Written By: Michael D. McClellan |Nobel laureate Arthur Bruce McDonald’s world, much like the solar neutrinos he’s so passionately studied for decades, changed in ways that could only have been theorized until October 6, 2015, when an early morning phone call from Stockholm, Sweden, transformed the unassuming, highly-regarded astrophysicist into a global scientific sensation. McDonald – who still prefers “Art” despite honors and awards that include the Order of Canada and the Fellowship of the Royal Society – woke to news that he, along with Japanese physicist Takaaki Kajita, would share 2015 Nobel Prize in Physics. What followed was a blizzard of phone calls, emails, and interviews, along with a realization that the flavor of McDonald’s life would be forever altered, relative anonymity replaced by mainstream celebrity. It was as if he’d gone to bed with a guitar in his hands and awakened as Buddy Holly.

“That phone call created a whirlwind,” recalls McDonald, without the faintest hint of hubris. “It took a long while for the media requests to settle down. Even today, I have to be selective about the frequent requests I receive for speaking engagements and travel, because my primary focus is once again on research.”

To appreciate what all the fuss is about, one must consider that everything in the universe – comets, crash test dummies, a Wayne Gretzky rookie card – is found to be made from a few basic building blocks called fundamental particles, governed by four fundamental forces. Our best understanding of how these particles and three of the forces are related to each other is encapsulated in something called the Standard Model of particle physics. That’s where McDonald comes in: The work by him and the team  at the Sudbury Neutrino Observatory in Northern Ontario led to the discovery of neutrino oscillations, which proves that neutrinos have mass, which, in turn, requires changes to the Standard Model at a very basic level. It’s the kind of revelation that not only turns heads, but turns an already brilliant career into the stuff of legend.

“Neutrinos are the basic building blocks that we know the least about because they are so difficult to detect,” says McDonald, professor emeritus at Queen’s University. “Therefore they were of substantial interest, because if one could measure their basic properties it would be of great significance for the fundamental laws of physics.”

Billions of neutrinos harmlessly pass through our bodies every second, yet we cannot see or feel them.  The existence of these almost massless, electrically neutral particles was postulated back in 1930, when Wolfgang Pauli suggested their existence to explain a loss of energy when neutrons decayed. For over two decades these particles remained hypothetical, but in 1956, two American physicists, Frederick Reines and Clyde Cowan, detected neutrinos streaming from a nuclear reactor, which proved their existence. Experimenters later found that there were actually three kinds of neutrinos – electron, muon and tau neutrinos – but confusion in the scientific community ensued, because models of the Sun predicted there should have been more solar neutrinos than detectors picked up. This led to the “solar neutrino problem” that aggravated physicists for so long, as well as concerns that perhaps our understanding of the basic laws of physics were wrong.

“As scientists, we needed a way to measure the type of neutrinos produced in the core of the Sun and also the sum of all types of neutrinos,” McDonald says. “That’s where SNO and the SNO Collaboration comes in.”

The detection of neutrino particles in labs is difficult because of the many other cosmic particles reaching the Earth, and the numerous natural radioactive decays taking place. So, in 1990, McDonald began supervising the construction of SNO, a giant neutrino detector built 2 kilometers underground in a nickel mine in Sudbury, Ontario. A huge transparent acrylic tank, containing 1,000 tons of heavy water worth $300 million, was surrounded by a geodesic sphere equipped with over 9,000 light sensors. This assembly, in turn, was sunk in a massive cavity filled with regular water. All of the construction was carried out in ultra-clean conditions.

By 2002, McDonald and his team were able to prove definitively that neutrinos produced in the Sun were changing into the other two types as predicted. In fact, two-thirds of the electron neutrinos had transformed into muon and tau neutrinos on the eight-minute journey from the core of the Sun.

“We had an extremely clear result that showed neutrinos do change from one type to another,” McDonald explains. “This meant that, first of all, we had a clear indication that neutrinos change their flavor and therefore have a finite mass. We were also able to determine that the model of how the Sun burns, and in particular the nuclear reaction that we were measuring, was being calculated very accurately. Those measurements were quite significant, it was felt, in the world of physics and astrophysics.”

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Let’s pump the brakes.

While there is no disputing the groundbreaking nature of the SNO Collaboration’s discovery, part of the success arises from the makeup of the man at the helm. Long before Art McDonald morphed into the tall, bespectacled, silver-haired scientist with a Nobel Prize in his dossier, he was a friendly, fun-loving teenager from Sydney, Nova Scotia, the kid who attended the local high school dances on Saturday nights. He met his wife, Janet, at one of those dances. They have been together ever since.

“And we still dance,” McDonald says proudly. “It helps keep us young.”

He was born Nova Scotia, Canada in 1943 and McDonald’s father left shortly afterward to fight in the war. The resulting void was filled by his mother, two aunts, and grandparents who lived close by. He played sports, held down a 104 house paper route (which he now recalls as being all uphill, particularly in winter), and did well in school.

“I had a very pleasant childhood, with lots of interaction with family and friends, and very nice teenage years,” McDonald says. “My family were congenial and warm, and encouraged open communication. They helped me understand that you can be far more productive if you got along with others. It was a great environment to grow up in.”

The lessons learned during his childhood help lay the foundation for the expert leadership he would later demonstrate at SNO. He chose Dalhousie University in Halifax, N.S. for his undergraduate and master’s degrees, trying chemistry, geology, and math before falling in love with physics. From there, McDonald landed at Caltech, where he had plenty of beam time running the Kellogg Lab’s Van de Graaff accelerator, and where he also got to know two famous scientists, Raymond Davis and John Bahcall. It was Davis and Bahcall, ironically, whose work in the Homestake Mine in South Dakota and on the theory of neutrino production in the sun led to the solar neutrino problem that McDonald and his team would ultimately solve.

“Ray Davis came to Caltech during the summers while I was there,” McDonald explains. “John Bahcall was a junior faculty member in theoretical physics at that time, so I was certainly quite knowledgeable about their work, and what came to be known as the solar neutrino problem.”

Where the Higgs boson has famously come to be called the “God Particle,” neutrinos have earned a  well-deserved nickname of their own.

“The Ghost Particle,” says Arthur Loureiro, study author and PhD student in the University College London’s Physics & Astronomy department. “Neutrinos are tiny, very weakly interactive ghosts, but they are also abundant – a hundred billion neutrinos fly through your thumb from the Sun every second, even at night.”

On the hunt for the elusive Ghost Particle, Davis set out to observe neutrinos by monitoring what happens on the rare occasion when a neutrino collides with a chlorine atom and creates radioactive argon, which is readily detectable. At the core of the experiment was a tank filled with 600 tons of a chlorine-rich liquid, perchloroethylene, a fluid used in dry-cleaning. Every few months, the scientists would flush the tank and extract about 15 argon atoms, evidence of 15 neutrino interactions. The monitoring continued for more than 30 years. Meanwhile, the solar neutrino problem continued to baffle scientists.

“Imagine trying to catch a mosquito traveling at the speed of light with a butterfly net,” says Giuliana Galati, physics professor at University of Naples Federico II. “Even though neutrinos are the second most abundant particle in the universe after photons, they are the most elusive.”

The most intriguing explanation for the missing solar neutrinos was that, while the Sun creates as many electron neutrinos as it should, on the way to the detector they change into their cousins, muon and tau neutrinos, and possibly back again. Muon and tau neutrino flavors were invisible to Davis’s detector. To test this “neutrino oscillation” hypothesis, new kinds of detectors were needed.

SNO was initiated in 1984 primarily to provide a definitive answer to the solar neutrino problem. The persistence of the problem motivated Herb Chen to contact Canadian scientist Cliff Hargrove, a former colleague, to explore whether enough heavy water could be made available on loan to perform a sensitive measurement and determine whether the neutrinos change their type in transit from the core of the Sun.

“Heavy water is deuterium oxide, rather than hydrogen oxide,” McDonald says. “Instead of having just a proton in the nucleus, you also have a neutron in the nucleus, which makes it heavier than ordinary water but chemically similar. The unique properties of deuterium made it possible to observe both the electron neutrinos produced in the core of the Sun and the sum of all neutrino types.”

With the immediate involvement of George Ewan, who had been exploring underground sites for future experiments, a collaboration of 16 Canadian and US scientists was formed in 1984, led by Chen and Ewan as Co-spokesmen. In 1985, David Sinclair brought the UK into the collaboration. Sadly, Herb Chen passed away from leukemia in 1987. The collaboration continued with Art McDonald and Gene Beier as US spokesmen and grew with the addition of other institutions in the US and Canada for a total of 13 institutions. In 1989, funding was provided jointly by Canadian, US and UK agencies with McDonald as Director of the project and the scientific collaboration.

All of which brings us back to McDonald, who not only had a reputation as a brilliant scientist, but who was also someone well-connected to the Canadian and United States scientific communities. As affable as the day is long and an expert consensus builder, McDonald was a good choice to lead the SNO Collaboration.

“Art was always soft spoken at collaboration meetings, and asked the simplest, but most important questions about each of the details being studied,” University of Winnipeg Associate Professor Dr. Blair Jamieson was recently quoted. “He valued the ideas and input from everyone involved. I was part of a small group at UBC working on the third phase of SNO where all flavors of neutrinos were detected in the SNO heavy water, and in a set of proportional chambers.  For that analysis I developed a new Bayesian analysis that simultaneously fit the data from both detectors.  Art had many questions about this new analysis method, and eventually was convinced that it should be adopted as the main analysis for extracting the solar neutrino fluxes.”

On Jan. 4, 1990, the SNO project was announced in Ottawa. Scientists, engineers and contractors descended in droves upon Sudbury, until then a city known primarily for the nickel-copper ore discovered in 1893 at the edge of the Sudbury Basin. It was this 19th century discovery that brought the first waves of European settlers, who arrived not only to work at Murray Mine and others like it, but also to build a service station for railway workers. Nearly 100 years later, the mines were of keen interest to neutrino hunters.

“We looked at a number of mines and developed a good relationship with the people at Inco, who owned and operated Creighton Mine at the time,” Ewan, now Professor Emeritus of physics at Queen’s University, was quoted as saying.

SNO gained momentum. With McDonald as Project Director, crews completed construction of the plastic acrylic tank, which was built two kilometers underground and 12 meters in diameter. The work proved tedious, as each of the 120 pieces had to be lowered down in the mine elevator and then seamlessly bonded. Sometimes the bonds would form bubbles; workers had to sand them down and start over. Whether it was complications with the tank, or issues encountered with the detector’s 9,600 light sensors, McDonald looked to the very competent scientific collaboration and technical team to solve the problems. Perhaps mostly importantly, McDonald kept his cool through the myriad crises cropping up on a weekly basis – not surprising to anyone who’d ever taken classes from him.

“He was an excellent professor,” says Dr. Ian Hill, who did his fourth-year undergraduate thesis on the SNO Collaboration. “Art is very genuine, down-to-earth, and humble – just a very nice guy. He conveyed information very well, and was never intimidating – which of course, theoretical physics certainly can be.”

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Stephen Hawking – yes, that Stephen Hawking – was there when SNO opened in the spring of 1998.

A rock star in the scientific community as well as an undisputed pop culture icon, Hawking brought a street cred to SNO that made the world take notice: Something important, and equally cool, was about to happen at this wintry Canadian outpost.

In 1999, SNO began collecting data. In 2002, the team announced its definitive results: Neutrinos from the Sun weren’t disappearing. Instead, the pathologically shy particles were oscillating, switching “flavors” on their journey – which is why Davis was unable to account for the anticipated number.

“We were able to show that we understand very accurately how the Sun burns,” McDonald says. “It was an exciting time for everyone involved.”

The SNO Collaboration’s results, in conjunction with the work led by Takaaki Kajita at the Super-Kamiokande (Super-K) detector, resolved the solar neutrino problem that had perplexed scientists for decades. As expected, the findings were met with great fanfare; the same year that SNO published its results, Raymond Davis won the Nobel Prize in Physics, alongside Masatoshi Koshiba of Japan, for the “detection of cosmic neutrinos.”

As for the Standard Model?

Gone was the Standard Model’s massless neutrino traveling at the speed of light. In order to exchange identities, neutrinos have to have mass, if only a little, with a different mass for each flavor. Despite the discovery of the Higgs boson, the Standard Model isn’t complete. The results from SNO and Super-K offer a pathway toward a new model, and a new way to understand the universe.

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Hawking returned to Sudbury in 2012 and went underground with McDonald. A specially designed railcar allowed the wheelchair-bound scientist to travel through “The Drift” and into the clean facility. Hawking toured the expanded surface and underground facilities at SNOLAB and put the science there back into the spotlight.

There were many who felt it was only a matter of time before a Nobel was awarded for the SNO measurements. McDonald, for his part, chose not to buy into the hype. “There are things beyond your control,” he says, “and the Nobel Prize is one of them. You can either worry over it every year the awards are announced, or you can get back to work. All of us involved with the project got on with doing science.”

Indeed.

And then, on October 6, 2015, everything changed with a 5:15 a.m. phone call.

“It was an incredible moment, but winning the Nobel Prize isn’t about one person,” he says quickly, pointing out that a maximum of three Nobel laureates and two different works may be selected for the Nobel Prize in Physics. “That is regrettable, because there were 273 others involved in our discovery, and they deserve as much credit for this award as I do. How do I bring 273 people with me to Stockholm?”

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Today, SNO has expanded into SNOLAB, the facility doing additional research into those maddeningly elusive neutrinos. There are also other experiments underway, including the hunt for dark matter, the mysterious substance that makes up more than a quarter of the known universe. McDonald, as you might expect, has rolled up his sleeves and is pouring his energy into the science currently underway.

“It’s an exciting time for the DEAP experiment,” McDonald says, touching on the direct dark matter search experiment which uses liquid argon as a target material. “The Nobel Prize and everything that has come with it has been wonderful, but I’m happy to be back doing research. I’m looking forward to what we learn about dark matter with DEAP and its successors and about neutrinos with the new SNO+ experiment.”

Let’s start at the beginning.  Where did you grow up?

I come from Sydney, Nova Scotia. It was, and still is, a city of about 30,000 people, and therefore a small town, so to speak. It’s interesting, actually, because there have been six Canadian winners of the Nobel Prize in Physics, and every one of them has come from a town of fewer than 40,000 people or so at the time they were born. So, in Canada at least, the Nobel Physics Prize appears to be a small town exercise.

Early on, who were some of the key people instrumental in your development?

My early childhood education was an important part of my later success. I attribute that to close relatives and the closeness of the family. I had a grandmother and grandaunts who taught me many things when I was very young. My parents also played very important roles in my development.

Interestingly, neither my father nor my mother went to university. This is partly because of what was going on in the world at the time – I was born in 1943, and shortly after I was born, my father went overseas as part of the European campaign. He received the Military Cross for the role he played in the liberation of Holland. It is the second highest military medal in the Canadian Armed Forces, so he was a strong achiever. He later earned an accountant’s degree, and went on to become the business manager of the newspaper, and also a town councillor.

My mother was a housewife. In those days, that was essentially the normal thing – when you got married, you stopped working, and you became a housewife. In this case, she stopped her secretary job, but she was a very intelligent person as well. Later, she and my father developed a subdivision in the city and she served as contractor for a number of homes. So, even though my parents never had a university education, they certainly were very inspirational, both to me and to my sister, who is ten years younger and obviously a post-war baby.

What were your high school years like?

It was the late 1950s, so it was the Rock and Roll generation. We had service clubs at the local YMCA, boys clubs and girls clubs that had meetings on Friday nights followed by dances afterwards. We also ran the dance for the high school every Saturday night. It was an ideal situation to meet people in a very nice atmosphere. Everybody was having fun. I met my wife through that group. We were married seven years later, after we finished university. My wife and I have continued to dance, even now that we are in our 70s. We still enjoy it, so that has certainly been a part of my life all the way along.

Let’s talk mentors. Who in high school inspired you?

I went to university with a significant interest in mathematics, rather than particularly science, or particularly physics, and that was mainly because I had an excellent mathematics teacher in high school. His name was Bob Chafe. We had a class of about 35 students during my senior year, and those were the ones that were interested in math and science. There were two others in that class besides myself who went on to get PhDs (in mathematics). Bob Chafe certainly had an inspirational effect on that class. Every now and again you are lucky to get a mentor like that.

Following high school, you went to Dalhousie University in Halifax, Nova Scotia. How did you become interested in physics?

Kids often ask me how to choose a career. I respond by saying, “You should choose a few things that you would be happy to do when you wake up in the morning. Then try them out. See what you are good at, and what you actually enjoy doing.” That has been basically my experience. I tried physics, chemistry, geology, and math in my first year. Physics just worked for me. I loved the way that you could apply math and calculate things that actually applied to the way the world worked. I did well in it and I had very good marks, and that was because I really enjoyed doing what I was doing. I went on to complete my master’s degree in physics, and that convinced me that I wanted to do experimental physics.

Please tell me about Professor Ernie Guptill, your first year Physics teacher.

Ernie Guptill, or Ernest Guptill, was also a great mentor. He was originally from Grand Manan, a small island located between Nova Scotia and New Brunswick. He loved outdoor activities, and he loved sailing. He lived on the shore of the harbor in Halifax, where he had a pet lobster tethered underwater in front of his house, which was quite interesting. To see it, you had to don a wetsuit. Once, he invited a close friend of mine and I over to see the lobster, and I actually got to go down and meet the lobster personally. I still have a picture from that day. I’m wearing the wetsuit.

It sounds like Ernie Guptill cared about his students.

He was the chairman of the department at the time, but he also went out of his way to teach first year physics. He inspired me, as well as a number of other people in my class, to go into physics as a serious activity. He was also very personal with the students, and took a vested interest in their work. He was a well-revered professor. It was that kind of friendliness that, coupled with the educational activity, made what was really a small university at the time a very warm and friendly place. It was filled with good people. It was an excellent place to do your undergraduate and post-graduate work.

Tragically, Ernie Guptill lost his life attempting to save someone who had fallen into the cold water in the spring in Halifax. They both perished from hypothermia. That happened about 10 years after I left Dalhousie. I was honored to go back at one point and deliver a lecture in his honor.

While at Dalhousie, you were introduced to engineering physics by working in the summer for Professor Ewart Blanchard, measuring gravity on the roads of Nova Scotia.

You have really done your homework. That is correct. Ewart Blanchard, one of my professors, was a pioneer in what these days is referred to as technology transfer. He went on to become the Director of the Nova Scotia Research Institute, which was a government agency supporting research – particularly supporting the connection between universities and industry. At the time he had two programs going, both of which turned out to be very productive. One in which I was involved was measuring gravity to a part per million over large areas. Professor Blanchard also had students involved in seismic activities, setting depth charges off boats in the surrounding waters, which eventually led to the discovery of a gas source off the coast of Nova Scotia. This was the 1960s, so this was somewhat pioneering activity at the time. I have a lot of respect for Professor Blanchard, not only as a person, but also for his foresight in moving in these sorts of directions, and for the way he used students in the process.

How did you conduct your measurements?

We had our own Jeep. I was using a device that was sort of like a coffee percolator, which was basically a thermos to insulate the inside material, the material being a quartz spring that was capable of measuring gravity to about a part per million. We actually discovered what eventually turned out to be a very productive gypsum mine in Nova Scotia. This was because gypsum, with a very low density, gave a big difference in the gravity. My friends were measuring elevation, because you have to correct for elevation when you are dealing with a part per million, so they played an important part as well. It was a wonderful summer experience. Professor Blanchard and I became very good friends.

You earned your Master’s Degree in Physics at Dalhousie University. What memories do you look back fondly on?

I was studying the lifetimes for positrons in metals. What we discovered in the year that I was there, which was followed on by a couple of other graduate students thereafter, was that the time that it takes a positron to find an electron and annihilate with it – which is time that is in the nanosecond region – depends strongly on whether or not there are defects in the material. That was the basic discovery. In fact, this work remains one of my highest cited papers. It has turned out to be very useful in situations where there is a buildup of defects in materials, like nuclear reactors, where the question is, “What’s the longevity of the material that you are using in terms of the integrity of the reactor?”

That is a significant research effort.

It was very practical. This was ten years before positrons were used for positron emission tomography, a medical diagnosis tool of great value. At the time I had a professor named Innes MacKenzie, who was a very good experimentalist, and very creative. He was a good mentor for me. I was in the laboratory 5-to-7 days a week, and he made the experience fun. The attitude was, “Let’s try this; let’s try the next thing; let’s follow our ideas as to what it is that’s causing this.” And I just discovered that having your hands on equipment, and being able to make these measurements, was great fun. That continued when I got to Caltech.

And Professor Innes MacKenzie is someone else who’s had a big influence on your life.

Professor MacKenzie did have a big influence, and we maintained contact for many years. To give you a feel for the sort of consummate experimental physicist that he was, we both happened to be back at Dalhousie speaking at an undergraduate physics conference there, and we had dinner together. During dinner we were talking about a problem that we were faced with at the Sudbury Neutrino Observatory project, where the team was preparing to spray layers of material on the cavity walls. This is a cavity that is 70 feet in diameter and 100 feet high – or roughly 22 meters in diameter and 34 meters high – and we needed it watertight and radon tight in order to keep the radioactivity in a very low level.

Innes MacKenzie was actually retired at the time, and he was working on the backscattering of x-rays from materials – in fact, he had developed a technique whereby you could measure bone density for bone density scans by looking at the backscatter of these x-rays.

He said, “I think my device would work for your situation.” Six months later we were delivered not only a device that measured the thickness of the layers that we were putting on, but also was coupled to a small, belt-suspended computer system to capture the data and store it.

I maintained contact with Professor MacKenzie for many years, including speaking to him after the award of the Nobel Prize. Sadly, he passed away a couple of years later, but he maintained contact with his students throughout his life.

Aside from science, what were some of the other things you were into while at Dalhousie?

I was on the junior varsity football team for a year at the university. I wasn’t part of the topline team, but that year I was probably in the best physical shape I’ve ever been in. Football just took too much time after I got into the third and fourth years of my studies. I also played golf and tennis recreationally. I wasted a lot of time in university playing ping-pong and pool, but that was the primary form of relaxation at the time.

The average layperson may think of highly successful scientists as being an arrogant and humorless lot. In your case, I don’t think the stereotype applies.

With respect to humor, I’ve always liked approaching things in a positive and humorous way whenever possible. Point of fact, I actually started every one of our collaboration meetings with a joke that, unfortunately, became known as “Art’s Joke,” which was not necessarily a positive term [laughs]. If anybody got into a serious argument during the process of our collaboration meetings, which often happened, anyone was allowed to yell out the punchline of that particular joke as a way of diffusing the tension. The point being, let’s not get too serious about ourselves here, we got into this to have some fun. That is one of the messages that I like to give to students. Science is fun. You really can have an enjoyable time doing science if you work hard, but also if you realize you’re in it to enjoy it.

I’ve read where Peter Nicholson, who served as head of policy in the Office of the Prime Minister of Canada, was one of your closest friends in college.

Peter Nicholson is a very remarkable individual. He was the other fellow who was at Ernie Guptill’s when we went to meet the lobster [laughs]. He was my college roommate after second year at university, which worked out because we got along really well, and still do. He and I went to a space physics course at Columbia University in 1963. It was a summer six week summer course run by NASA. A poster described where you could apply, so we did. It was amazing. They accepted a couple of Canadians at the time, and we not only got the space physics course, but we got to tour the NASA facilities in Houston and Cape Canaveral as they were back then.

Is it true that the two of you hatched a plan to visit various graduate schools on the US east coast?

We were both looking at United States universities for post-graduate studies, so we went together to the chairman, who happened to be Ernie Guptill at that point. We said, “Look, we would love to go and visit these places. We will make you a deal. If you’ll write some letters on our behalf and support our trip, then what we will do is go and interview people at the universities and evaluate each school’s facilities. When we come back, we’ll give a seminar that highlights the possibilities at these various institutions. We’ll give our impression of what it’s like at these places, and we’ll do it for students and faculty alike.” And that is basically what we did. You have to remember, it was a much different time back in the ‘60s. There were no smartphones. Social media didn’t exist. You didn’t have a wealth of information at your fingertips.

What memories from those trips stand out?

The interesting people. Jim Peebles won the Nobel Prize this year – he was a colleague of mine at Princeton, and is still a good friend. He and Bob Dicke – Peebles theoretically and Dicke experimentally – were working on various astrophysical measurements. One of the things that they were trying to do was to measure the cosmic microwave background. As it turns out, this was actually measured for the first time down the road at Bell Labs by Arno Penzias and Robert Wilson, who then came to Princeton in 1965 and learned what it was they were observing as background in their experiment. Penzias and Wilson later got a Nobel prize for it. This year, the committee finally recognized that Jim Peebles was one of those motivating Penzias and Wilson back then, but that he has further developed a real understanding of how the world has evolved since the Big Bang. We met Dicke and a number of other top physicists because the department chair in each case had received a letter from our chair, and they introduced us to top scientists. Those are the things that stand out.

You decided to go to Caltech.

We applied to the places we were visiting, and we were actually accepted into several of them, but I was also accepted at Caltech and Peter Nicholson was accepted at Stanford. After Peter completed his Master’s in Theoretical Physics, he decided that he wanted to do operations research. We both looked at the map and said, “Boy, California looks kind of neat.” There was a friend of Innes MacKenzie’s at Caltech by the name of Charles Barnes – he was known as Charlie Barnes – that wrote back to me and said that he was willing to take me as a student. He turned out to be a wonderful supervisor. So, Peter and I both went to California. Going to Caltech was great, because I got to do experimental nuclear physics in the laboratory, which was headed by William Fowler, who later received the Nobel Prize for having pioneered the calculations of how elements are produced in the Sun.

Peter Nicholson also went on to do great things.

He went on to an illustrious career, both as a teacher, businessman and as key figure in the Canadian government.  He was a financial advisor to Canada’s Minister of Finance starting in 1993, and was very actively involved in establishing the Canada Foundation for Innovation, which is the principal source of funding for infrastructure for basic science and engineering in Canada. He eventually became the founding president of the Council of Canadian Academies, which provides advice to government on various scientific questions. So, it has been interesting to maintain contact over the years as we went through our separate careers. It all started back in our university days.

At Caltech you got to work in the Kellogg Laboratory.

There were actually three Van de Graaff accelerators in the laboratory; two smaller ones, and a larger, EN Tandem Accelerator. The EN Tandem Accelerator was nothing compared to the scale of the Large Hadron Collider, which has collaborations with about 4,000 scientists who are building enormous, complicated experiments, but it was extremely useful for our purposes.

What was some of the research that you were into at the time?

I worked with a graduate student, Eric Adelberger, who is an exceptional scientist and is now still an active emeritus professor at the University of Washington. He has gone on to make the definitive measurements of gravity at short distances with very ingenious devices to balance out the systematic uncertainties that you get in those sorts of experiments, so he has had a wonderful career as well.

Eric and I worked on experiments that were looking for symmetries in nuclei, which were an indication of how the electromagnetic interaction worked inside the nuclei and whether there were any differences in what you might expect from conventional theory. It turned out that we were able to constrain it very strongly and found no significant differences. We also worked on measurements where we looked for the weak interaction in nuclei as a test of the Standard Model, which was relatively new at the time. We were able to make measures of the weak interaction between quarks that were a part in a million effects in some cases.

Were these measurements made using the Kellogg lab accelerators?

Yes, many of them. The nice thing was that you had access to a very large amount of beam time. It had to be scheduled, but you could be running your experiment a couple of days a week on that accelerator. You actually had to run the accelerator yourself, so that was a good experience as well. And we had a little fun with it; there was one knob on the control console that wasn’t connected to anything, so the new graduate students were asked to control the beam using that knob [laughs]. We didn’t let that go on for very long before we let them in on the joke, but it was fun at the time.

I think that debunks the myth that scientists don’t know how to have fun.

The experience of that laboratory really made me understand the importance of social activities. The seminars at Caltech at that time were on Friday nights at 7:30 PM, which were always followed by a party at one of the professor’s homes. There was also something called the Kellogg Band, including Charlie Barnes on the piano, and students and Post Docs who went on to have very good careers, so that was a great social time as well.

The solar neutrino problem was big at Caltech while you were there.

Ray Davis, who was the one who originally detected neutrinos from the Sun, came from the Brookhaven National Laboratory to Caltech during the summers to work with John Bahcall, who was a theorist and junior faculty member at that time. So, I was quite knowledgeable about what came to be known as the solar neutrino problem.

John Bahcall’s calculations predicted something three times larger than what was observed in the Homestake experiment, which was an underground detector built in a South Dakota gold mine – essentially, a very large tank filled with cleaning fluid. Bahcall did the theoretical calculations and Davis performed the experiment. Davis eventually received the Nobel Prize for his work in pioneering the detection of solar neutrinos.

Could you have ever imagined that your work at SNO would vindicate Bahcall’s solar neutrino theory?

When we solved the solar neutrino problem with the Sudbury Neutrino Observatory, a New York TV station interviewed John Bahcall. They asked him what he thought when he heard that the results verified his calculations after 30 years of scrutiny. He replied, “I feel as though the DNA evidence has just overturned my conviction by the scientific community for having got it wrong.” [Laughs]. He got it right, to within 10% or so.

Physics and mathematics are inextricably linked, but creativity can be equally important.

I do agree. As Director of the Sudbury Neutrino Observatory I was involved with hundreds of very intelligent scientists, engineers and technicians. It was a real pleasure to see the creativity that resulted, creating solutions to very difficult technical problems. As a result we were able to build an ultra-clean detector the size of a ten story building, housing 1,000 tons of heavy water safely 2 kilometers underground in an active nickel mine, and observing one burst of light from a solar neutrino per hour with little or no interference from radioactivity of any kind. There were many creative firsts in this project. I am very careful to point out, that while they give the Nobel Prize to one individual, there are many authors on our papers plus many other technical people that don’t get on the author list. And I am a representative of them. So I would say, working with collective creativity has been what I have been mostly involved in for many years.

Please tell me about the SNO Collaboration.

Herb Chen, who was from the University of California at Irvine, had a very good idea, which was, if you could get enough heavy water, then you could potentially use it to solve what had come to be known as the solar neutrino problem. In fact, he had worked with Ottawa scientist Cliff Hargrove at the Los Alamos Meson Physics Facility using heavy water for this purpose. So, at one point he called Cliff and said, “Do you think it would be possible to borrow 4,000 tons of heavy water?” Well, that would have been $1.2 billion in 1984 dollars. But to our amazement, Atomic Energy of Canada had 5,000 tons of heavy water in reserve for the use in CANDU-style nuclear reactors and said, “Well, maybe not 4,000, but maybe 1,000. Do you think you can do it with that?” And sure enough, that’s what we eventually designed, which was a detector that used $300 million worth of heavy water to do the measurements.

Unfortunately, Herb Chen passed away about three years later. He was at a collaboration meeting and said that he didn’t feel well, and that turned out to be leukemia. He passed away within a year, in his mid-40s. We still like to preserve his memory, because he was a founding member of the collaboration and an inspiration to all of us.

How did you become involved in the SNO Collaboration?

I was a professor at Princeton at the time and became involved at the beginning of the project along with 15 other scientists. I took over as the US spokesman for the project when Herb passed away in 1987 and was soon joined by Gene Beier from the University of Pennsylvania. I then became the overall director when the Canadian spokesperson, George Ewan, was about to retire, and at that point I moved from Princeton to Queens University. It took us from 1984 to 1990 to eventually get the funding because the project was very large, particularly in Canadian terms in those days. So I was involved early on, but it really was Herb Chen, George Ewan and those original 16 people that got everything going.

As Director of SNO, how would you describe your leadership style, and how did you resolve conflict?

Conflict is unavoidable whenever you are dealing with people. I attempted to resolve conflict by having everyone focus on the facts associated with what they were doing, and the objectives that they were trying to accomplish. And, in some cases, you had to factor in the timeline in which you are trying to make it happen.

I try not to get overly involved in the emotional approaches that people take in decision-making. People love their own ideas. You have to sit there and recognize that the reason that this person is being so adamant is that they clearly think that they are right. They wouldn’t be saying it if they didn’t think they were right. So, in some cases, you have two people who think they are right who are directly opposed to each other. How do you resolve that? Well, you try to get around the subjective aspects of it, and get down to the basic facts. You try to determine which is the right way, or the best way, to do something. If you can direct the conversation towards the factual topics that are necessary to resolve a dispute, and reduce the rhetoric associated with it – then you can come to a decision. This is especially true if you keep in mind that we are in this for some enjoyable science. This isn’t supposed to be a knockdown, drag out fight. It’s something where, if we can come to a conclusion and get on with things, then we can really have some good experiences and do some excellent science. That’s my approach.

Were any of your classmates involved in the SNO Collaboration?

Absolutely. The SNO Collaboration included a number of people who had worked together internationally. Hay-Boon Mak, who is a friend and who was just behind me as a student at Caltech, is a good example. He eventually moved to Queens University and became a significant part of the Sudbury Neutrino Observatory project. Bob Stokstad is another example from Caltech days. Hamish Robertson, with whom I had worked starting in the 1970’s and who joined Gene Beier as US Spokesperson and George Ewan, with whom I worked at Chalk River, are other examples at a more senior level. So working with your friends is another way you can enjoy science.

Tell me a little about SNOLAB.

SNOLAB is the deepest cleanroom facility in the world. It’s 2 kilometers underground to reduce cosmic rays. What is distinctive about SNOLAB is that the entire laboratory is a class 2000 cleanroom, just as it was when we were building the Sudbury Neutrino Observatory. That makes it easier to achieve class 100 in our actual experiments, as good as the table-tops in a semiconductor fabrication facility. So, SNOLAB is ultra-clean, as well as being ultra-deep.

Let’s talk solar neutrinos.

Neutrinos change from the moment they are created in the core of the Sun. Electron neutrinos are the only one of the three neutrino types that are produced by the nuclear reactions that power the Sun. Then, as one of these electron neutrinos traverses the Sun, it changes to a different composition, in which if you make a measurement of it, it is only one-third electron neutrinos at that point. Two-thirds of the time it behaves like muon or tau neutrinos. And so, that is where our experiment comes in. When the electron neutrinos eventually got to the Earth, we were able to measure, using heavy water, that that change had taken place.

How does the SNO detector work?

The way we did it was the following: Heavy water is deuterium oxide, rather than hydrogen oxide – it is D₂0 rather than H₂O. The D means that it is heavier than ordinary hydrogen, because, instead of having just a proton in the nucleus, you also have a neutron. Because it doesn’t change the charge of the nucleus, it doesn’t change the number of electrons going around nucleus, which match the charge of the proton. So chemically, D₂O is essentially the same as H₂O because deuterium behaves chemically like hydrogen. That extra neutron is the secret to being able to resolve the question of whether these neutrinos actually changed from electron neutrinos to another type…because, with the extra neutron, there are two reactions we can observe: One being explicitly caused by only electron neutrinos, and one which is equally sensitive to all neutrino types.

What we found was that the number of neutrinos observed with the reaction that was sensitive to all neutrino types, matched exactly the calculations of how many were produced in the Sun according to Bahcall and his coworkers. Whereas, the number of electron neutrinos that survived were only one third of the total. By being able to do this  we were able to have an explicit measurement, independent of calculations of the number produced in the Sun, proving that neutrinos had, in fact, changed from one flavor to another. The change from one type to another also implies that neutrinos have a finite mass, which is outside of the Standard Model of particle physics. This means that you have to extend the Standard Model beyond what we have so far, perhaps even going back to things that are of relevance for how the universe evolved in the early days. So that was the significant contribution that we made, and the reason the Nobel Prize was awarded.

The Standard Model has been the definitive source of truth for a long time.

The Standard Model of particle physics has been absolutely remarkable. The discovery of the Higgs particle was predicted from the mathematics that underlie the set of particles that make up the Standard Model. It was predicted to be in the range where it was discovered – 125 giga-electron volt mass – which was an absolutely remarkable confirmation that the Standard Model works, except that the Higgs particle is not the origin of mass for neutrinos. It’s the origin of mass for all of the other particles in the Standard Model.

It isn’t every day that the scientific community is presented with evidence that changes the Standard Model.

You might think that scientists say, “Oh my, our model is broken.” Instead, they take the attitude that this is interesting, because we really need to have a more extensive theory of everything. And that’s because the Standard Model that we know has its limitations. Right off the bat, it doesn’t include gravity. It also doesn’t also explain the so-called dark matter particles, which are the object of our current searches, so it isn’t a final, total model.

Neutrinos were long thought to be massless.

It turns out that one of the best models for how neutrinos do get their mass, the so-called seesaw mechanism, is regarded as one of the best contributors to what happened to all of the antimatter in the early universe. We think that the Big Bang was energy being converted into equal amounts of matter and antimatter, but all of the antimatter has decayed away. It is thought that there are other massive neutrinos that are at the highest mass that you can imagine. They are thought to be involved when the energies in the original Big Bang were high enough to have them participating in the processes that would’ve led to all antimatter decaying away. So, in many respects, neutrino physics has become an extremely big topic now. In fact, Fermilab, the highest accelerator laboratory United States, is basically doing almost all neutrino physics.

Where is this new physics taking us?

One of the things that scientists are still attempting to determine is that question of how antimatter decayed away in the early universe. We, at our underground laboratories at SNOLAB, which is an outgrowth of the Sudbury Neutrino Observatory, are studying what’s called the SNO+ experiment. By replacing the heavy water used in the SNO experiment with liquid scintillator, the detector will be able to study low energy solar neutrinos, geoneutrinos and reactor neutrinos as well as conduct a supernova search. The SNO+ experiment will also add tellurium to search for neutrinoless double beta decay. Neutrinoless double beta decay occurs if there is a neutrino symmetry property, which contributes to the overall theory of how the antimatter decayed away in the early universe. So, neutrino physics has become a very big part of particle physics. We are fortunate in Canada to have a laboratory that enables us to continue to do things that are at the frontier, both for neutrinos and for dark matter studies. That’s why I’m still doing it, and still having fun.

Dark matter is equally elusive.

In addition to the SNO+ experiments studying neutrinoless double beta decay, we have four different experiments studying dark matter at SNOLAB. The experiment that I have been working on is a DEAP experiment, which uses liquid argon as a way of detecting the dark matter particles we know… or at least, if you assume that the phenomena you observe and call dark matter are particles, and in particular, weakly interacting massive particles – WIMPS, as we call them, with a bit of whimsy –  then the way that our galaxy is held together by these dark matter particles implies that we have millions of them going through us right now, with very weak interaction…even weaker than neutrinos. So, you need a large amount of material, and you need a very low radioactivity environment, and we can do both at SNOLAB.

Why liquid argon in the search for dark matter particles?

Liquid argon is an interesting material to use, because, if a dark matter particle hits a nucleus causing it to recoil, then the light comes out in about 10 ns (nanosecond). If it’s typical gamma and beta background radioactivity, then the light comes out over about 10 (microseconds) µs, around 1000 times longer. So, if you digitize each one of the pulses you observe in a detector where you are observing the light, you can compare the short time versus the full time of the pulse and discriminate against background by factors of greater than a billion, which we have been doing.

The experiment that is running now has 3 tons of liquid argon. We have a major international collaboration of over 400 scientists that are working on a 50 ton detector that is due to start in about three years in the Gran Sasso Laboratory near Rome, Italy. Both the CERN laboratory and Fermilab are involved in the work. The decision has been made by the collaboration to push to an even larger detector, on the order of 400 tons. The location of choice for that project will be SNOLAB. So, I’m having fun doing things that I will probably be too old to participate in. Nevertheless, I’m in on the early stages and I’m enjoying it.

In 2015, you shared the Nobel Prize in Physics. That is an immense award, but one you accepted with grace and humility. Please share some of the memories of being nominated, and receiving, this incredible honor.

Well, the phone call came about 5:15 AM. My wife answered the phone and was about to say, “What you mean by calling at this time of day?” [Laughs.]  I picked up the extension and heard the Swedish accents, and at that point I realized what was going on. Each of the committee members congratulated me and then they said not to call anybody, because they wanted the line open for a news conference in the next twenty minutes. So I hung up and gave my wife a hug and said, “Wow, what are we involved in now? What’s next?”

Any fun memories from that phone call?

I have been a fan of the Toronto Maple Leafs for years and still follow them closely. When the committee called me, one of the committee members – the secretary of the committee – happens to be a hockey fan. At the end of the conversation announcing to me that I had won the Nobel Prize, he said to me that the last time we had spoken, we also spoke about hockey as well as physics. I said that I wished that Mats Sundin from Sweden was still the Captain of the Maple Leafs as they were not doing so well at the time. So, after the exchange on the phone that morning, one of the articles written said something like, “Typical Canadian. He learns he wins the Nobel Prize, and all he wants to talk about is hockey.” [Laughs.]

What came next?

An absolute whirlwind. I received 1,500 emails the first day, the media coverage continued until about 5 o’clock that evening, and then the university held a great reception in my honor. And then we had a wonderful week in Stockholm. I managed to arrange for 35 people, including mostly collaborators and their spouses to have the experience with us by sharing the opportunities for events during the week. My wife and I were treated literally, like the royalty we were meeting. Since then I’ve received many invitations to many places in the world, which I’m having to restrict because I’m interested in research again. I’m trying to get back to active participation at the design level in the experiments that I’m working on. I no longer have my own students, because I am doing too much traveling to do justice to what a supervisor should do. On the other hand, I have wonderful opportunities to participate and interact with all of these wonderful students that are in our collaborations. So, I’m trying to make some scientific contributions as well as simply interacting with people and trying to inspire students to do good science.

What activities are you into today?

I like to stay active. At my present age, I try to get moderate exercise on a regular basis. I live in a condo that has an indoor pool and exercise equipment. I try to do that at least three times a week, and I still play a bit of golf. My wife and I have started playing pickleball, which I suppose you could say is a step down from tennis, but it’s fun if your mobility gets a little lower as mine is now.

I have four children, as well as nine grandchildren ranging in age from one to 20. They are a great joy to my wife and myself. Over the years I’ve tried to be a family man to the degree that I can within my scope of activities as a scientist. I was a manager of the downhill ski teams that my children participated in. I was a Cub Scout leader and helped my children  build 9 wooden Cub Scout cars to race on tracks over the years. So, we’ve had a very close family situation for many years, and I have enjoyed it immensely.

Final Question: You’ve achieved great success in your life. If you could offer one piece of advice to aspiring scientists, what would that be?

How about two, two-word statements. For a scientist, I think they are both important. One is: Be nice. And the second is: Be curious. I think in terms of success in life and having a happy existence, the first one is very important. As a scientist, you should never lose your curiosity, and that will enable you to have a great career.