Expert Dialogue: Gravitational Wave Research
A PhD student and a veteran researcher at LIGO discuss the past, present, and future of their field.
Bangalore Sathyaprakash is a researcher and professor at Cardiff University’s School of Physics and Astronomy. He has been involved in the field of gravitational wave research for 27 years and is a member of the LIGO Governing Council.
Maximiliano Isi is a PhD student at the California Institute of Technology’s Division of Physics, Mathematics, and Astronomy and is part of the LIGO Data Analysis Group.
Maximiliano Isi: First, thanks for this chance to talk to one of the leaders in gravitational wave research at a very exciting time for the field. The detection announced in February marks the culmination of over 30 years of work by many extremely talented people all over the world, and you’re one of the few who have been around since the beginning. You've been doing gravitational wave research for nearly three decades. How did you first get involved?
Bangalore Sathyaprakash: My PhD was very theoretical—quite esoteric and not applicable to experimental work. So I was looking for opportunities to work in other fields and became interested in cosmology and gravitational waves. My involvement in gravitational wave research was almost accidental. There was a professor at the institute where I was working in India who came back from sabbatical and asked me, “How would you like to go to Britain and spend three months working on some data analysis?” At that time, I didn’t know anything about programming. I used to be a pencil and paper person. But I took on the challenge, came to Britain, and I started working on the analysis of the first ever coincident data between two interferometer detectors.
Isi: That’s very encouraging, because I’ve been wondering about the transition from theory to experiment myself. I’m reassured that it went well for you! I know you’ve worked in a lot of areas over the years, but I wonder what has kept you interested in the gravitational wave field in particular?
Sathyaprakash: I think the draw is that the problem is much, much more challenging than anything else you could imagine. It’s multi-dimensional, and it was a field where there were very few people at that time. It was an area where I found a niche. So that’s what kept me going: one could influence the field from the beginning and create a paradigm for the way analysis was being done.
Isi: Did you ever imagine when you started that the field would grow to the scale that it has?
Sathyaprakash: Not at all. At that time, the field basically did not exist. Both LIGO and Virgo collaborations were run by just a few labs. There was no broader external participation. It was Barry Barish who brought people together. He understood that LIGO data analysis could not be done by just a few labs. It needed to involve broader collaboration, both for the sake of the experiment itself in terms of research and development going forward, but also to build a community that could deploy the data to advance science. That great vision is what helped us in come together and without it, I can’t imagine that the field would be where it is today.
“Everyone was hugging, and some of my students were in tears”
Isi: It’s interesting that the decision to expand the scale of the effort was a conscious one, that at some point there was a moment where the group decided it needed to organize itself better. After seeing the results of that decision, after watching the field grow from the beginning, what has last September’s discovery meant for you?
Sathyaprakash: First of all, it has not always been a very smooth ride. There have been lots of difficulties and frustration as well as joy. There were funding setbacks. There were delays. But finally we got Advanced LIGO running, and we made the detection in the very first week. Even now, I don’t think I have realized the full impact of this discovery. When it happened, we were all busy trying to verify. Was it really a detection? Or a malicious injection? Or was it an artifact, or something going on with the detector? We weren’t sure. Even at the press conference it hadn’t really sunk in for me yet—there was too much going on. We held a colloquium at our department the week after the press conference and had a party afterward. Everyone was hugging, and some of my students were in tears. They’ve grown with me. Some of them have become professors in the department, and they were saying what a great achievement, and so on. It’s then that I really realized. That was a moment of joy. This discovery has helped us verify many things that we predicted in the collaboration. It used a method on which I wrote a paper in 1991—24 years ago—so that was fantastic for me personally. But we also made predictions in 1996 about how general relativity could be tested that played out in this observation. Seeing work pay off over such a long period has been immensely satisfying for many people—not just me.
Isi: We knew that this discovery was going to be picked up by the press, but for me at least, the magnitude of the reception and the huge interest were extremely positively overwhelming. What’s it been like for you to see the field suddenly become “mainstream?”
Sathyaprakash: The press coverage is actually something that completely baffles me as well. Before the discovery, when I gave talks at astronomical societies, they used to ask, “When are you going to discover something? How are you going to tell us?” I used to say, “Look, when we make a detection, you will know. There’s no way this is not going to be news.” But even I never imagined it would be covered in the way it was. I know people who have said it’s the greatest discovery since Galileo Galilei. This is only the second fundamental wave that we actually have detected in a laboratory. Can you imagine? It’s mind bogglingly fundamental. We have opened up a new window not just for astrophysics—because people would like to say “hey, this is a new observational window for astronomy”—but also for fundamental physics. And cosmology. We have opened up a lab for all three.
“That’s where the holy grail of our field is, which is to observe the very early moments of the big bang. And it could happen in any window.”
Isi: Yes, we wear many different hats! It’s actually the ramifications for fundamental physics that are the most exciting for me. On the one hand, this discovery is the culmination of three decades of work, but on the other it also marks the beginning of a completely new field. We want to keep improving sensitivity, pushing the science further. Where do you see the field in the next five, ten years?
Sathyaprakash: I think we should think not just in terms of five to ten years, but 20 or 30 years ahead. In the next five years, we will see more and more binary black holes, probably binary neutron stars, and so on. To do more in the area of fundamental physics, we need to make very precise, uncontaminated measurements. And that’s when we can probe very deep into the interiors of neutron stars, or things that happen near black holes.
It’s an alternative tool to classical astronomy. Just as telescopes have been continuously improved on over time—the way the receivers work, the way their back-end works—Advanced LIGO and Einstein Telescope will become facilities with increasingly sensitive detectors. That’s the future I imagine for ground-based and underground detectors. Of course, space-based interferometers would be wonderful. And then there’s the Laser Interferometer Space Antenna (LISA) that could observe supermassive black holes. I think it would be fantastic if we start detecting gravitational waves at even lower frequencies with pulsar timing arrays and signature of primordial gravitational waves in the polarization pattern of the cosmic microwave background. That’s where the holy grail of our field is, which is to observe the very early moments of the big bang. And it could happen in any window. It could happen in CMB polarization, pulsar timing array, ground-based detectors, or LISA. That would be the ultimate goal.
Isi: And hopefully, this recent discovery will encourage further support for all these future and present projects. It confirms the investment is worth it!
Sathyaprakash: At the moment, everyone is excited. I don’t know how long that will go on, but at the moment, yes. They are talking about expanding the group and bringing on new people.
“Always take a step back and ask, is it worth doing?”
Sathyaprakash: I have a question for you as a young member of the project. Our collaboration is very big. When we started, everybody got visibility, but how is it now for younger researchers like you? Do you feel that you are getting enough recognition? That you can find a niche and work on problems that are interesting to other people? How do you feel the collaboration is treating its younger colleagues?
Isi: There are definitely challenges to working with so many people, but I think that’s just an inherent, unavoidable aspect of just such a big collaboration. Bureaucracy and some conflict are unavoidable. That said, everyone is fundamentally interested in making the science happen. Everyone is willing to help and get the work done, so in that respect it’s welcoming. Though I can’t speak for every young person in the collaboration, my experience has been very positive, even if there are frustrating moments—especially in big groups, where there are a lot of opinions and a young person might get lost. But the mentorship, which is fundamentally important, is definitely there. People like you, who put effort into educating and cultivating the next generation, are extremely valuable. In terms of showcasing our work, I actually think I get more attention than I deserve.
Sathyaprakash: That’s good to hear! But I think we have to be mindful of the fact that most of the work actually gets done by PhD students and postdocs. We older guys did our work 20-30 years ago. Now we’re just bossing others around! And we have to be mindful of your young colleagues and provide them with opportunities. So, do you feel that you are heard by senior people within the collaboration?
Isi: I do. Though of course, everyone is different. I think some senior people are more interested in listening to younger generations and giving opportunities than others. In the group here at Caltech, it’s very easy to get guidance. I don’t feel alone or exploited. In the spirit of your question, I wonder if you could speak a bit now about advice you have for young PhD students, young scientists like me starting their career?
Sathyaprakash: Oh, what advice can I give you? You guys know a lot more than I do! Looking back on my own career, I wish I’d more often asked the question: is this problem I’m working on worth investigating? If I do this, will the world be any better? If I don’t, will it be any worse? Not in terms of any direct material benefit, but intellectual benefit. Can I make the world intellectually richer by gaining a better understanding of the universe? It’s always good to take one step back. I’m not talking about the technical problems, but about the actual crux of the research problem. Always take a step back and ask, is it worth doing?
“Don’t just talk to your supervisor.”
Isi: Especially for young students just getting into the field, that’s not so easy to do; you can’t always tell. So that’s where guidance and mentorship also play a crucial role, to give you that perspective.
Sathyaprakash: Absolutely. If you can’t answer that question yourself, knock on the door of a postdoc next door. Don’t just talk to your supervisor. Because supervisors have ulterior motives; they want to get things done. Talk to your friendly neighbor who has absolutely no interest in your work. They can give you the best advice on whether a problem sounds like a worthy PhD problem, or how they would go about investigating it. I completely agree with you, it’s not easy for a young person to know whether a PhD problem is worthy, so it’s always good to seek outside input.
Isi: Ask for the opinion of a third party—I’m writing this down! It’s very valuable advice. Thank you again for taking the time to chat. It’s been great to hear your perspectives.
Sathyaprakash: Thank you as well. It’s been my pleasure.
Featured image courtesy of LIGO Caltech.