INTERNATIONAL

Gravitational wave astronomy’s finest moment

The event where the announcement of the detection of gravitational waves was made was transmitted live at the Inter-University Centre for Astronomy and Astrophysics (IUCAA) here. Representatives of the collaborating Indian institutions were present. The announcement was received with thunderous applause here too because it was a proud moment for the Indian gravitational wave community as well.

Groups at IUCAA and the Raman Research Institute (RRI), Bangalore, have made significant contribution in the analysis of the LIGO data, which has enabled it to be pinned down to a coalescence of two black holes consistent with Einstein’s theory. As many as 34 Indian scientists are contributing authors in the landmark paper about the discovery that has been published online in the journal Physical Review Letters .

Although indirect evidence for the existence of gravitational waves had been seen from the decaying orbital period of objects called binary pulsars — which Russel Hulse and Joseph Taylor discovered in 1974 and for which they were awarded the Nobel Prize in 1993 — a direct detection of gravitational waves had till now proved to be extremely difficult. This required enormous advances in technology to enable instruments with sensitivity sufficient to detect distortions of space-time as tiny as 10-18 m, which is a thousandth of the diameter of a proton, and less. That is like measuring the distance between the Earth and the nearest galaxy Andromeda, which is 2.5 million light years away, to hair-width precision.

This is what the upgraded or advanced LIGO, which began its first run only in September 2015, achieved and within days it made this spectacular literally earth-shaking discovery. The gravitational wave signal struck the detector on September 14, 2015, and the signal had the unmistakable stamp of a black-hole binary merger, a phenomenon that has been extensively studied through simulations.

The LIGO is the most precise instrument that has ever been built. It consists of two identical L-shaped laser interferometer systems, one at Hanford in Washington and one at Livingston in Louisiana. There are two systems to ensure that detection at both the instruments that are about 3000 km apart with the calculated time delay ensures that the detected signal is not due to any spurious seismic signal or any other local vibration.

Each of the arms of the L is a 4 km tunnel in which laser beams bounce back and forth between two highly sensitive suspended mirrors. The laser beams are tuned to be perfectly in opposite phase so that there is total interference when the beams arrive at the intersection of the arms and no light passes through the beam splitter at the intersection into the photo-detector behind. But when a gravitational wave passes through the detector, the space-time gets distorted much like a squeezed ball, oscillating between the two states compressed in one direction and elongated in the other. So the effect of this oscillatory compression of one arm and elongation of the other is that there is no total cancellation of the interfering laser beams and a net signal gets through to the photodetector.

According to Gabriela Gonzalez, the chief spokesperson of the LIGO at Livingston, the signal was received precisely 7 miliseconds later as calculated. “The coincidence is remarkable” she said.

The total signal lasted for about 0.4 s with the “ringing down” that is characteristic of two orbiting black holes in-spiralling towards each other, shrinking of the orbit, merger of the two, coalescence and finally settling down as a single black hole, he said.

The data is consistent with one black hole with 36 solar masses merging with another of 29 solar masses giving rise to a single black hole of 62 solar masses. A total energy of 10 49 watts, equivalent to the missing 3 solar masses, has been radiated away as gravitational waves. This would be the most luminous astronomical source ever observed noted P. Ajith of the International Centre for Theoretical Sciences, Bangalore, who is part of LIGO collaboration and was involved in the analysis.

According to him, the probability of it being a false alarm is less than 2x10 -7.

The biggest victory for the Indian gravitational wave astronomy community as a result of Thursday’s discovery has been the in-principle approval from Prime Minister Narendra Modi for setting up of the Indian component of the advanced LIGO, which has been hanging fire for more than three years since the proposal was approved by the National Science Foundation (NSF), U.S.