Comprising the world's largest precision optical instruments and one of the world's largest vacuum systems, LIGO is a marvel of engineering and human ingenuity. Read on for some quick facts about LIGO, its past, and its exciting future.

Evolution of LIGO's Detectors

Construction of LIGO's original gravitational wave detectors (dubbed Initial LIGO or iLIGO) was completed in 1999. The first search for gravitational waves began in 2002 and concluded in 2010 during which time no gravitational waves were detected. Nevertheless, much was learned from the experience to prepare for the next phase of LIGO’s search for gravitational waves.

The lessons learned during Initial LIGO's operation led to a complete redesign of LIGO's instruments, which were rebuilt between 2010 and 2014. This redesign and subsequent improvements will ultimately make LIGO's interferometers 10 times more sensitive than their initial incarnation. A 10-fold increase in sensitivity means that LIGO will be able to detect gravitational waves 10 times farther away than Initial LIGO, which translates into 'sampling' 1000-times more volume of space (volume increases with the cube of the distance, so 10 times farther away means 10x10x10=1000 times the volume of space), and 1000-times more galaxies containing sources of gravitational waves.

This deeper search for gravitational waves began in September 2015, and within days, LIGO's "advanced" detectors achieved what Initial LIGO could not accomplish in 8 years of operation: On September 14, 2015, LIGO's interferometers in Livingston, LA and Hanford, WA made the world's first direct detection of gravitational waves, heralding a new era in astronomical exploration. The gravitational waves detected by LIGO on that day were generated by two black holes colliding and merging into one nearly 1.3 BILLION light years away!

LIGO and Cutting Edge Discovery Science

LIGO is one of the most sophisticated scientific instruments ever built. LIGO's detections will provide physicists with the means to answer key scientific questions, such as:

What are the properties of gravitational waves?

Is general relativity the correct theory of gravity?

Is general relativity still valid under strong-gravity conditions?

Are nature's black holes the black holes of general relativity?

How does matter behave under extremes of density?

What happens when a massive star collapses?

How do compact binary stars form and evolve, and what can they tell us about the history of star formation rates in the Universe?

For more information on LIGO's impact on the broader scientific community, visit LIGO's Impact on Science.

LIGO's Extreme Engineering

LIGO exemplifies extreme engineering and technology. LIGO consists of:

Two “blind” L-shaped detectors with 4 km long vacuum chambers...

situated 3000 kilometers apart operating in unison...

to measure a motion 10,000 times smaller than an atomic nucleus (the smallest measurement ever attempted by science)...

caused by the most violent and cataclysmic events in the Universe...

occurring tens-of-millions or billions of light years away!

A few of LIGO's most remarkable engineering facts are listed below.

Most sensitive: At its most sensitive state, LIGO will be able to detect a change in distance between its mirrors 1/10,000th the width of a proton! This is equivalent to measuring the distance to the nearest star (some 4.2 light years away) to an accuracy smaller than the width of a human hair.

World's third-largest vacuum chambers: Encapsulating 10,000 m3 (350,000 ft3), the air removed from each of LIGO’s vacuum chambers could inflate 2.5 million footballs, or 1.8 million soccer balls. LIGO's vacuum volume is the third largest in the world, surpassed only by the Large Hadron Collider (LHC) in Switzerland, and NASA's "Space Simulation Vacuum Chamber".

Ultra-high vacuum: LIGO's vacuum chambers may be the third largest of all vacuum chambers, but they are the second largest "Ultra High" vacuum chambers (the first being the LHC). The pressure inside LIGO's vacuum tubes is one-trillionth of an atmosphere (10-9 torr)--in other words, one trillionth the air pressure that you would encounter at sea level. It took 40 days to remove all 10,000 m3 (353,000 ft3) of air and other residual gases from each of LIGO’s vacuum tubes. This process was only conducted once. LIGO's vacuum tubes have endured this pressure for over 20 years.

Air pressure on the vacuum tubes: 155-million kg (341-million pounds) of air press down on each 4 km length of vacuum tube. Remarkably, the steel tubes that hold all that air at bay are only 3 mm (0.12 inches) thick.

Curvature of the Earth: LIGO’s arms are long enough that the curvature of the Earth was a factor in their construction. Over the 4 km length of each arm, the Earth curves away by nearly a meter! Precision concrete pouring of the path upon which the beam-tube is installed was required to counteract this curvature.