Gravitational Waves And The Move To Multi-Messenger Astronomy, Explained

Gravitational waves are disturbances in the fabric of space-time that are created when certain violent events take place in our universe. There occurs a repeated expansion and contraction of space-time like a rubber band, similar to the kind of disturbance we see in a body of water after a stone is dropped in it.

Gravitational waves were a prediction of Albert Einstein’s general theory of relativity, which gave a new perspective on how space and time are structured in our universe. As with many other of Einstein’s predictions, gravitational waves were detected not long ago and, since then, regularly.

Watching The Cosmos

Astronomy is mankind’s quest to understand the seemingly endless universe in which we live. It all started with ancient humans using their eyes to look at little lights, stars, in the sky. Then came telescopes to augment the eyes to see further out into space.

As other forms of radiation were discovered, of which visible light turned out to be only a small part, telescopes were built to detect radio waves, infrared waves, ultraviolet waves, x-rays, and gamma rays emanating from parts of our universe, near and far.

Light along with other radiation helped us look at different astronomical objects in the universe — stars, galaxies, black holes, pulsars, quasars, and so on. Gravitational waves became the latest addition to our toolbox with which we “see” and decode our universe.

It takes us to the concept of “multi-messenger” astronomy, giving us multiple pictures, or information, of the same object in different lights.

The Force of Gravity

Gravity is the attraction between objects in space, which famously makes an apple on a tree fall to the ground. Einstein postulated that gravity is a very powerful force that sculpts not only space but also time. Space-time can curve where large masses are present, like stars and planets and even galaxies. This causes light to bend as it travels across space.

Similarly, time can dilate or quicken in the presence of large masses. This can also happen if objects are moving very fast at hundreds of thousands or millions of miles per hour. Thus, space and time are relative, not the seemingly fixed quantities we experience in our everyday lives. It has been confirmed experimentally that light does bend and that time does slow down or speed up.

Einstein, a theoretical physicist, described the space curvature and time dilation in the form of mathematical equations, using a new kind of geometry of curvature devised by Bernhard Riemann. There were several solutions to the equations leading to other predictions. One such prediction was the existence of black holes, which is a very dense object containing several star masses into a volume a few miles across. Light bends so much near a black hole that it can never escape. Hence the name.

Several black holes have been discovered, and a few years ago, a black hole was pictured using a combination of several radio telescopes scattered across Earth.

Another prediction from solutions to Einstein’s equations was the existence of gravitational waves. They are a result of ripples in space when two massive objects collide, like two black holes merging into one. Such events are the most disturbing phenomena in our universe. The fabric of space-time is stretched and compressed repeatedly, resulting in gravitational waves. These waves then travel across the universe at the speed of light, which is about six trillion miles per hour. Yet, the universe is so large that such waves can take millions or billions of years to reach us. And the waves become increasingly feeble as they travel such vast distances.

Detecting such feeble waves is an engineering challenge. In that case, why not look for stronger waves? If such violent events happen in our neighbourhood or just a bit further out, we would not be around to experience them nor to read or write about them. Everything in the vicinity would be totally destroyed. Fortunately for us, these are distant events.

Detecting Gravitational Waves

A basic gravitational wave detector is conceptually simple. Have a laser beam split into two parts and traverse the same known distance in two perpendicular directions. If the space is stretching and compressing in one direction, but not the other, one arm will be longer than the other. Although by a tiny fraction, the laser beam will travel different distances and this differential can be measured.

The challenge is the precision. The differential can be as minute as the radius of an atom. Besides, there are a lot of local disturbances, like the movement of vehicles or wind. The apparatus needs to be insulated from all such events and the detectors have to be ultra-sensitive to detect the minute differentials.

Such an engineering accomplishment has ensured that we detect gravitational waves from distant sources in the universe. The detector is called LIGO (Laser Interferometer Gravitational-wave Observatory). It is located in two different centres in the United States (one in Louisiana and another in Washington state), each having two arms each of 2.5 miles and perpendicular to each other. VIRGO is another such detector in Italy.

Gravitational waves from a given source have a single frequency. From another source, it could have a different frequency resulting in a range from millihertz to one kilohertz. Each detector can be tuned to a narrow range of frequencies. Tuning different detectors to different ranges, combined with the quest to make each new detection more sensitive helps to capture more events from the universe.

A laser beam fired in tubes along both the arms is reflected by end mirrors. Any discrepancy in the arrival of the beams to the starting point could indicate the presence of gravitational waves. The mirrors are highly polished and guarded against any other terrestrial vibrations. The tubes themselves have a high vacuum and are very cold. To verify that a signal is not due to any other source of vibration, comparisons are made between two detectors from different parts of the Earth.

In the future, a detector will be set up in space that provides a good vantage point to avoid Earthly distractions. Named LISA (Laser Interferometer Space Antenna), it will be a set of three satellites with lasers and mirrors. Any discrepancy between the time the laser beams take to travel between the three satellites will point to gravitational waves. Planned to be completed in a decade, it promises to be a quantum jump over the existing detectors.

Looking at information gained from various detectors and telescopes offers a complete picture of what surrounds us. It removes the limitations of human senses like eyes and ears. It is like availing different pieces of information from various blindfolded persons touching different parts of the elephant. Combining the bits and bytes helps us form the full image of the elephant.

Gravitational waves help us better understand cosmic collisions from the past. Because looking further out into space means looking back into the past. When our universe was in its infancy, there was only fog and no light. Only gravitational waves from such early times can escape and reach us.

We are living in exciting times. Advances in science and technology are helping us to learn ever more. And learning is a quest, not a destination.

This article has been published as part of Swasti 22, the Swarajya Science and Technology Initiative 2022. Read other Swasti 22 submissions.

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