What is LIGO?

The acronym, LIGO, stands for "Laser Interferometer Gravitational-wave Observatory". Wholly supported by the U.S. National Science Foundation, LIGO 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).

NSF LIGO (and other detectors like it, e.g., Virgo, GEO, and KAGRA) is not an electromagnetic (EM) observatory and thus bears no resemblance to the kind of place most people probably think of when they hear the word "observatory", for example, the Mount Palomar Observatory telescope dome at right. Instead, LIGO is configured as a very large L-shaped detector with 4km (2.5 mi) long arms, as shown in the aerial photo of LIGO's Livingston Louisiana detector below right.

LIGO is fundamentally different from electromagnetic observatories in three primary ways:

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 or other objects or regions in space, it doesn't need to be round or dish-shaped like optical telescope mirrors or radio telescope dishes. Nor does it have to be steerable, i.e., able to move around to point in a specific direction. Instead, each LIGO detector consists of two 4km (2.5 mi.) long, 1.2m-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 by the two LIGO detectors, and some also by Italy's Virgo detector. But now that LIGO scientists better understand signal sources and how the instruments respond to gravitational waves, in some cases, a detection can be made with one instrument. However, to help electromagnetic astronomers find a possible light source associated with a gravitational wave detection, 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 cellphonecan 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 interferometers special, visit:

• What is an Interferometer and
• LIGO's Interferometer