What is LIGO?
LIGO is the world's largest gravitational wave observatory and a marvel of 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 it will look something like the photo below: 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 intererometer 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 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 confirm a detection.
LIGO is blind. Unlike optical or radio telescopes, LIGO cannot see electromagnetic radiation (e.g., visible light, radio waves, microwaves) nor does it have to because gravitational waves are not part of the electromagnetic spectrum. In fact, electromagnetic radiation from space 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 includes two 4 km (2.5 mi.) long, 1.2 m diameter steel vacuum tubes arranged in an "L" shape, and covered by a 10-foot wide, 12-foot tall concrete 'enclosure' 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), gravitational wave detectors like LIGO's cannot operate solo. The only way to confirm a gravitational wave detection is by operating in unison with another detector. This ensures that local vibrations are not mistaken for gravitational waves. A single detector can, in principle, sense a gravitational wave, say from a supernova explosion. But to confirm the detection, a coincident electromagnetic signal of some kind would also have to be made, again, verifying that what the LIGO detector sensed was not just some random noise in a single detector.