Interview with Arthur B. McDonald on 6 December 2015, during the Nobel Week in Stockholm, Sweden.

What’s your story? What brought you to science?

Arthur B. McDonald: What brought me to science started back in high school when I had an interest in science, but really a more substantial interest in mathematics which was generated by a really excellent teacher who went out of his way to deal with us outside class and really get us fascinated by the whole subject of mathematics. When I went to university I was inspired by another physics teacher and I went to university thinking I wanted to do science and, with physics, the whole idea that you really could use mathematics to calculate how things work in detail intrigued me. I found out I was good at it, so what brought me to it was a combination of the fact that it was fascinating and that I seemed to be able to do it so I continued with physics through a masters degree and then a PhD and then my career. I have continued to feel exactly the same way, I am fascinated by it, I am still fascinated by the fact that you can do calculations that actually show how the universe works in great detail from the most microscopic to the largest scales. It is just great fun to have the opportunity to work in that sort of area.

Describe your Nobel Prize-awarded work in 1 minute.

Arthur B. McDonald: What we received the prize for was determining brand new properties of neutrinos, fundamental particles that along with electrons and quarks we can’t subdivide any further and in the standard model of physics were thought to have zero mass and not change from one of the three types into another. We use neutrinos from the sun with a measurement using a material called heavy water that enabled us not only to measure the number of neutrinos of the type produced in the sun, the electron neutrinos, but also the sum of all neutrino types. If you compare those two you can tell whether or not there has been a change. We saw only one third surviving of the electron neutrino types showing that they change and that they therefore have a finite mass.

What’s the toughest challenge you’ve faced? How did you overcome it?

Arthur B. McDonald: In 1989 I was presented with the challenge of being the director of an experiment to measure neutrinos from the sun using an enormous detector, the size of a 10 story building, two kilometers underground in an active nickel mine where they were simultaneously taking about five thousand tons of ore out of the other side of the same shaft. A very small shaft, about three meters by three meters by four meters into which we had to parcel all of the roughly one million parts of our detector. We had to do it in an ultra-clean environment because we were seeking to observe one neutrino an hour and we had to restrict all of the remaining parts to have radioactivity levels that did not interfere with this. We had created an ultra-clean environment, that environment was equivalent of what is created for producing semiconductor chips in a factory, in this enormously dusty environment. We also were able to borrow three hundred million dollars’ worth of a material called heavy water, which is about one in seven thousand water molecules have an extra neutron in the hydrogen nucleus, it was that neutron that made our experiment possible.

It was an enormous engineering task, you had to combine science and engineering to get the best of both in the overall process. You had hundreds of people working together in order for this to happen. I guess the biggest challenge was to try to get the tremendous creativity that each individual had working in a co-operative manner so that you were able to accomplish something that was truly unique and had not been done before in many of its different elements. So it was really a combination of making sure you didn’t compromise your scientific principles at any point, but practicing the art of the possible and dealing with the human element of such a large group of varied individuals and make sure that you get the best of all those talents applied to a successful project. So that was a challenge, and how did I accomplish it? I just had to deal with one day at a time and the questions that you came up with. But the biggest thing was having a tremendous group of very accomplished collaborators.

What motivated you to pursue your research?

Arthur B. McDonald: I was a member of the original sixteen people who came together to start this project. We were inspired by a wonderful scientist named Herb Chen from the University of California at Irvine and a scientist from Canada named George Ewan. Herb Chen said You know if we had enough heavy water we could potentially solve a real problem for neutrinos from the sun in which it was observed that too few were being measured compared to what was calculated. One of the possible explanations of that was that neutrinos were changing from one type to the other and escaping previous experiments and that could be rectified by the use of heavy water. So it was a tremendous scientific concept that you could really answer once and for all a basic question in which if it were true you would be dealing with physics that goes beyond the very basic standard model and brings in new properties of neutrinos requiring a change to the most fundamental laws of physics.

We were all inspired by that idea that we could really go beyond the known because neutrinos have a very big influence in our universe. They affect how the universe evolves, the mass of neutrinos affects how stars and galaxies are formed. Neutrinos form a big part of supernova explosions where all of the elements greater than iron in mass are formed. We also knew that we could answer very fundamental questions about the sun if we were able to make these measurements. So the physics was very compelling, we as a group went on to add a number of other institutions, we ended up altogether with about 270 physics authors on our papers and that really indicates the number of people we were able to educate in the process and that was the other thing we thought was great. This a wonderful educational opportunity if we could possibly carry it off and so that is what motivated us. So, it was a very fundamental question to be answered.

What questions remain to be answered in your research?

Arthur B. McDonald: There are number of very interesting questions remaining to be answered. We know now that neutrinos do change from one type to another and that means that they have a finite mass. We don’t know how large that mass is although we have some limits. Fortunately, we have an opportunity right next door in our laboratory in the same underground location with ultra clean environment, two kilometers underground to be able to address a number of these questions and we are doing so. One of the questions is this question of neutrino mass and also symmetries of the neutrino relating to the relationship between antimatter and matter, between neutrinos and antineutrinos. That may be an explanation for why it is in the early universe all of the antimatter decayed away leaving us with a matter dominated universe. Both of those questions, that and the absolute mass of neutrinos, can be answered by studying particular elements, in our case tellurium, that can undergo a process in which two neutrinos potentially could be emitted, it’s called double beta decay. By studying that very rare radioactivity in a reconstituted Sudbury Neutrino Observatory, with a so-called liquid scintillator in the middle with tellurium in, it we can address this question.

In other areas of our laboratory we are addressing another very big question which is what are the dark matter particles that in many ways behave similarly to neutrinos. They are weakly interacting, they pass through material very easily, but we know they have to be much more massive than the neutrinos that we measured from the sun or the atmosphere and those particles we think go well beyond our current knowledge of particles, they are a realm that we’d like to explore by observing the ones that are left over from the original Big Bang, because we feel that perhaps there was enough energy then to produce them and there is good evidence for them in our galaxy. People at CERN are attempting to produce them for the first time since the Big Bang so there is a lot of co-operative effort there. But we feel with about four or five different approaches to dark matter detection in our laboratory we have an excellent chance to perhaps observe them for the first time.

Where do you do your best thinking?

Arthur B. McDonald: These days if I have a question that I really need to address in detail what I do is to sit with my computer which through google gives me access basically to the information that currently exists in the world. I used to do this in a library. In other words, I need to know what is already known on a topic that I am attempting to consider. I then write what I’m attempting to figure out in a very organised way. When I come to a given point in the problem I then look at what is already known about it and learn as much I can, attempt to work my way through the physics of it using my own knowledge of physics and my hands-on background in terms of how one does experiments. And in that way I simply organise myself over a period of hours, typically sitting by myself in a room in front of my computer attempting to come to an answer for the particular thing that I’m attempting to address. On the other hand, there are instances in which I wake up at three o’clock in the morning and all of a sudden I have the answer to something that I have been trying to figure out. That’s happened to me a number of times and I try to write it down and then get back to sleep. Sometimes I can’t get back to sleep and I end up sitting up and writing what has suddenly occurred to me. But I do find that every now and again there is an inspiration that comes in the middle of the night.

Have you had a eureka moment?

Arthur B. McDonald: We are very fortunate in our experiment to have a very specific eureka moment because, and we have had several of them actually in our experiment, because we had several phases in which we approached the measurements that we were doing with different techniques. But in each case we worked out a way such that we were blinding, as we say, the people that are analysing the data. In other words, we didn’t want people to start with a preconceived notion of what the answer is for neutrinos where you are trying to make sure that you are seeing neutrinos in this very complex detector and not something else that is similar to them, but really is a radioactive background or something like that. So we introduced, we did several things, we only analysed part of the data in order to define all the parameters in which we were going to approach our final analysis. Or in some cases we actually added in extra data that looked like what we were looking for, but only one person in the collaboration knew how much of that had been added in. So that nobody really knew what the answer was until on one day all of a sudden, we removed the blindness.

Then everyone in the collaboration all at once knew and this happened particularly in 2002 for our major paper. Everyone all at once knew that we had an answer and not only was that an answer that said that neutrinos had changed from one type to another, but it had what we referred to as a five standard deviation uncertainty in it, meaning that there is less than a chance in ten million that this is not what you see as the result that neutrinos do change from one type to another and so yes, there was a collaboration-wide eureka moment. At any given point, I think on that particular paper there were 176 authors and so the majority of them had the opportunity to say eureka all at the same time. In some cases, students who had signed up for their PhD thesis on this and when you get a eureka moment like that as a part of your graduate education, it can be great fun.

What advice would you give yourself at 20 years old?

Arthur B. McDonald: I guess there is partly a question whether I am meeting myself as a twenty-year-old student when I was actually twenty years old or whether I was presuming that today I am twenty years old and in the present circumstances I will give some advice. I will take the second of them and the advice I would give to that person is both general and it is specific. The specific will be to pursue the field in which I have worked because there’s still many things still to be learned. I will speak about that in a moment. The general would be train yourself at age twenty as broadly as you can, train yourself in the basic science that you are attempting to pursue. Read all those books, understand all those equations, work out all those examples and the problems that your professor gives you. That basic information is going to be essential for you to have overall the ability to be an accomplished researcher in this field. But in my case for experimental physics, learn how to be hands-on, learn how to work a lathe to build a piece of equipment, get into the machine shop and actually physically construct things, because then you will learn what is possible and what is not possible, everything from the most microscopic to the largest piece of equipment and this goes in to computers and electronics and so on.

Train yourself technically in order that you can be confident in all of these things. Don’t neglect the interpersonal elements of collaboration in science. Both in terms of the person next door and in terms of international collaborators. You have to develop a certain amount of what these days has come to be known as emotional intelligence as well as analytic intelligence in order for you to be able to gain the most benefit from an extremely skilled set of people out there in the world. If you can choose very good collaborators in your work, you are going to end up, all of you, in the collaboration able to accomplish very substantial things and so make sure you pay attention to how to get along with other people, how to understand what their motivation is for what they are doing and how to assess their skills such that you can have a complementary group of people accomplishing what you are trying to accomplish.

And I would say to you to consider the field in which I have been working, it is called particle astrophysics. It is a way of understanding particles that in many cases come from astronomical sources. It is a way in which we attempt to understand the most basic laws of physics. The ones that were defining things back in the very beginnings of the universe when the environment was very different than it is today. There are many very important questions still to be answered, we don’t know what dark matter is, we don’t know what dark energy is. We know it is there, it influences how our universe has evolved. Dark matter is accessible in a number of different ways, very important experiments going on to address it either by observing it directly or inferring its presence in both accelerator experiments and also astronomical measurements. It is a very big question because there’s five times as many dark matter particles as there are the particles that make us up in ordinary matter. And so a very fundamental question that’s I think addressable in a not too distant future so I would certainly recommend that type of study to people today. We have a universe that is becoming understood much more in the last twenty years or so than it ever has before. I would recommend that they become a part of that exercise.

Would I like to have obtained the advice I just stated when I was twenty years old? I think actually I did get that advice. I had the benefit of excellent teachers who approached things in just the way that I described. When I talk about learning how a machine works in the machine shop, actually all of us as graduate students at Caltech had to take a course in the machine shop from a truly excellent machinist. With respect to getting along with other people, I learned that from my parents when I was very young and also from my professors. In terms of studying hard and trying to understand things broadly that’s certainly also advice that I got at that time. So yes, I accepted all of that and I think I would accept it again

What does intelligence mean to you?

Arthur B. McDonald: Intelligence, I think is the ability to size up a situation and attempt to understand what the essence of the thing you are dealing with is. Which in some instance requires you to be very analytical, requires you to go in to the scientific background that has been developed and in some cases be very creative in order to make that jump to the next level of synthesising all of the information that you have obtained both from past experience of yourself and others and also the information that you see in front of you. That analytic intelligence is something that the scientist can benefit from greatly.

I think there is another kind of intelligence as well which is tended to be called emotional intelligence these days, which is a way of assessing a situation that involves interpersonal relationships as well. And that is something if you are able to understand what’s happening in terms of other people’s approach to a topic, if you understand other people’s motivation, understand where they are coming from on a given situation and able to understand therefore how you and they can work together to obtain a solution to this particular problem that you are confronting or even simply to end up becoming friends and proceeding. That’s something I think is type of intelligence that the people who mastered well have quite a happy life, so I think there is a lot of different kinds of intelligence.

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