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For over 100 years, gravitational waves remained just a hypothesis. Now it is a fact. The discovery could open new channels of understanding our universe in myriad ways
A secret whispered 1.3 billion years ago finally reached Earth on September 14, 2015, after traversing over 100 trillion km. As the cosmic who-osh reached the intent ears of hundreds of scientists across the world, a historic discovery was made and a new page in science was turned. Gravitational waves (GWs) were transformed from a hypothesis to a fact on February 11 this year. GWs had suddenly become as real as the paper containing the equations hypothesising their existence. While the historic discovery brought us closer to our understanding of the universe, it also opened a Pandora’s box on a range of possibilities.
In 1915, Albert Einstein theorised the existence of tiny ripples in the space-time fabric of the universe due to objects accelerating in the gravitational field. But Einstein thought these waves would remain in the realm of hypotheses as the waves would be too small for humans to capture and study. For instance, the gravitational wave power radiated by Earth revolving the sun is just 200 Watts. It has taken top scientific minds more than 100 years since Einstein’s suggestion of the general theory of relativity to get an actual glimpse of the “ethereal” ripples.
All that was needed was an apparatus large enough with fine sensitivity to detect an infinitesimally small change. And that is exactly what the Advanced Laser Interfero-meter Gravitational-Wave Observatory (LIGO) near Washington is. Scientists believe the twin detectors installed at two separate locations will unlock the doors to the mysteries of the universe.
In September 2015, LIGO detected waves caused by the merging of two black holes 1.3 billion light years away. “By studying sound and seismic waves, we understand the properties of the world around us. Light and the entire range of the electromagnetic (EM) spectrum are like the second eye. By analysing the light from stars and other cosmic objects, we learn a lot about them. The GWs discovery has given us information about two initial black holes and their subsequent merger into one. Studying GWs is like getting the third eye—with this we can understand objects such as black holes and dark matter, which do not interact with the EM spectrum,” T V Venkateswaran, a scientist with Vigyan Prasar, told Down To Earth. He says the image of the universe has changed into a much more dynamic one, where stars explode and galaxies collide. The study of GWs may explain why there was a sudden enlargement of the universe, called inflation, in the wee moments of the Big Bang. “The universe may be weirder than we ever imagined,” says Venkateswaran.
What are these waves?
In 1915, Einstein shook the scientific dispensation of his time by challenging Newton’s mechanistic clockwork universe. Einstein argued that space and time are not distinct, but are woven into a single space-time fabric. Einstein claimed that gravity too, like light, was not an instantaneous force, but the curvature in the space-time fabric. Just as a charge moving in the EM field produces EM waves, GWs too are described in the form of tiny ripples or waves in the space-time fabric emanating from an accelerating body.
Over the years, all the phenomena predicted by the general theory of relativity, with the exception of these waves, have been experimentally verified. But gravity is such a weak force and the ripples so small that these eluded detection till now. The waves are extremely tiny, just a millionth of an atom in size, but they throw up exciting possibilities in the realm of space science research and the Big Bang theory.
Says Geraint Lewis, professor of astrophysics at the University of Sydney, “Telescopes have been able to observe the universe in (an) optical light, as well as X-rays and radio waves, but GWs reveal a different side of the universe; ultraviolent events revealed by their shaping of space and time themselves.”
To understand how GWs work, consider a tiny pebble dropped in a still lake. The act would certainly cause ripples to form that would move towards the other end, but these would be too small to detect. However, if one throws a big rock, the resulting waves would be detected at the other end of the lake. An asymmetric gravitational system, like the moon and Earth orbiting each other, also produces GWs, but these are too small to detect at current capabilities. Even when massive or super massive bodies disturb the space-time fabric, creating ripples, they are incredibly faint which makes them hard to detect and measure. “The detection of GWs will allow scientists to infer the processes at work that produced the waves,” Venkateswaran adds. These waves generate a “strain”—a fractional change in the length of anything that they travel through.
Alex Nielsen, a researcher at the Max Planck Institute for Gravitational Physics (Albert Einstein Institute), Germany, says that the detection was unexpected. “Scientists had expected the first GWs detections in 2017 or even in 2019.”
So is the discovery significant? According to Nielsen, the fact that the detection happened early implies that these types of signals (merger of black holes) are more common than we expected. This may imply that heavy black holes are more common than we ever thought of, which in turn, would change our understanding of how stars are formed, particularly large ones around 100 times the mass of the sun.
Even after this momentous discovery, we are still a long way from a unified theory of matter and gravitation. “Matter by itself is still well-modelled by the standard model of particle physics and nothing in these LIGO results changes that,” says Nielsen. Varun Bhalerao of the Inter-University Centre for Astronomy and Astrophysics (IUCAA), Pune, who is also a member of the LIGO scientific collaboration, says, “We have a different channel to understand the universe now.” (See ‘We can now track the birth, evolution and death of stars’ and ‘We are enriching our knowledge of the cosmos’.)
|`We can now track the birth, evolution and death of stars'
So this event is already telling us something about how stars form and evolve to produce heavy binary black hole systems.
It was very hard to detect the waves. We had indirect proof before, but this is the first direct proof of their existence. Since events like binary mergers are rare, we have to be able to search a large volume of the universe, and the ones we detect tend to be rather far away. The gravitational-wave strain is very tiny when it reaches Earth. That's why it took many years of research to arrive at detectors which are sensitive enough.
This discovery is one more confirmation of our understanding of mass and gravity through general relativity, but this measurement does not say anything about other interactions of matter.
In the future, we will be able to detect things that aren't seen in other ways. Mergers of binary black holes and binary neutron stars, for instance. We'll also learn about the population and properties of those things in our universe. That will give us more information to test models of birth, evolution and deaths of stars and their galactic environments.
(As told to Down To Earth)
GWs are not electromagnetic waves, so we cannot “see” them with our eyes or with telescopes. “But this is the first time we saw heavy black holes collide. We were ‘blind’ to certain aspects of the universe, but with this discovery will be able to study them in the coming years,” adds Bhalerao.
The detection could lead to the opening up of an entirely new discipline of science—gravitational astronomy that would depend purely on GWs, just as radio waves engendered radio astronomy. Scientists are eager at the prospect of such a development as it open up a new side of the universe, one that is yet to be explored.
According to Sanjeev V Dhurandhar, professor emeritus at IUCAA, “Our current knowledge of the universe is almost entirely obtained from EM waves. EM waves are easily scattered or absorbed. On the other hand, GWs are not easily scattered, so it would give high fidelity information. This difference in their properties implies that we would be able to obtain knowledge which was hitherto not available to us.”
Nielsen says we may be able to see relatively nearby neutron stars (hundreds of light years away) spinning and emitting GWs in the future, which will tell us about the matter that make up these stars. If we are very lucky, we may even see a supernova explosion in the Milky Way and GWs will explain how this happens. Nielsen says that to take maximum scientific benefits of GWs requires a worldwide network with detectors placed widely across Earth, all scanning the sky together and combining their results.
Detectors under construction or planned in Italy and Japan will help greatly. In fact, India has approved a proposal to set up a gravitational wave detector. Scientific groups in Spain, South Korea and Australia too have provided important contributions in modelling expected signals and identifying background disturbances that reduce our sensitivity to GWs. With this discovery, we could be reinventing our knowledge wheel about the universe.
|`We are enriching our knowledge of the cosmos'
We were "blind" to certain aspects of the universe, but now we will be able to study them as LIGO detects more such events. It is also the only way we can test Einstein's general theory of relativity in the "strong gravity" regime. Detection of GWs was one of the final remaining evidence of Einstein's theory of relativity. These waves generate a "strain"—a fractional change in length of anything that they travel through. Measuring this small change in length required one of the most sensitive instruments ever built with the best lasers, vacuum and other technical apparatus.
During the early stages of LIGO, we demonstrated technology, learnt lessons and adapted them to the next generation of instruments. After LIGO's first run, we implemented several tweaks at all levels—from hardware upgrades to better signal processing—to make detections possible.
The detection of GWs is a long-awaited confirmation of Einstein's predictions. However, it does not deal with, say, the quantum nature of matter. This discovery improves our understanding of gravity, but does not give us a grand unified theory just yet.
Further data from the GW detectors will tell us more about black holes, certain types of binary neutron stars and neutron star binaries. This will further our understanding about the formation and death of stars, and enrich our knowledge of our cosmic neighbourhood.
(As told to Down To Earth)