactions deep inside the sun and how many would be captured in a huge tank; and Ray with his huge tank would boil through helium every day to flush out the few atoms of argon that were in there and every two or three months he would purge something like hopefully a dozen or 10 of these argon atoms, separate them from the rest of the material that he had, and put them in a small glass counter and wait for a few months to count the three or four or, as it turned out, maybe one atom of argon that was in this glass vial containing gas of argon and carrier gas.

"I said it was time for us to declare the solar neutrino problem solved, and that was a big mistake."

It's an incredible connection between scribbles on a sheet of paper and flashes that you see when an argon atom decays in a gas contained in a small glass vial deep underground. But that's the conceptual connection that we made and, surprisingly enough, we convinced people that the number of atoms in his little glass vial had something to do with the number of lines that I drew on a sheet of paper.

### And yet there was a nagging discrepancy between your results and his, right?

Well, right from the beginning it was apparent that Ray was measuring fewer neutrinos events than I had predicted. He came to Caltech in early 1968 to spend a week with me while he and I wrote our papers up describing for me a refined calculation, for him the first measurement of the rate in his tank. It was clear that the rate that he was getting was a factor of three smaller than I was predicting, and that was a very serious problem.

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."

For me I think of all of the walks or conversations I have had in my professional life, that was the most important, because I was a young man without tenure, and [while] I'd done many calculations by that time, this was the one that was most visible and people had paid the most attention to, and it looked like it was wrong. I really was feeling very, very, very discouraged. And for a person whom I so enormously admired, Dick Feynman, to tell me "You haven't done anything that's visibly wrong, maybe you've done something important"—for me that was a huge boost.

### But there were plenty of scientists who did think there was something wrong with your model of the sun.

Well, initially very few people paid any attention to this discrepancy, but the discrepancy persisted. ... And every year for 30 years I had to look at different processes that people would imaginatively suggest that might play a role in the sun, and it didn't matter how convinced I was that they were wrong. I had to demonstrate scientifically that these processes were not important in order to convince people [that] yes, the expectation from the sun was robust and therefore you should take the discrepancy seriously. It took I would guess three and a half decades before I convinced everybody.

### When did things start to change?

Well, we had information beginning in the late 1980s, around 1988, that measurements made on the surface of the sun about how the sun vibrates were giving us information about the interior of the sun. And the first indications were that the measurements were in agreement with our predictions using our model of the sun. So that was very encouraging to me, and I began sort of speaking my mind. Until that time I'd been quite reserved, at least for me. I would state the facts as I knew them, but I never tried to make very strong claims. But once this evidence from the surface of the sun seemed to confirm our predictions based on how the sun vibrated in its interior, I began being much bolder in my calculations.

"That took me off the hook. I was no longer the person who had done the wrong calculation."

I remember a meeting in Toledo, Spain—I think it was in 1991—where, based on these measurements of what we call helioseismology, I said it was time for us to declare the solar neutrino problem solved. It was time for the astronomers to declare a victory, that it was clear that our models were in sufficient agreement with the sun that that could not be the source of the discrepancy.

And that was a big mistake on my part, because the summary speaker at that conference was a very eloquent, humorous speaker who also had the ability to make very beautiful and very humorous drawings. He made several caricatures of me which he showed in the viewgraphs in the summary, and he had the whole auditorium, including me, laughing at the bravado, the hubris of this guy claiming that he could say something about particle physics based on this complicated sun. I tapered down my comments for a few years based on that rather humiliating personal attack. It wasn't a scientific attack, but it was a very, very effective attack.

### Meanwhile, other experiments started to find a similar neutrino deficit.

The first experiment that was done after Ray's experiment was done by the Japanese-American collaboration called Kamiokande, which converted a water detector designed to see the decay of the proton into a very sensitive detector of neutrinos from the sun and from supernovae. Just in time they made the conversion so that they could see the neutrinos from Supernova 1987A. That was a very spectacular achievement.

### And their results supported Ray's?

Yes, when their first results came out, I was absolutely thrilled, because they got a result which showed that the flux was definitely less than what I had predicted and that was a confirmation of Ray's result. My feeling was aha, we've eliminated the possibility of experimental results being wrong, and I'm confident in my theory. I think we're onto something good.

In fact, two years later one of my idols and heroes Hans Bethe and I used the first results from the Kamiokande experiment together with Ray's results and a very, very basic result from our solar models to argue very strongly that either one of the two experiments was wrong or we needed new physics, that it couldn't be something wrong with my solar models. Hans and I (Hans is the guy who first worked out, in 1939, the nuclear reactions that we think make the stars shine) compared the results from the chlorine experiment and the Kamiokande experiment and showed that on very general grounds either one of the experiments had to be wrong, which didn't seem likely by that time, or there had to be some new physics, and that took me off the hook. I was no longer the person who had done the wrong calculation.

### What about the Sudbury Neutrino Observatory? What role did it play in finally putting the solar neutrino question to bed?

The SNO experiments had been in development for I guess almost a decade and a half. They were designed to finally solve this problem clearly, so that it wasn't a matter of stacking up one argument against another argument against another argument—to prove that it had to be new physics but to demonstrate it all within one or two clear experiments.

The SNO experiment could look at a very particular high-energy branch of neutrinos, which only is about a hundredth of a percent of all the neutrinos I think come from the sun. They could find how many of those neutrinos were in a form called electron-type neutrinos.

### So neutrinos come in different forms?

Neutrinos can come in different flavors. For ice cream we have, for example, chocolate (which is what I prefer), vanilla, and strawberry. Neutrinos can come in the flavor of electron type, associated with electrons, or with other types of particles called muons and taus. What the Sudbury Neutrino Observatory was uniquely able to do was observe neutrinos in the electron type only. They were able to determine how many neutrinos of the electron type got to us at Earth, in this huge tank of heavy water. And then they made use of the result from a much larger tank of water called Super-Kamiokande in Japan, which measured primarily electron-type neutrinos but had a little sensitivity to muon and tau neutrinos.

"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."

So we had two measurements, one of just the electron type from SNO, and one from mostly the electron type but a little from muon and tau neutrinos from the Japanese-American experiment. And combining those two data points the SNO people together with the Japanese-American collaboration could work out two things—how many neutrinos of the electron type got here and how many neutrinos of all types got here. And it's the one which is neutrinos of all types that's really exciting.

Because what I can calculate for the sun is the neutrinos of all types that start in the interior of the sun, and you want to know what's the total number of neutrinos reaching the Earth. That's what the Sudbury Neutrino Observatory was able to measure together with the Japanese-American experiment in 2001. And their answer was bang on our prediction, I mean so close that it was embarrassingly close.

### How did it make you feel?

For me personally it was the most exciting time after the understanding of how to increase the rate of the neutrino capture in Ray's tank. It was just enormously exciting for me. In fact, I was called right after the announcement was made by someone from The New York Times and asked how I felt. Without thinking I said "I feel like dancing I'm so happy." The one thing my kids kept sending each other e-mails about all week was, "Did you see where it said in The New York Times that Dad felt like dancing?" They kept making fun of me about that, but I was deliriously happy.

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.

### Editor's Notes

This feature originally appeared on the site for the NOVA program The Ghost Particle.

National corporate funding for NOVA is provided by Draper. Major funding for NOVA is provided by the David H. Koch Fund for Science, the NOVA Science Trust, the Corporation for Public Broadcasting, and PBS viewers.