Context: The Government of India has given ‘In-Principle’ approval for Construction and Operation of Laser Interferometer Gravitational Wave Laboratory – India (LIGO) in India in collaboration with LIGO Laboratory, United States of America.

Gravitational Waves 

A gravitational wave is an invisible (yet incredibly fast) ripple in space. Gravitational Waves travel at the speed of light. 

• General Relativity proposed by Albert Einstein predicted that moving objects would generate gravitational waves in spacetime, just like a moving boat produces ripples in water.
• Because these are ripples in spacetime itself, gravitational waves have the effect of causing a temporary deformation in a body when it comes in contact.
• Since spacetime itself elongates or contracts during the propagation of the gravitational wave, everything lying in that spacetime also goes through the same experience.

• For example,
  • This effect is like a ball being slightly squeezed along any of its diameters. 
  • The ball flattens a bit in the direction of pressure that is applied, while it bulges out in the perpendicular direction.

• When a gravitational wave passes the Earth, for example, the Earth gets similarly squeezed in one direction, and bulges in the perpendicular direction.
• Because gravity is the weakest of all natural forces, the deforming effect of gravitational waves is extremely tiny, the reason why it could not be experimentally verified for 100 years.
• There are many astrophysical phenomena that are either very dim or completely invisible in any form of light that astronomy has relied on for 400 years.
• Gravitational waves are a powerful new probe of the Universe that uses gravity instead of light to take measure of dynamical astrophysical phenomena.

Gravitational Waves Tell Us About

• Studying gravitational waves gives enormous potential for discovering the parts of the universe that are invisible by other means, such as black holes, the Big Bang, and other, as yet unknown, objects. 
• It provides experimental foundation to the general theory of relativity proposed by Einstein.
• Gravitational waves allow us to directly observe some of the most cataclysmic events in the universe, such as black hole mergers and neutron star collisions.
• Gravitational waves can potentially help us study the mysterious components of the universe, such as dark matter and dark energy.

Causes of the Gravitational Waves 

The most powerful gravitational waves are created when objects move at very high speeds. Some examples of events that could cause a gravitational wave are: 

• When a star explodes asymmetrically (called a supernova). 
• When two big stars orbit each other.
• When two black holes orbit each other.
• It can be created by the mergers of neutron stars and black holes.
• Continuous gravitational waves are expected to be produced by a single spinning massive object like a neutron star.

• Compact binary inspiral gravitational waves are produced by orbiting pairs of massive and dense (“compact”) objects like white dwarf stars, black holes, and neutron stars.

Laser Interferometer Gravitational-wave Observatory (LIGO) 

LIGO stands for “Laser Interferometer Gravitational-wave Observatory”. LIGO exploits the physical properties of light and of space itself to detect and understand the origins of gravitational waves (GW).

Measurement of Gravitational Wave by LIGO

The observatory comprises two 4-km-long vacuum chambers, built perpendicular to each other. Highly reflective mirrors are placed at the end of the vacuum chambers.

• Light rays are released simultaneously in both the vacuum chambers. They hit the mirrors, get reflected, and are captured back. 
• In normal circumstances, the light rays in both the chambers would return simultaneously.
• But when a gravitational wave arrives, one of the chambers gets a little elongated, while the other one gets squished a bit.
  • In this case, light rays do not return simultaneously, and there is a phase difference (one returns after some time than the other). 
• The presence of a phase difference marks the detection of a gravitational wave.

Instrumental Accuracy in Measurement 

The precision of the measurements required to detect gravitational waves is mind-boggling. 

• At a 4-km scale, the changes in distance that light must travel because of the gravitational wave are 10,000 times smaller than the width of the proton (approximately one-millionth of a nanometer), and LIGO instruments are designed to pick this up. 
• According to the LIGO website, this is like measuring the distance to a neighbouring star 4.2 light years away with an accuracy smaller than the width of human hair.

Unique Features of LIGO Observatory 

• LIGO is blind: 
  • Unlike optical or radio telescopes, LIGO does not see electromagnetic radiation (e.g., visible light, radio waves, microwaves). 
  • But it doesn’t have to because gravitational waves are not part of the electromagnetic spectrum. 

• LIGO can’t point to specific locations in space: 
  • Since LIGO doesn’t need to collect light from stars (in fact, it can detect gravitational waves coming from below!), it doesn’t need to be round or dish-shaped like optical telescope mirrors or radio telescope dishes. 
  • Instead, each LIGO detector consists of two 4 km long, 1.2 m-wide steel vacuum tubes arranged in an “L” shape (LIGO’s laser travels through these arms), and enclosed within a 10-foot wide, 12-foot-tall concrete structure that protects the tubes from the environment.

Limitation of LIGO 

• The longer the Better: the longer the interferometer’s arms, the bigger the absolute changes that gravitational waves make to total arm length (i.e., total distance traveled by the lasers).
  • As a result, longer arms and greater laser travel amplify the impact of even small gravitational waves, increasing their detectability in the interferometer.
• Need more power: Increasing laser power also enhances its performance.
  • In this case, the more laser photons that merge from each arm the sharper the fringes that are measured by the photodetector.
• External Noises: It uses a seismic isolation system to protect the LIGO from unwanted vibration that can hamper the detection of Gravitational waves.
  • The instruments at these observatories are so sensitive that they can easily get influenced by events like earthquakes, landslides, or even the movement of trucks, and produce a false reading. 
• Multiplicity of Results: It is difficult for a single LIGO detector to confirm a gravitational wave signal on its own. Multiple instruments (ideally 3 or more) must be able to detect the signal to localize the source in the sky.
• Availability of Land resources: The interferometry needs a very large tract of land which may result in conflict over transfer and compensation.

Benefits of LIGO India 

• Though LIGO’s mission is to detect gravitational waves from some of the most violent and energetic processes in the Universe, the data LIGO collects may also contribute to other areas of physics such as gravitation, relativity, cosmology, astrophysics, particle physics, and nuclear physics. 
• Socio-Economic benefits: the construction of an observatory will provide employment opportunities in the adjoining areas such as jobs for guards, construction work, tourist guide etc. 
• International Collaboration: The LIGO provide an opportunity to the scientific community of India to integrate with world leading to adoption of best practices and can lead to effective utilisation of resources.

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