What is LIGO?
LIGO is the world's largest gravitational wave observatory and a marvel of precision engineering. Comprising two enormous laser interferometers located thousands of kilometers apart, LIGO exploits the physical properties of light and of space itself to detect and understand the origins of gravitational waves.
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 'traditional' astronomical observatory: LIGO is blind, it is not round, and a single detector cannot make a discovery all on its own (another gravitational wave detector or other astronomical instrument must confirm a detection).
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. In fact, electromagnetic radiation is so unimportant to LIGO that its detector components are completely isolated and sheltered from the outside world.
LIGO is the opposite of round. 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 focus EM radiation to produce images. Each LIGO detector consists of two 4km (2.5 mi.) long, 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.
A single LIGO detector cannot confirm gravitational waves on its own. While an astronomical observatory can function and collect data just fine on its own (though some do not, by choice), a single gravitational wave detector cannot make a discovery on its own. A random, local vibration (what we call "noise") could conceivably create a signal that looks like a gravitational wave. So the only way to verify a gravitational wave detection is to operate in unison with another detector. There are special cases where a single gravitational wave detector could make a discovery, but it would still need help from the electromagnetic astronomical community. For example, a single LIGO detector could sense the gravitational waves from a supernova (an exploding star). But even in this case, a coincident electromagnetic signal of some kind would also have to be made by an astronomical observatory (ground- or space-based), verifying the detection.