The universe wobbled, sending quivers which were received by two massive machines on the Earth, thereby giving us a new tool to perceive the cosmos, and opening up new realms in the world of physics. From the times of Einstein to last year’s Nobel Prize in Physics, this is the story of gravitational waves, our new “tool.”
Gravitational waves are ripples in the fabric of space-time that propagate outward from the source that created them, travelling at the speed of light. As a gravitational wave passes an observer, that observer will find spacetime distorted by the effects of strain. As the wave passes,distances between objects increase and decrease rhythmically, corresponding to the frequency of the wave. In very simple words, it distorts your shape – stretching and squeezing you in the direction the wave is travelling!
The tale of gravitational waves is a long one. Albert Einstein is commonly praised for bringing forward the idea of gravitational waves when he published his ‘General Theory of Relativity’ in 1915. However, gravitational waves were first proposed by Henri Poincare ten years prior to that!
Space-time is a mathematical model that joins space and time into a single idea called a continuum, and massive objects warp space-time around them. According to the general theory of relativity, curvatures in space-time are due to the presence of mass: the more the mass, the greater the curvature of space-time. In certain circumstances, massive accelerating objects can cause ripples in this curvature, and these ripples travel outward as gravitational waves. Stars exploding asymmetrically, coalescing neutron stars, or two black holes orbiting each other and colliding, are some examples of causation of gravitational waves.
Gravitational waves are invisible and not easily detectable; their sources are very far away, and they have a very small amplitude by the time they reach Earth. According to Cardiff University researchers,
“A supernova explosion in our own galaxy would emit pretty strong gravitational radiation, yet a 1 km ring would deform no more than a one-thousandth the size of an atomic nucleus.”
This calls for extremely sensitive detectors!
Recently, the development of sensitive instruments that use interferometry (merging two or more sources of light to create what’s known as an interference pattern) has facilitated the creation of sensors which can detect tiny amounts of gravitational radiation. Currently, the most sensitive one is called LIGO – Laser Interferometer Gravitational-Wave Observatory. By initiating LIGO’s construction 20 years ago, humans took a giant leap towards the dawn of a new era in astrophysics. There are two LIGO observatories which have been functional since 2015. A single LIGO observatory has two arms, each 4 km long and perpendicular to each other. Laser beams are shone along these arms, which reflect off mirrors at each end. A passing gravitational wave will slightly stretch one arm as it shortens the other, and allow the detection of waves as shown in the diagram.
After a hundred years of mathematical and theoretical proofs, the physical evidence arrived in the form of twin tremors in two detectors that were 1,865 miles apart. On September 14, 2015, ripples through space-time sent quivers down the L shaped arms of a detector in Louisiana. Only a fraction of a second later, another detector received the tiny-amplitude waves in Washington. The origin of these waves was a black hole merger 1.3 billion light years away! Since then, a few more gravitational waves have been detected, produced by other black hole mergers.
Consequently, the 2017 Nobel Prize in Physics was awarded to Rainer Weiss, Barry Barish, and Kip Thorne for their significant contributions to LIGO, adding a new chapter to their decades-long quest to detect gravitational waves.
However, an even bigger discovery was yet to be added to this tale. On August 17, 2017, a massive event in the heavens caught the eye of more than 70 laboratories around the world. Two neutron stars merged 130 million light years away in a cataclysmic collision that not only released gravitational waves but also a bright gamma-ray burst. The detection of this once-in-a-lifetime event was revolutionary for humanity; in addition to being noticed by LIGO, it was simultaneously photographed by optical telescopes, allowing us to see the explosion as well as detect the waves. Cross checking different data from different telescopes, such as the Swope telescope in Chile, confirmed the nature of this explosion and enriched our understanding of this groundbreaking phenomenon that was detected worldwide by showing that the explosion released tonnes of gold and other heavy metals.
Prior to this, light and other forms of electromagnetic radiation were the key to gazing into the history of our universe. While telescopes such as The Hubble Telescope were effective, they faced an optical limit – there was not much to view in the dark. However, gravitational waves can be used to look back at the opaque moments of the fractions of a second after the Big Bang, unravelling a few mysteries while generating some more. Moreover, gravitational waves are weak but hard to mask and can give us insight into what happens during some of the Universe’s most violent explosions such as supernovae and gamma-ray bursts. In the words of Professor Paul Hardaker, chief executive officer of the Institute of Physics:
“For as long as we have had astronomy we have used light in some form or another to understand how our universe works. This significant result marked the beginning of another way of viewing the universe, using gravity, which is what makes it such a major step forward, and so deserving of a Nobel prize.”
The story is far from over: with each advancement in technology, we will open a new chapter in this saga.
“At some point, not with the detectors we have now, we hope to be able to look at the beginnings of the universe,”
said Rainer Weiss, the physicist at MIT who shared Nobel Prize in Physics with other members of the LIGO team.
With countries like Japan and India planning to build their own gravitational wave detectors, and European Space Agency intending to send an observatory into space to detect far weaker gravitational waves than is possible on Earth, we are embarking on a journey into the known as well as into the unknown, and there is an excitement about what we are yet to learn about the cosmos, and our place in it.
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