Scientists spot elusive space-time ripples

Gravitational waves from two black holes reach Earth.

The collision of two black holes holes - a tremendously powerful event detected for the first time ever by the Laser Interferometer Gravitational-Wave Observatory, or LIGO - is seen in this still image from a computer simulation released in Washington February 11, 2016. Photo courtesy: Caltech/MIT/LIGO Laboratory

The highly elusive ‘gravitational waves’ have finally been detected. Understandably, and justifiably, there is great elation within the global physics community, astrophysicists and cosmologists in particular.

After decades of search for these ripples in space-time, which Albert Einstein predicted exactly 100 years ago, scientists working with the gigantic optical instruments in the U.S. called LIGO [Laser Interferometer Gravitational-wave Observatory], have detected signals of gravitational waves emanating from two merging black holes 1.3 billion light years away arriving at their instruments on the Earth. That is to say, this cataclysmic event of two black holes merging occurred 1.3 b yrs ago, when multi-cellular organisms were just beginning to form on the Earth, the gravitational waves from which are being received now on the Earth.

Indeed, “We have detected gravitational waves,” were the opening remarks of David Reitze, the Executive Director of LIGO at Caltech, while making the announcement of the discovery to the media at the National Press Club in Washington that was received with rousing ovation.

(“Piled Higher and Deeper” by Jorge Cham. www.phdcomics.com)

The announcement was beamed across all the laboratories of the world participating in the LIGO Science Collaboration (LSC). LSC comprises about 1000 scientists from 16 countries.

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 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.

Gravitational waves Explained

• What are gravitational waves? Gravitational waves are small ripples in space-time that are believed to travel across the universe at the speed of light. They are like tiny waves on a lake — from far away, the lake’s surface looks glassy smooth; only up very close can the details of the surface be seen. They were predicted to exist by Albert Einstein in 1916 as a consequence of his General Theory of Relativity.
• What does Einstein say about gravity? While Sir Isaac Newton visualised gravitational force as a pulling force between objects, Albert Einstein opined it to be a pushing force due to the curvature of four dimensional spacetime fabric. The curvature of spacetime stems from the dent heavy objects produce on spacetime fabric, which can be compared to the dent one could see on a plastic sheet when a massive ball is placed.
• How are these waves detected?

  Scientists have been trying to detect them using two large laser instruments in the United States, known together as the Laser Interferometer Gravitational-Wave Observatory (LIGO), as well as another in Italy.

  The twin LIGO installations are located roughly 3,000 km apart in Livingston, Louisiana, and Hanford, Washington. Having two detectors is a way to sift out terrestrial rumblings, such as traffic and earthquakes, from the faint ripples of space itself.

  The LIGO work is funded by the National Science Foundation, an independent agency of the U.S. government.
• Why is the study of gravitational waves important? Discovery of gravitational waves would represent a scientific landmark, opening the door to an entirely new way to observe the cosmos and unlock secrets about the early universe and mysterious objects like black holes and neutron stars.
• Did scientists ever detect gravitational waves?

  Although, physics supports the existence of gravitational waves, the strength of such waves even due to astronomically heavy bodies is awfully weak to be detected.

  On March 17, 2014, Harvard-Smithsonian Centre for Astrophysics erroneously claimed discovery of gravitational waves. The Harvard group, working at BICEP2 (Background Imaging of Cosmic Extragalactic Polarisation) telescope, had reported that they had observed a twist in the polarisation of ancient light that goes back to the time of the big bang. But within a month, studies pointed out flaws in the study.