What is LIGO?

LIGO stands for “Laser Interferometer Gravitational-wave Observatory”. It is the world’s largest gravitational wave observatory and a marvel of precision engineering. Comprising two enormous laser interferometers located 3000 kilometers apart, LIGO exploits the physical properties of light and of space itself to detect and understand the origins of gravitational waves (GW).

LIGO (and other detectors like it) is unlike any other observatory on Earth. Ask someone to draw a picture of an observatory and odds are they will draw a gleaming white telescope dome perched on a mountain-top. As a gravitational wave observatory, LIGO bears no resemblance to this whatsoever, as the aerial photo of the LIGO Livingston interferometer at right clearly illustrates.

More than an observatory, LIGO is a remarkable physics experiment on the scale and complexity of some of the world’s giant particle accelerators and nuclear physics laboratories. Though its mission is to detect gravitational waves from some of the most violent and energetic processes in the Universe, the data LIGO collects may have far-reaching effects on many areas of physics including gravitation, relativity, astrophysics, cosmology, particle physics, and nuclear physics.

Nevertheless, since the “O” in LIGO stands for “observatory”, below we describe how it differs from the observatories that most people envision. Three things distinguish LIGO from a stereotypical astronomical observatory: LIGO is blind, it is not round and cannot point at a particular part of the sky, and it is rare for a single detector to make a discovery on its own.

LIGO is blind. Unlike optical or radio telescopes, LIGO does not see electromagnetic radiation (e.g., visible light, radio waves, microwaves). It doesn’t have to because gravitational waves are not part of the electromagnetic spectrum. They are a completely different phenomenon altogether (though in some cases, we do expect to see some form of EM radiation coming from GW sources). In fact, electromagnetic radiation is so unimportant to LIGO that its detector components are completely isolated and sheltered from the outside world.

LIGO isn’t round and can’t point to specific locations in space. Since LIGO doesn’t need to collect light from stars, it doesn’t need to be round or dish-shaped like optical telescope mirrors or radio telescope dishes, both of which need such structures to focus EM radiation onto a detector. Each LIGO detector consists of two arms, each 4km (2.5 mi.) long, comprising 1.2m-wide steel vacuum tubes arranged in an “L” shape, and covered by a 10-foot wide, 12-foot tall concrete shelter that protects the tubes from the environment. LIGO can also detect gravitational waves coming from any direction (even below)!

A single LIGO detector could not initially confirm gravitational waves on its own. The initial discovery of gravitational waves required that similar signals arrive quasi-simultaneously in multiple detectors. Happily, GW150914 fulfilled that requirement, and we have now seen many signals which appeared in the two LIGO detectors and also in Italy’s Virgo detector. Now that we understand both our signal sources and our instruments better, some detections can be confidently made with a significant signal in just one detector – a great step forward for our field. However, to help electromagnetic observers find a possible light source associated with our detections, we must have multiple detectors – ideally 3 or more – to localize the signal in the sky. This was the case for the first binary neutron star signal, GW170817.

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