National Science Day| An astrophysicist’s journey from starlight to solving the solar neutrino puzzle

The Sun is right under our noses, so to speak, and when one thinks of astrophysics, one thinks of more distant celestial objects. What made you curious about the Sun?

When I did my Ph.D. many years ago, it wasn’t on the Sun. It was on our galaxy, and how different parts of the galaxy evolve. At that time, there was very little data about the composition of stars and the ages of stars, how far they are, it’s a completely different field now. But at that time, there was nothing and I was quite tired that the data did not allow me to discriminate between models. I had just submitted my thesis, and I wasn’t as enamoured with the field anymore.

After your thesis, what made you lose motivation? Was it the lack of data?

Yes, and I had a slightly rocky relationship with my thesis supervisor, which I learned much later was nothing unusual. I mean, lots of people have rocky relationships with their thesis supervisors. In the Indian context at that time, we didn’t have thesis committees where there are other faculty members you could talk to. So if you had a hard, difficult relationship with your supervisor, then it really made everything else difficult.

So what did you do?

For a long time, I couldn’t figure out what to do. In those days, it took a long time for a submitted thesis to be evaluated. I realised there were a few people in the department at TIFR [Tata Institute of Fundamental Research] who were exploring this completely new field of seismology of the Sun… helioseismology. It seemed very interesting. So I asked them if I could join them and do a project with them. And that was it.

And why physics? Was it by design?

Oh, yes, completely so, because I knew in my third- and fourth-grade that I wanted to do astronomy. And the only way at that time to do astronomy in India, most places actually, was to do physics, to do mathematics.

And what made you fall in love with astronomy in the first place?

I had a fantastic schoolteacher in the third- and fourth-grades, and she would bring us news about the Russian launches to Mars and the American launches to Venus and beyond, such as the Voyager missions. And she would write to the two embassies and get glossy pictures for us to see. That’s what made me interested.

I think having a good teacher makes such a difference. It makes a world of a difference, particularly in the lower grades.

And speaking of your school experience, did you have to pick physics over other subjects?

No, because I did my schooling in India. We had to do everything till class 10. And after class 10, we had to go either into the science stream or the humanities stream. The science stream had physics, chemistry, biology, mathematics. I did my 10th with the CBSE [board], but my 11th and 12th were with the West Bengal board. And then I did my Bachelor’s degree in Chennai.

Did you ever regret going into this field?

Not really, because I was quite focused on what I wanted to do. I did have a fallback plan if I didn’t succeed in getting an academic position after my Ph.D. And that was to appear for UPSC (Union Public Service Commission) exams. Because I was always interested in general affairs and what’s going on [in the world]. So that was my fallback.

When did you go to the U.S.?

So that was much later. After my Ph.D., I was in London for a year, then in Denmark for three years. And then I went to the U.S.

And what was that like?

London felt just like Mumbai, frankly. And Denmark was very different, but it was also much smaller — lovely and peaceful and very safe. The bad part was the language, but if I tried to speak in broken Danish, people would answer me in fluent English, because by then, English was very common in Denmark.

In Denmark, they don’t dub TV programmes or movies into English, they just subtitle them in Danish, unless they’re programmes for very young kids, because they start teaching English, from the age of six or seven. So for kids below that age, they dub the show, and for everybody else, it’s subtitled. So I actually learned to read Danish by looking at the subtitles.

That’s amazing! I’m curious, how real was the gender gap for you? Did you encounter it, and in what ways?

There was always a gender gap, there were so few women around, but I went into the field with my eyes open, knowing that this was the case. I think the first-ever female astrophysicist I met was in the second year of my Ph.D., I’d never met one before that. But I never let that bother me. I had always been prepared for the fact that I might have to carve my own path, instead of depending on female role models.

So being aware of the gender gap made it easier for you to deal with it?

Yes, and to not let it bother me. You have to, of course, command respect. In Denmark, that wasn’t a problem. It was nice and easy and slightly laid-back. When I went to the U.S., though, it was suddenly a very hyper-competitive environment.

So that is where you had to hold your own. Yes, I got my faculty position. And after that, of course, everything was self-driven.

What are the highs and the lows of a career in astronomy and physics?

If you’re speaking in terms of validation, then the highs are when you solve a particularly difficult problem, and the lows would be when you realise — after spending a year or two trying to solve it — that you can’t solve it.

Another high point was when I was offered a position at the Institute for Advanced Study in Princeton, which is when I moved from Denmark to the U.S., because that is where [Albert] Einstein worked. That is where people like [John] von Neumann worked and [Kurt] Gödel worked. That was fantastic, that feeling of, oh my goodness, I’m going to work in such an institute!

And the idea that you’re now joining all those people you’ve only ever read about in books — the very people who inspired you — suddenly makes it feel like a dream.

Yes, exactly!

And what was it like being at the Institute for Advanced Study in Princeton?

The Institute has no students. It has, I think even today, only about six faculty members in various subjects. The rest are postdoc positions, people who have finished their Ph.Ds, but not gotten a faculty job yet. And my particular batch of postdocs were a competitive bunch. Most of them hold high positions in US science now.

What advice would you give to other young girls? And how do you think they can make better use of opportunities? What would you want to tell your past self?

The first thing I would say is, it is your dream. Don’t let people’s comments, and frankly, unasked advice, spoil your dream for you. It is your dream. It is your job to go and follow your dream.

Just because somebody makes a snide remark about your work, or about your place in the sciences, don’t let that derail you. It’s your dream. Why should anybody else be in charge of breaking your dream? Yes, of course, things happen in personal life, which might derail you for a while, but that is different from somebody else trying to spoil your dream for you.

And how did it feel going down this not-so-beaten path? Did you have a lot of support from your family?

My family, somewhat, but I had really good mentors. I chose people to advise me and I wasn’t hung up on the fact that they were men. And the thing with choosing mentors is to make sure your mentor is a reasonably successful person. Otherwise, like in the case of my thesis supervisor, I realise now, after 30 years of being in the field, that he was just insecure. I was his first student and he was very, very insecure. And maybe that’s a part of the problem. So always have multiple mentors. So that you can take advice, find somebody to advise you on your career path, independent of your supervisor. So you want a third-party opinion.

Somebody to advise you on how to navigate personal matters while you’re working or while you’re studying. So multiple mentors is always a good thing. Also have a close circle of colleagues who have similar issues so that you can bounce ideas off each other.

That makes sense. Going back to what you said about one of the highs being able to solve a problem, I’ve read a lot about your work with the solar neutrino problem. What was that experience like?

I was still too inexperienced to realise how important the solution was. I didn’t realise how important it was till I went and talked to my mentor, and he said, “Do you know what you have done?” That’s an amazing feeling.

Would you explain the solar neutrino problem to anyone reading this interview?

So we know that stars shine by nuclear reactions, but how do we know that’s happening in the heart of the star? How do we know it’s nuclear reactions? And this is where a couple of people, John Bahcall and Ray Davis Jr., came together and said, if we could observe neutrinos streaming from the sun, we would know that there are nuclear reactions because the only way neutrinos would be produced is in a nuclear reaction.

So they built an experiment to observe neutrinos — and neutrinos are notoriously difficult to observe! They just don’t interact much with matter. And what they found was the number of neutrinos they observed was one-third of the expected number.

So that was the first solar neutrino problem. They built a few more [experiments], and all of them showed that you only see about a third [of the expected number of neutrinos], but each detector detected neutrinos of a particular energy range.

The second neutrino problem, which is much less known, except for the people who worked on the problem, is that the results of one detector were not consistent with the results from another detector.

There was already this hypothesis that neutrinos are not massless particles, that there are three types of neutrinos: the electron-type, the muon-type, and the tau-type, and that if they didn’t have mass, the electron type would forever remain electron type and the muon type would forever remain muon type. But if they had mass, the electron type could convert to a muon type and vice versa.

Most of these detectors were sensitive mainly to electron-type neutrinos, not to muon-type neutrinos. So another theory at that time said that if neutrinos that have mass had to travel through material, then the conversion from one neutrino type to another would be more efficient.

One reason why we see fewer neutrinos from the Sun is because the electron-type neutrinos convert to muon-type neutrinos [as they travel from the core of the Sun to the Earth]. As our detectors are not sensitive to muon-type neutrinos, we won’t see them. That’s when a completely different neutrino detector was built in Canada, the Sudbury Neutrino Observatory, which can detect both electron-type and muon-type neutrinos.

Okay, so once the problem had been identified, an observatory came up to solve for it.

Yes, and the first result [from the observatory], published in 2001, showed that there’s no solar neutrino problem. [The missing neutrinos are neutrinos that have changed their type.] 

The problem was that the particle physics folks at that time claimed that neutrinos did not have mass, but now this was unequivocal proof that they do.

That’s pretty crazy!

And my role was not in the experiments. I’m a solar physicist and a helioseismologist, so one of the suggested solutions of the solar neutrino problem is that our solar model, the model we think of to explain the structure of the sun, is wrong.

People spent a long time trying to make what we call non-standard models, just to solve the solar neutrino problem. But with seismology, I can figure out exactly what the structure of the sun is. And what we found is that our normal models of the sun are perfectly fine.

You pointed out that the problem was the neutrinos and not the solar model …

Yes, and that took a while because particle physics theory was so well-established that it took their own experiments to actually convince them.

Wow. So that must have definitely been one of the highs of your career.

Yes, that was, particularly after the Sudbury neutrino results came out, it was.

I was just about to ask you, what are you most proud of? Is this it?

Oh, I don’t know. I think figuring out the structure of the sun, its dynamics, I would say those have been the highlights of my career.

My students have been trying to make inroads into the details of modelling stars. What are the physical ingredients that we are missing? I’m still continuing in that line, but in a different way.

And now that you’re a professor yourself, how does it feel to have this full-circle moment? You started out studying with a teacher who inspired you to go into the sciences, and now you get to occupy that role …

I had always considered this as something I must do. It’s a duty rather than a task. Which is one of the reasons I give public lectures, which is one of the reasons I talk to people like you. It’s my way of giving back. My science teacher has passed away, that particular one from my childhood, but if she had been alive, I think I would have gone and thanked her for making me enthusiastic about the subject.

I think it’s amazing, the impact that someone can make even when you’re so young and you don’t realise it at the time.

That is when the fire needs to be built.

Yes, completely. And would you say the gender gap in STEM is improving?

It is. Not as fast as I would like to see it happening. Part of it is the system, though I would also put part of [the onus] on women not being confident enough.

Because of the so-called imposter syndrome, women think they don’t know anything. They don’t realise this syndrome hits everybody. If you’re any good, it’ll hit you. If you’re not good, you’ll think you’re fantastic.

I find too many women not being able to navigate this. And also not negotiating enough with their families. Yes, I’ve had a child—so what? It’s not my child alone. So why should I let it derail my career?

How did you navigate this?

I did it the easy way. I chose not to have kids.

And the imposter syndrome?

I just, you know, internalised it a bit. And then the moment I solved the problem, I said, maybe I can do it. Yes, it’s a cycle. You get a new job, and you have to teach a large class for the first time. And you think, oh, I won’t be able to do it. And then you just do it because you have to do it. And then you realise, no, of course I can do it.

What you’re saying is that self-doubt vanishes when you push yourself to do it anyway.

Exactly. You have to do it anyway. It’s not avoidable. The easiest way to counter the syndrome is to realise that anybody who’s good has at some point in their career, in their lives, experienced this. I think that is probably very comforting as well. To know there’s people who have come before you and will come after you with the same experience. It’s normal.