Our Milky Way galaxy. [Image Credit: NASA/S. Brunier]


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. Initial LIGO's "range" (the radius out to which LIGO could detect at least a binary neutron star (BNS) merger) was 15 megaparsecs (Mpc) -- a parsec is a unit of distance used by astronomers that equates to 3.26 light years, so a megaparsec is 3.26 million light years, and 15 Mpc = 48.9 million light years. LIGO did not detect any gravitational waves during iLIGO's run, but much was learned from the experience to prepare for the next phase of LIGO’s search for gravitational waves.

Lessons learned during Initial LIGO's operation were applied in a complete redesign of LIGO's instruments. This upgraded version of LIGO was dubbed Advanced LIGO (aLIGO), and during its operation between 2015 and 2020 (with the help of additional upgrades implemented in that time frame), covering three observing runs, O1, O2, and O3, LIGO achieved a BNS range of 135 Mpc (440 million light years). Subsequent improvements will ultimately make LIGO's interferometers more than 10 times more sensitive than iLIGO, i.e., able to detect gravitational waves from BNS mergers 10 times farther away than Initial LIGO. This translates into LIGO probing 1000-times more volume of space than iLIGO (volume increases with the cube of the distance, so 10 times farther away means 10x10x10=1000 times the volume of space).

LIGO volume increase

Ten times distance reached equates to 1000 times more volume of space probed.



aLIGO began its search for gravitational waves in September 2015, and within days, 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, generated by two black holes colliding and merging into one nearly 1.3 BILLION light years away!

LIGO's instruments have undergone additional upgrades in its (to date) three full observing runs, the last concluding in March, 2020. By the end of that run (O3), LIGO had confirmed the detection of 90 merger events, including the first ever detection of two neutron stars colliding.

LIGO's fourth observing run, O4, will commence in 2023, at which time its sensitivity range is expected to reach 190 megaparsecs, surpassing the 10x-more-than-iLIGO goal, and reaching nearly 2.4 times farther out into the Universe than the aLIGO detectors reached in O1.

Below is a simple animation showing LIGO's range evolution between Initial LIGO and what is expected in O4, encompassing multiple superclusters of galaxies. Click the full screen button in the lower right corner of video frame for the full-sized graphic.

















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.