What is LIGO?

LIGO Livingston. The arms you see are concrete structures that protect the vacuum tubes, which reside just inside. Each arm is 4 km (2.5 miles) long. This concrete 'enclosure' shelters the critically-important steel vacuum tubes from the environment. (Caltech/MIT/LIGO Lab)

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: Virgo, GEO, and KAGRA) 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, probably much that of the 200-inch Hale Telescope at Palomar Observatory seen in the photo below. LIGO bears no resemblance to this whatsoever, as illustrated by the aerial photo of the LIGO Livingston interferometer at right.

An iconic telescope dome. The 200-inch Hale Telescope at Palomar Observatory in northern San Diego County, California. (Tylerfinvold/Wikimedia Commons)

Since the "O" in LIGO stands for "observatory", how does it compare to the observatories that most people envision?

Three things fundamentally distinguish LIGO from a stereotypical astronomical observatory:

(1) LIGO is blind to the light from the Universe.

(2) It doesn't need to focus starlight or point at a particular part of the sky.

(3) It is difficult for a single detector to make a discovery on its own.

Each of these differences is explained below.

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. They are a completely different phenomenon altogether (though in some cases, EM astronomers hope to see some form of light coming from GW sources, like that which occurred immediately following the binary neutron star merger detected in August 2017). In fact, electromagnetic radiation is so unimportant to LIGO that its detector components are completely isolated and sheltered from the outside world.

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 (2.5 mi.) 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.

It is difficult for a single LIGO detector to confirm a gravitational wave signal on its own. The initial discovery of gravitational waves required that the signal be seen in both detectors (Hanford and Livingston). Happily, GW150914 fulfilled that requirement and since then, dozens more signals have been observed in the two LIGO detectors, and some also in Italy's Virgo detector. Now that we better understand signal sources and how our instruments respond to gravitational waves, in some cases, a detection can be made with one instrument as long as the signal is especially powerful. However, to help electromagnetic astronomers find a possible light source associated with our detections, multiple instruments – ideally 3 or more – must be able to detect the signal to localize the source on the sky (in a process similar how a cellphone can be located with three or more nearby cell towers). This was the case for the first-ever binary neutron star merger detection, GW170817.

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. In this way, LIGO is also a physics experiment on the scale and complexity of some of the world's giant particle accelerators and nuclear physics laboratories.

To learn more about interferometers in general and what makes LIGO's interferometer special, visit What is an Interferometer and LIGO's Interferometer.