In August 1970, in a paper in Nature, relativist C. V. Vishveshwara first published a calculation and plot of the signal that would be given out by a single perturbed black hole. This was the disturbance, or gravitational wave, that would emerge from a black hole when it was hit by a bunch of radiation. It implied that when a black hole is struck, it gives off a sound like a bell, only that it muffles quickly – more like a wooden bell than a silver bell.

When he did this calculation, Vishveshwara was a Research Associate at the Institute of Space Studies at New York. “Those were the times when the concept of the existence of black holes itself was not easily accepted. It took a couple of decades to believe in their existence,” recalls Saraswathi Vishveshwara, spouse of VIshveshwara, who is a professor emeritus at the Molecular Biology Unit of Indian Institute of Science, Bengaluru.

Quasi-normal mode

Many years later, Vishveshwara would tell his then PhD student, Rajesh Kumble Nayak, “You can disturb a black hole with anything and out comes this clean, well-defined signal.” This signal is what is known as the quasi-normal mode. In contrast with a normal mode, which can be a sine wave, which consists of a train of waves of the same amplitude (or height), this wave form looks like a wave of high amplitude followed by a series of waves with diminishing amplitude. Prof Nayak is now the head of Center of Excellence in Space Sciences, Kolkata.

In January 2016, when LIGO’s big discovery – the first detection of the merger of binary black holes was to be announced, Vishveshwara and Prof Saraswathi were invited to IUCAA, Pune, where it was going to be live-streamed. “Although he was aware of LIGO efforts, he was neither actively involved in the experiments nor the numerical [activity] required to detect the signal,” says Prof. Saraswathi. “All the friends and well-wishers wanted to surprise him and were trying to hide what it was about… Soon after the LIGO finding was announced, they stopped the streaming and asked Vishu to say a few words. It was like a dream coming true for him.”

For his PhD thesis, Vishveshwara had been given the problem of calculating the gravitational waves emanating from a black hole merger. Exactly what LIGO and other detectors have observed now.

Halfway through his defense of the thesis, the examiner from the mathematics department raised a query - why should one bother to prove the stability of an object that was impossible to observe and was of doubtful existence in the first place? Visheshwara’s advisor took exception to this and a verbal wrangle followed.

Relevant question

But in Vishveshwara’s mind only one question remained - how do you observe a solitary black hole? To him, the answer was obvious. It had to be through scattering of radiation, provided the black hole left its finger print on the scattered wave. During the next years, as a postdoc, he arrived at the answer to this question.

“We have still not detected the spectrum of quasi-normal modes from these coalescing binary systems that Vishu predicted,” says Bangalore Sathyaprakash, professor at Penn State University, U.S., and a senior member of the LIGO Collaboration. He refers to the fact that LIGO and VIRGO have so far only detected with significance the inspiral and merger portions of gravitational waves from binary black holes but not similarly the quasi-normal modes from the remnant black hole. “But I am confident we will get there within the next 10 years.”