Interview: Sanjeev Dhurandhar, a Doyen of Gravitational Wave Physics in India

Sanjeev Dhurandhar is one of the giants of gravitational wave physics research worldwide, and has been leading the Indian research and experimental efforts in the area for three decades

Gravitational. Credit: Ita Mehrotra

Credit: Ita Mehrotra

Sanjeev Dhurandhar is a physicist, one of the giants of gravitational wave physics research worldwide. He has been leading the Indian research and experimental efforts in the area for the last three decades. Dhurandhar works at the Inter-University Centre for Astronomy and Astrophysics (IUCAA), Pune, on problems in general relativity and blackhole thermodynamics. He played an important role in the first direct discovery of gravitational waves, announced in February 2016, by the Laser Interferometer Gravitational-wave Observatories (LIGO), as well as helped ensure India will get a LIGO of its own in the mid-2020s. Scharada Dubey caught up with him for an interview on all these developments and more.

Tell us something about your work on gravitational waves and where it fits into the present findings.

I have been working on gravitational waves since 1987. I got interested because the detector aspects involved both general relativity and the engineering aspects as well, since I was involved in some theoretical antenna problems with the radio telescope. Where my work fits into the present is in the extraction of the gravitational signal from the noise. There is a vast volume of data being received, and the gravitational signal itself is weak, so sophisticated mathematical and statistical techniques have to be designed to extract the signal from noise.

In 1991, my then colleague [B.S. Sathyaprakash] and I were working on the data analysis of gravitational waves and we set up a basic method for the extraction of the inspiraling binary signal from detector data. Some of the mathematical techniques have stood the test of time. In the first detection of September 2015, which led to the announcement being made in February 2016, as well as the second event in June 2016, where the signal was weaker as it involved a smaller pair of black holes, the basic technique we set up had to be used.  The feat achieved by modern technology is amazing – in order to detect a [gravitational wave], it is necessary to measure an effective length of a thousandth or a ten-thousandth of a proton, for instance, and this has been shown to be possible. My contribution can be considered important: of devising a method for the extraction of the gravitational signal from compact inspiraling binaries from detector noise. More updated versions of the method are still being used.

Some reports after the announcement of the observation of gravitational waves in February 2016 spoke of your work and how you had worked in relative isolation in the early years…

Well, no one then believed that the waves could be detected, even though it had been predicted with [Albert] Einstein’s general theory of relativity in 1916. It was relatively isolated work, but I was never isolated at IUCAA. As soon as I joined IUCAA, I was encouraged and supported by Prof. [Jayant] Narlikar to build my own team. In fact, he and his PhD guide Fred Hoyle were those who believed very strongly in academic freedom, and supporting such efforts. It is rumoured that Fred Hoyle would say, “Do a hundred completely crazy things. Even if one of them succeeds, it would have been worth it.” I drew encouragement from this.

Do the mathematical equations involved in gravitational wave astronomy represent a departure from more traditional areas?

Although gravitational waves are a part of Einstein’s predictions, they do represent a difference from the way things have been studied in the electromagnetic spectrum.  Even from Einstein’s own work, the equations used to describe the merger of black holes are very complicated. They are not only non-linear equations, they are also coupled, partial differential equations – difficult to solve. In fact, in blackhole mergers, there are parts for which the calculations can be done with linear equations, the propagates for instance. In the beginning of the inspiral, analytical methods are possible, but very very difficult to source. Non-linear equations must be solved. For the latter part – the merger – we need to solve the equations numerically on powerful computers.

Sounds to me like a leap of imagination is required, apart from just number-crunching!    

Yes, very much so.

Are you one of the scientists who have won the Special Breakthrough Prize in Fundamental Physics? What do you think of this award?

The Special Breakthrough Prize [comes with] two-and-a-half times the amount of the Nobel Prize. It has been instituted by Yuri Milner and is given to scientists who have made a significant breakthrough. The good thing is that it can be given at any stage of one’s career – if you have managed to make a breakthrough even at a older age, you are eligible for the prize. This year, after the detection was announced in the centenary year of Einstein’s theory of relativity, in April 2016, the Special Breakthrough Prize was awarded. The whole amount of three million dollars is distributed among the team of scientists who have made the discovery. One million dollars is for the LIGO founders Ronald W. P. Drever, Kip S. Thorne and Rainer Weiss. The remaining two million is being given to over 1,000 collaborating scientists, of which I am one. There are about  37 of them from India, I think, and eight are from IUCAA. Several of them have been my PhD students. I think this is an important award in terms of the validation and encouragement it gives to one’s work. We need more such awards to support the sciences. In fact, closer home, the Vijnan Bhushan H.K. Firodia award will be conferred on me later this year.

Kip Thorne, in a recent interview featured in The Wire, spoke about community and continuity in scientific work and the importance of communication. What are your views on these?

There is a big community now for work in gravitational waves astronomy. Many of my students have gone to other universities and centres. Bala Iyer had created another team at the Raman Institute in Bangalore, whose members moved out similarly in many directions. In fact, now I can think of so many, like Rajesh Nayak at IISER Kolkata, or Archana Pai at IISER Trivandrum. Bala Iyer’s student Arun K.G. is at the Mathematical Institute in Chennai, R.R. Sengupta is at IIT, Gandhinagar. All of them have become hubs for further work in this field, and it is a growing community.

In 1989-1990, I had proposed a detector for gravitational waves in India in collaboration with the Centre for Advanced Technology, Indore (now RRCAT). The then director, [Dilip Devidas] Bhawalkar, was very interested in the proposal. In fact, the enthusiasm I had over twenty years ago has remained and continues to be felt by others in the field. In terms of communication, we scientists need to work on that, but at IUCAA we have had the benefit of working with someone like Prof. Narlikar, who is a master in communicating our ideas and discoveries with proper analogies. In fact, IUCAA has an active Science Popularisation Centre, and recently, in a public forum, Prof. Narlikar described the detection of gravitational waves of being similar in difficulty as detecting a tiny flea sitting on an elephant.

How did the IndIGO consortium come about? Tell us something about the plans for an Indian detector.

It was during the 20th anniversary of IUCAA, when we were meeting here in 2009, that Bala Iyer, myself and some experimental scientists got together and decided to press ahead for a proposal to build a detector on Indian soil. So the IndIGO consortium was essentially formed to foster [gravitational wave] astronomy in India and build a detector on Indian soil. LIGO-India is the name given to this detector, which will be built in collaboration with US. Currently, the sites are being considered. My colleague Tarun Souradeep is chairman of the site selection committee and more equipped to provide updates on that. The LIGO-India has been approved in principle.

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How will the third detector being located in India impact the detection results overall?

The main improvement will be in being able to localise the source [of gravitational waves]. With the two present detectors [two LIGO detectors based in the US each with two four-kilometre long detector ‘arms’], and the Virgo three-km based in Italy, a detector in India will provide a baseline almost the diameter of Earth. Because we are placed at an almost opposite side of Earth from the US detector, for example, we get a baseline of 39 milliseconds [multiplied by the speed of light], whereas the diameter of Earth is 41 or 42 milliseconds. So it is close to Earth’s diameter. But with the third detector, the localisation of the source will be much improved. Also, the sky coverage will be better because we will have a different orientation. While we were able to localise the first detection to 600 square degrees with the two LIGO detectors – with a third detector in India operating, the same source would have been localised to between 2 and 7 degrees: an improvement of a factor of 100.

Also, two detectors are not enough. The binary black holes have an orbit in a particular plane which cannot be detected with two detectors. With three or more detectors having different polarisations or orientations, we can solve for the orientation of the source as well. Overall, having a network of detectors means improvement in terms of sky coverage, duty cycle; signal becomes stronger as the signal to noise ratio improves. So the third detector in India will be very significant.

How did the findings of the LISA Pathfinder impact the field of gravitational wave astronomy? And what are your views on the very long-term prospects for this emerging field?

The LISA Pathfinder was actually testing the technology of space-based detectors. There was a surprise there, and luckily it was a good surprise. It was found that what had been theoretically predicted as the range for the Pathfinder – the actual Pathfinder was much, much more sensitive. In fact, with space-based detectors, one can go to frequencies lower than Earth-based detectors. This is comparable to electromagnetic astronomy, where the radio and optical telescopes operate in different frequency windows and bring in complementary information. You see the universe in different wavelengths with the two different instruments. Similarly, LIGO detectors can detect up to a 10 hertz lower limit frequency, whereas space based LISA can go to millihertz or tenth of a hertz. The two can complement each other, just as the radio and optical telescopes do.

As for the long term prospects of gravitational wave astronomy, a new astronomy has opened up, with a whole new window to the universe. Prof. Narlikar compared it to Galileo turning his telescope to the skies for the first time. The possibilities are endless, and it is an exciting time to be an astrophysicist. In fact, the personal satisfaction and excitement for me, having been part of the whole journey, is tremendous. I am sure others feel the same interest and excitement.

What could the future technological applications of this be like?

Work in gravitational wave astronomy essentially pushes technology to the limits. There have to be immense improvements in lasers, vacuum technology for example. For a common man, these could bring in a revolution in technology for the industry. Also, high-performance computers and computing are needed to extract and search for data in this field which will impact the lives of all through advances in computers.

What would be your advice to young scientists and researchers?

Basically, to be a scientist, you have to have an enquiring mind. I would go on to say that be prepared to learn by yourself. There will be institutions, teachers, but whatever you do, you have to do it yourself. Also, you have to add on to knowledge or it becomes stale. I think it was Einstein who gave the analogy about knowledge being a marble statue standing in a desert, subject to erosion, if it is not tended to and polished diligently by other knowledge-seekers. Every time we have the satisfaction of making some discovery in science, it brings additional questions, takes us further. So the key for scientists is to make every effort to add to it, to be thankful for the immense earlier contributions, and to enable others to make more discoveries. Science does not have room for complacency. It is a selfless discipline.