'We may have tracked one of the sources of the gravitational waves'

Earth is not big enough for the apparatus required to sense gravitational waves of the kind scientists announced having detected on June 28. In fact, the solar system is not big enough for it. Scientists overcame the problem by turning the Milky Way galaxy into an observatory

- ROHINI KRISHNAMURTHY

Gravitational waves-ripples in space that stretch and squeeze everything in the universe were detected first in 2015 and have been noted about 100 times since then. But the gravitational waves detected were only of high frequencies and short wavelengths, indicating the relatively minor events that caused them, such as collisions of small black holes or dead stars. The latest detection is of gravitational waves of low frequency and long wavelength (running into lightyears), which indicates that their sources are perhaps some of the universe's biggest objects-supermassive black holes billions of times the mass of the Sun.

Scientists tracked these massive gravitational waves by monitoring for 15 years an array of pulsars-dead stars that emit radio waves as they spin with such exceptional regularity that scientists know when the radio waves are supposed to arrive on Earth. As gravitational waves warp space, the distance between these pulsars and the Earth changes, altering the arrival timing of the radio waves. By tracking the pulsars, scientists could infer that they were hit by gravitational waves. This method did not reveal which black hole collisions or exploding supernovae caused them. But it did indicate that they were coming from not one but many such events, a kind of perpetual background "hum" going on in the universe. The findings could mean that there are more (or larger) of such gargantuan objects and events than scientists had thought of, or maybe there are sources of gravitational waves that remain unknown. India's Giant Metrewave Radio Telescope in Pune was one of the world's six telescopes used in the study. GOPAKUMAR ACHAMVEEDU, professor of astronomy and astrophysics at Mumbai's Tata Institute of Fundamental Research, and one of the researchers involved in the international collaboration, talks to ROHINI KRISHNAMURTHY about the findings. Excerpts: