By Divya Swaminathan

On October 3, 2017, the Royal Swedish Academy of Science awarded one half of the Nobel Prize in Physics to Dr Rainer Weiss and the other half jointly to Dr Barry C. Barish and Dr Kip S. Thorne for and I quote “decisive contributions to the LIGO detector and the observation of gravitational waves”.

About 1.3 billion light years ago, in some corner of our universe, two black holes collided and merged emitting gravitational waves in space and time – just like ripples from a pebble thrown in still waters. On September 14, 2015, gravitational waves generated by this event passed through Earth becoming the first ever to be detected by the twin LIGO detectors – an experiment all three of this year’s Nobel laureates were an integral part of.

Gravitational waves

About a hundred years ago, while working on his general theory of relativity, Albert Einstein predicted that any mass distorts space and time around it consequently affecting other moving masses in its neighbourhood. Gravity, he predicted, results from this distortion of space and time. Or simply put accelerating masses (stars, rotating black holes, you and me) lead to radiating ripples in space and time called ‘gravitational waves’ that are permeating through our universe.

Though predicted by the general theory of relativity, Einstein himself was not quite sure if they were real or a ‘mathematical illusion’. In any case, he believed the waves would never be detected as the signal would likely be extremely weak.

Indeed, the signal that came in on the morning of September 14, 2015, was a weak one. It needed extremely clever engineering, decades of dedicated work by a fleet of scientists and millions of dollars in funding before the elusive gravitational waves were detected exactly 100 years after they were predicted.

Figure Legend 1- The Collision. Two black holes rotating around each other emitted gravitational waves for millions of years before they came closer and merged. The gravitational waves resulting from this collision were detected 1.3 billion lightyears later on Earth by LIGO.

LIGO

The Laser Interferometer  Gravitational-Wave Observatory was built with the specific intention of running experiments designed to detect gravitational waves.

By the mid-1970s, Rainer Weiss had analysed potential noise sources that could drown weak gravitational wave signals and designed a detector to separate signal from noise.  Not only was it a signal versus noise problem but the real challenge was to be able to detect a change in length much smaller than an atom’s nucleus. His idea was the laser-based interferometer that is now the heart of LIGO.

To understand how the interferometer works, one must understand what the terms ‘in phase’ and ‘out of phase’ mean in the context of waves. If two waves are in phase, that is their peaks and troughs completely align, the net sum would be a wave twice the size of the individual waves or a sort of magnification takes place.  If two waves are out of phase, i.e. the peak of one wave aligns with the trough of another, then the net sum is a flat line as they cancel each other out. One positive and one negative equals zero. A passing gravitational wave distorts space and time. Therefore, if two identical light beams get caught in a gravitational wave, they will be stretched and pulled differentially picking up a phase difference detectable by LIGO.

A single laser beam is split into two identical daughter beams and sent off, bouncing back and forth, in four-km-long L-shaped tunnels. Sophisticated mirrors at the end of each tunnel reflect the daughter beams back to the corner of L where the detector is. In the absence of a gravitational wave, the daughter beams meet at the detector ‘out of phase’ and no light is detected. But in the presence of gravitational wave, following space-time distortion, the long L-shaped arms of the interferometer are disturbed. One arm may extend as the other compresses, changes registered by the daughter beams. They now arrive at the detector having travelled a different distance each – resulting in a light signal – a signature of space-time distortion.

LIGO consists of twin observatories, one in Hanford, Washington State, USA and another  about 4000 km away in Livingston, Louisiana, USA. The rationale being a gravitational wave passing through Earth should be detected by both observatories albeit at different points in time whereas a false positive would only be picked up by one. It took a total of 40 years from theoretical principle development to actually getting the observatory up and running by 2002.

Figure Legend 2:-  The inner workings of LIGO. The detector is hypersensitive and can measure unbelievably faint signals, i.e. disturbances in spacetime about a thousand times less than an atomic nucleus.

On September 14, 2015, the first gravitational wave was detected in Livingston and travelling at the speed of light, seven milliseconds later in Hanford. Since then three others have been detected.

Individual contributions

So how do we know what the signal from a gravitational wave should look like and what cosmic events cause it? In the Sixties, theoretical astrophysicist Thorne realised that collisions between black holes should be detectable via gravitational waves and went on to pioneer the development of mathematics necessary to analyse data that would be collected from LIGO. He put the theoretical tools for understanding and analysing results in place. Together with Weiss, they figured out what the magnitude of the gravitational wave signal should be and also worked on the design of the interferometer.

Rainer Weiss was the engineering mastermind. The LIGO is a hypersensitive detector and the cosmic world a noisy place. How does one ensure the laser beams aren’t picking up the wrong signal? How does one ensure precise measurements at subatomic length scales? A lot of creative engineering went into solving the problem of signal vs. noise and designing the detector and is credited to Weiss.

The LIGO project’s initial takeoff was a failure and in 1994, Barry Barish took charge. Under his audacious leadership, the small group of scientists quickly grew into a large-scale international collaboration with over a thousand participants. He searched for required expertise, nurtured valuable collaborations and won significant funding from the National Science Foundation to get LIGO up and running.

What Lies Ahead 

One must pause and reflect on the magnificence of LIGO and the people behind this great scientific effort – it is now possible to detect colliding black holes from millions and billions of light years away.  This opens up an entire new field of study as astrophysicists can now use gravitational waves to ‘see’ disruptions in space-time and study the dark unseen parts of our universe.

Across the world, other LIGO-like observatories are under development. LIGO-India is in the works too with funding approved and is expected to begin operations by 2024.