Sixty Indian scientists from across nine Indian institutes are involved in the experiment that led to the discovery of the gravitational wave as predicted by Albert Einstein a century ago.
HT caught up with the leading lights in dull-grey jackets that read ‘Listening to Cosmic Whispers’ at Pune’s Inter University of Astronomy and Astrophysics (IUCAA).
MEET THE SCIENTISTS
As a PhD student in his early thirties in Bangalore, Sanjeev Dhurandhar made a presentation seeking funds for a science project that was unheard in 1988-1989 – to build the world’s biggest100-metre interferometer to detect gravitational waves. A senior astronomer told him he lacked credibility and he didn’t get funding.
Interferometers are investigative tools used in many fields of science and engineering, according to this website. They are called interferometers because they work by merging two or more sources of light to create an interference pattern, which can be measured and analysed; hence “Interfere-ometer”.
Today, Dhurandhar is referred to as the pillar holding up India’s gravitational wave research. “Gravitational waves fascinated me since it was a good combination of both theoretical and experimental aspect. There was no technology then to detect these waves because they are so feeble though they had a lot of energy,” said Dhurandhar. “The project never got the funding.”
Realising that this field of astrophysics would be the next frontier in science, Dhurandhar went on to develop novel algorithms on how to extract gravitational wave signals from the noise created from sources such as black holes, and how to do it with several detectors – all of this was used for the discovery of the gravitational wave.
“I wasn’t sure if the discovery would come through in my lifetime,” said an elated Dhurandhar, who has built a strong data analysis group in IUCAA over 25 years.
Parameswaran Ajith was on a vacation in Kerala in September 2015 when he received a call from his colleagues informing him of a “trigger” in the LIGO detectors that looked like it was from a binary black hole. Ajith cut short his vacation. “The next three weeks were pretty sleepless till we got out the first results. Our preliminary results were out in ten days which then went in for further revision and review,” he recalled.
Just three months before the discovery, the group at ICTS-TIFR had written a paper laying down the method to infer the mass and spin of the black holes that determines the shape of the gravitational wave. “We used simulations to infer the mass and spin of the final black hole mass which went into the paper. But at that time we didn’t imagine we would receive the signal in September,” said Ajith. “It was a coincidence. I am feeling relaxed now.”
When Anand Sengupta received the news of the first gravitational wave, he thought it was an “injection”. “I thought it was a textbook injection to ensure that search groups are doing their job, protocols are in place for due diligence and checks are being done by running various programmes,” laughed Sengupta. “I didn’t believe it at first, and then I was zapped.”
His group worked on matched filtering algorithm to pull out weak signals. “Basically, central to the search analysis pipeline is a part when the signals from two interferometers are compared against each other to make sure they are generated from astrophysical sources,” said Sengupta. “We developed an algorithm to figure out how signals are compared across the detectors.
The group is now working on algorithm that can separate background noise from the true gravitational wave event using machine learning.
Sengupta summed up, “While historic, this is an isolated discovery. We need more detections and also locate sources using other telescopes.”
More than the event, what is even more significant for Archana Pai is the fact that gravitational wave was discovered in the 100th year since Albert Einstein first predicted it. “We did have an idea that detecting gravitational wave needed sensitivity and that would happen in the advanced LIGO. The first detection wasn’t however expected this soon,” said Pai.
Pai added, “It was a surprise, and we started following what the data is trying to tell. The images were quite unbelievable at the first time and we did a series of checks to see if it was worth following.”
Having developed algorithms for detecting gravitational waves from double neutron stars and black holes, Pai’s group tested gravitational wave from the event and found it consistent with the Einstein’s theory of general relativity.
“Detection science is parallel with developing algorithms. Theorists study wave forms, scientists develop algorithms for different kinds of sources, and engineers work on detectors. The quest has been a journey that will continue,” said Pai.
For India, according Pai, it is a new way of looking at the universe and opens gravitational wave astronomy which is different from usual astronomy from a telescope.