By Parameswaran Ajith
Ten years ago, on September 14, 2015, the Laser Interferometer Gravitational-wave Observatory (LIGO) in the United States observed a tiny disturbance. Further analysis revealed that this was a signal of gravitational waves produced by the collision of two black holes in a distant galaxy.
The existence of gravitational waves was predicted by Albert Einstein a century ago as part of his General Theory of Relativity. According to this theory, gravity is the curvature of spacetime produced by massive objects. Motion of such objects will generate ripples in spacetime that propagate at the speed of light, known as gravitational waves.
The early decades of gravitational-wave research were muddled with confusion. Many eminent scientists, including Einstein, worried whether his prediction was a product of certain mathematical choices. Only by the 1950s did theoretical physicists start to agree that gravitational waves are indeed a physical phenomenon that could be observed in nature.
Around the same time, American physicist Joseph Weber started working on their detection using massive aluminium cylinders. When gravitational waves pass through these bars, they vibrate. Weber claimed that he detected several signals. However, other scientists were unable to reproduce Weber’s results using different detectors, casting doubt on his claims. The future of gravitational-wave detection appeared bleak.
By the 1980s, radio astronomers studying a binary pulsar system brought a new excitement to the story. Pulsars are rapidly rotating neutron stars producing pulses of radio waves, like a lighthouse. Their rotation periods are extremely stable, which makes them excellent clocks. Astronomers showed that the orbital period of this binary system (two pulsars orbiting each other) was changing by a tiny amount, at a rate predicted by Einstein’s theory due to the emission of gravitational waves. This gave a new push to the quest for gravitational waves.
In the 1970s, Rainer Weiss, a young American physicist started thinking about using light to detect gravitational waves — a technique called interferometry. He developed this idea while he was teaching a course on General Relativity. He discussed his ideas with Kip Thorne, a young theoretical physicist. This collaboration eventually gave birth to LIGO, which was later shepherded by particle physicist Barry Barish.
LIGO’s detection of gravitational waves was lauded as one of the most important scientific discoveries of our times. Not surprisingly, Weiss, Thorne and Barish were awarded the 2017 Nobel Prize in Physics for their decisive contributions.
This discovery not only verified the century-old prediction of Einstein’s theory but also opened up a new window to the Universe. Till now, the vast majority of astronomical observations were done using light — electromagnetic waves of different frequencies. Gravitational-wave observations enable us to observe the universe in a new light.
In the past decade, LIGO and its European sister observatory, Virgo, have detected over 200 signals from different black-hole collisions. These events are excellent laboratories for testing General Relativity — a pillar of modern physics. These massive black holes would be dashing with lightning speeds before smashing into each other. These are cosmic-sized versions of particle colliders such as the Large Hadron Collider.
These observations also open up new puzzles for astronomers. It is not clear how these black holes are formed in nature. Some of them are too massive to be formed by the collapse of normal stars. Scientists have speculated that these could be “primordial” black holes, formed soon after the Big Bang, when some ultra-dense regions of the universe imploded under their own gravity.
One of the signals detected by LIGO and Virgo was produced by the collision of two neutron stars. This collision also produced flashes of light that were detected by several telescopes. This “multi-messenger” observation provided a new way of measuring the expansion rate of the universe, and told us how elements such as gold and platinum are formed in the universe.
LIGO detectors measure tiny disturbances in space — smaller than one billionth of the size of an atom. The scientific and technological capabilities required for this are beyond the scope of any single lab or even a country. The LIGO scientific collaboration includes over 1,200 scientists from 18 different countries.
Gravitational-wave astronomy is inherently an international endeavour. A single detector cannot identify the arrival direction of a signal in the sky. Sky localisation is achieved by using data from multiple detectors located at different parts of the world. This is similar to how we localise sound using our two ears. This means that different observatories have to collaborate, rather than compete. LIGO operates jointly with Virgo in Europe and the nascent KAGRA detector in Japan, showcasing a fine example of international cooperation. These detectors will continue to improve their sensitivity over the next decade, observing thousands of new signals.
Indian scientists have made significant contributions to gravitational-wave science over the past three decades. Their work played a major role in enabling LIGO’s discovery. Currently, over 100 scientists from 17 Indian institutions are part of the LIGO-Virgo-KAGRA collaboration.
Perhaps, the most impactful Indian contribution to gravitational-wave astronomy is yet to come. Work is ongoing to build a third LIGO observatory in Maharashtra in collaboration with the US and other international partners.
LIGO-India will significantly improve the ability of the international network of gravitational-wave detectors to localise the signals in the sky. This is due to its favourable geographic location, which is antipodal to the existing detectors in the far north. LIGO-India will make the Indian scientific community a leading player in this research frontier. This project will also give us early access to a host of advanced technologies involving lasers, precision optical systems and quantum metrology.
Indeed, executing a project of this magnitude comes with its share of challenges. However, this is an opportunity that is too good to miss. While celebrating the first decade of gravitational-wave observations, Indian astronomers remain cautiously optimistic.
The writer is an astrophysicist at the International Centre for Theoretical Sciences, Bengaluru. Views are personal
