As per media reports, dated August 16, 2020, a paper on the calculation of how a perturbed black hole rings out, by Dr. C.V. Vishveshwara, was first published in the journal Nature, in 1970. This was the gravitational wave, that would emerge from a black hole when it was hit by a bunch of radiation. He concluded that when a black hole is struck, it gives off a sound like a wooden bell and muffles quickly.
Vishveshwara was a Research Associate at the Institute of Space Studies at New York when the concept of the existence of black holes was not easily accepted. A couple of decades later, people started believing in their existence.
Quasi-normal Mode
Vishveshwara would tell one of his PhD students, Rajesh Kumble Nayak that he could disturb a black hole with anything and cause a clean, well-defined signal to come out, known as the quasi-normal mode. On the other hand, a normal mode can be a sine wave, which consists of a train of waves of the same amplitude (or height). It looks like a wave of high amplitude followed by a series of waves with diminishing amplitude.
Vishveshwara and Prof Saraswathi were invited to Inter-University Centre for Astronomy and Astrophysics (IUCAA), Pune, in January 2016, when Laser Interferometer Gravitational-wave Observatory’s (LIGO) big discovery—the first detection of the merger of binary black holes—was to be announced. He was aware of LIGO efforts but was neither actively involved in the experiments nor the numerical (activity) required to detect the signal. Soon after, the LIGO finding was announced, which was like a dream coming true for him.
Vishveshwara’s PhD thesis was on the problem of calculating the gravitational waves, emanating from a black hole merger, exactly what LIGO and other detectors have observed now.
Question of Relevance
Vishveshwara was preoccupied with a single question—how is a solitary black hole observed? The answer was—through a scattering of radiation, provided the black hole left its fingerprint on the scattered wave. And during the next years, he arrived at the answer to this question.
According to Sathyaprakash, Professor at Penn State University, U.S. and a senior member of the LIGO Collaboration admit that they have not detected till now the spectrum of quasi-normal modes from these coalescing binary systems that Vishveshwara predicted. 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.
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