The Waves from Eternity

Technothlon
Dec 8, 2017 · 6 min read

You don’t need to be a hardcore physicist to appreciate the recent developments in the subject. And if the development involves something which could change the way we look at the cosmos, even the ‘non-scientific’ junta is bound to get goosebumps. It is not tough to guess that I am talking about the detection of Gravitational Waves by the LIGO. Moreover, the Nobel Prize in Physics 2017 was awarded for this very discovery. Hence I feel highly obliged to write this article.

The monumental milestone in modern physics was achieved on September 14, 2015 at 0551 hrs Eastern Time (1521 hrs IST). The waves were detected by both of the twin Laser Interferometer Gravitational-wave Observatory (LIGO) detectors, located in Livingston, Louisiana, and Hanford, Washington, USA. But before proceeding with the details of the event let us discuss briefly about gravitational waves.

Gravitational Waves

Gravitational waves are the ‘ripples’ in the fabric of space-time caused by super energetic processes occurring in the universe. In physics, space-time is a mathematical model that joins three dimensional space and time into a single idea called the space-time continuum. In two dimensions, space-time can be vizualised as an infinite sheet of stretched fabric. In the absence of matter, the sheet is sans any distortion or bends. Wherever matter exists, it distorts this sheet.
This results in a curved shape of space-time which can be understood as gravity.

Fig. Presence of matter distorts the space-time fabric

When an object accelerates, it creates ripples in space-time, just like a boat causes ripples in a pond (and also similarly an accelerating electric charge produces an electromagnetic wave). These ripples propagate in space-time as gravitational waves. These waves travel at the speed of light through the universe carrying valuable information about their origin and about gravity itself. Albert Einstein predicted the existence of gravitational waves in 1916 in his general theory of relativity. The strongest gravitational waves are produced by catastrophic events such as colliding black holes, the collapse of stellar cores (supernovae), coalescing neutron stars or white dwarf stars, the slightly wobbly rotation of neutron stars that are not perfect spheres, and the remnants of gravitational radiation created by the birth of the Universe itself. Although the waves originate from tremendously energetic cosmic events, they are very difficult to detect when they reach Earth. This is due to the fact that the waves travel the entirety of the Universe to reach us and a big big portion of their energy gets dissipated on their way to Earth. In fact, by the time gravitational waves from the first detection reached LIGO, the amount of space-time wobbling they generated was thousands of times smaller than the nucleus of an atom! LIGO was designed to make such infinitesimally small measurements. Let us see how.

Laser Interferometer Gravitational-wave Observatory (LIGO)

Fig. A very basic depiction of the LIGO

The key to building an interferometer lies in the way a gravitational wave distorts the fabric of space-time. It stretches space-time in one direction and compresses it in the perpendicular direction.

A laser beam generated by the source S is split and directed into a pair of two long perpendicular tubes, both of precisely the same length(4 km). The mirrors on the end of these tubes reflect back the laser beams which recombine at the base B. The recombination takes place in such a way that the two beams cancel each other completely. Thus, no light gets detected in the photodetector D. But when a gravitational wave comes along, it distorts space-time and alters the distance between the mirrors M1 and M2; one arm gets longer and the other a bit shorter. An instant later, they switch. The back and forth stretching and squeezing happens over and over until the wave has passed. As the distances change, so does the alignment between the crests and troughs of the the two recombining waves, and now, they don’t cancel each other completely. Hence, some light intensity is detected at D. The intensity varies as the distance between the mirrors varies. Measure the intensities, and you’re measuring gravity waves! Just to make sure that the detection is not due to local terrestrial disturbances we have two LIGOs, one at Hanford and one at Livingston.

Fig. Actual picture of LIGO at Hanford

Based on the observed signals, LIGO scientists estimate that the black holes for this event were about 29 and 36 times the mass of the sun, and the event took place 1.3 billion years ago. About 3 times the mass of the sun was converted into gravitational waves in a fraction of a second — with a peak power output about 50 times that of the whole visible universe. By looking at the time of arrival of the signals — the detector in Livingston recorded the event 7 milliseconds before the detector in Hanford — scientists can say that the source was located in the Southern Hemisphere.

Nobel Prize in Physics 2017
The Nobel Prize in Physics 2017 was awarded — one half to Rainer Weiss and the other half jointly to Barry C. Barish and Kip Thorne for their critical contributions to the LIGO detector and the detection of gravitational waves.

Rainer Weiss born 1932 in Berlin, Germany.
Ph.D. 1962 from Massachusetts Institute of Technology, MIT, Cambridge, MA, USA.
Professor of Physics, Massachusetts Institute of Technology, MIT, Cambridge, MA, USA

Barry C. Barish born 1936 in Omaha, NE, USA.
Ph.D. 1962 from University of California, Berkeley, CA, USA.
Linde Professor of Physics, California Institute of Technology, Pasadena, CA, USA

Kip S. Thorne born 1940 in Logan, UT, USA.
Ph.D. 1965 from Princeton University, NJ, USA.
Feynman Professor of Theoretical Physics, California Institute of Technology, Pasadena, CA, USA

Let us hope that these developments strengthen our understanding of the universe further and push us closer to the answer to the QUESTION OF EXISTENCE. As active members of an educated society, it is our foremost responsibility to honour the men behind the machines. At an individual level this can be done by trying to know more about them and their ventures and spreading this knowledge to as many people as we can. All we need to do is to acknowledge our role in building a strong scientific society.

“The whole science is nothing more than the refinement of everyday thinking.”- Albert Einstein

Ishan Azad, Chief Organizer, Technothlon’ 18

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Technothlon is an international school championship organized by the students of IIT Guwahati. Technothlon began in 2004 with an aim to ‘Inspire Young Minds’