The physics of the world’s largest gravitational-wave observatory: LIGO

Since the confirmation of detection, talk of gravitational waves has drastically increased in the public forum. In February 2016, the Laser Interferometric Gravitational-wave Observatory (LIGO) Collaboration announced that they had sensed gravitational waves, or ripples in spacetime, caused by the collision of two black holes approximately 1.3 billion light years away. Such an amazing feat quickly became globalized news with many asking how it could be physically possible to detect an event occurring at an unimaginable distance? For some, the entire situation feels incomprehensible.

Although named an observatory, LIGO looks quite different from observatories such as the late Arecibo Observatory in Puerto Rico, the Very Large Array (VLA) in New Mexico, or the Lowell Observatory in Arizona. Instead of being related to the traditional telescope concept, LIGO is comprised of two interferometers, one in Hanford, Washington and the other in Livingston, Louisiana, that use lasers to detect vibrations in the fabric of spacetime. 

An interferometer is an L-shaped apparatus with mirrors at the end of each arm specifically positioned to split the incoming light waves, specifically in this case laser waves, into an interference pattern. This pattern is then detected by a device called a photodetector, which converts the pattern into carefully recorded data. When an incredibly violent event occurs, two black holes colliding, for instance, that action results in a massive release of energy that ripples across the fabric of spacetime. The energy from the event vibrates the laser light causing a change in the recorded light pattern. This change is also recorded by the photodetector and stored as data, which scientists can collect to analyze as needed.

Because the LIGO detector is so sensitive, there are a number of systems in place to maintain its functionality and reliability. The apparatus is comprised of four main systems:

1) seismic isolation that focuses on removing non-gravitational-wave detections (also called ‘noise’)

2) optics that regulate the laser

3) a vacuum system preserving the continuity of the laser by removing dust from the components

4) computing infrastructure that manages the collected scientific data. The collaboration of these systems helps to minimize the number of false detections.

False detections are also kept at a minimum with the effective communication between the Washington and Louisiana LIGO sites. It took months for the official announcement of the 2015 gravitational-wave detection because both locations had to compare data to ensure that the detection of one apparatus was also accurately detected by the other apparatus. Because of human activity on Earth, there can be a number of vibrations similar to gravitational-wave ripples, but ultimately are shown to be terrestrial events rather than celestial ones. So, while LIGO physics itself is fairly straightforward, the interpretation of the gathered data tends to be tricky.

Written by Amber Elinsky

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