Comprising the world's largest precision optical instruments and utilizing 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 led to a massive redesign of LIGO's instruments. The subsequent upgraded version of LIGO was dubbed Advanced LIGO (aLIGO). During aLIGO’s operation between 2015 and 2020 (in which time, additional upgrades were made), three discrete ‘observing runs’ were conducted (O1, O2, and O3), and LIGO achieved a maximum BNS range of 135 Mpc (440 million light years), nearly three times deeper than iLIGO. Continued improvements are intended to make LIGO's interferometers 10 times more sensitive than iLIGO, which translates into LIGO’s ‘advanced’ detectors probing a volume of space 1000-times greater than Initial LIGO (volume increases with the cube of the distance, so 10 times farther away means 10x10x10=1000 times the volume of space).
LIGO’s history is one of alternating times of instrument upgrades interspersed within extended observing runs. With an instrument as sensitive and complex as LIGO, limited, deliberate changes to the instruments must be carefully planned and made to ensure it functions at its best, and significant scientific gains are being made.
After the major overhaul transforming iLIGO into aLIGO, LIGO’s search for gravitational waves resumed in September 2015. This time, within days, aLIGO 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 nearly 1.3 BILLION light years away! aLIGO’s first observing run (O1) lasted just 4 months, and in that time, LIGO successfully detected 3 gravitational waves, all from colliding black holes.
Between January 2016 and November 2016, aLIGO underwent further improvements to prepare for its second observing run, O2, which commenced in November 2016 and concluded at the end of August 2017. It was in O2 that LIGO made its most revolutionary discovery: the first ever detection of two neutron stars colliding. All in all, in O2, LIGO made 8 more detections.
After still more extensive instrument upgrades, LIGO's third observing run (O3) began in April 2019, and ended in March 2020. In this extraordinary run, LIGO nabbed a whopping 79 detections, demonstrating the value of LIGO’s continued and careful stepwise upgrades.
In three observing runs, spanning just 25 months of total observing time, LIGO had confirmed the detection of 90 merger events!
LIGO’s two instruments have undergone another major upgrade since March of 2020, preparing for LIGO's fourth observing run, O4, which will commence in 2023. With the help of this latest upgrade, LIGO hopes its sensitivity range will reach 190 megaparsecs, which would surpass LIGO’s goal of probing 10x deeper into space than Initial LIGO.
Below is an animation showing LIGO's range evolution between O1 and what is expected in O4. 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 are providing 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 (530,000 ft3), and NASA's "Space Simulation Vacuum Chamber" (800,000 ft3).
Ultra-high vacuum: LIGO's vacuum envelope may be the third largest by volume in the world, 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 maintained 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. The tubes are encircled by steel support 'strips' every meter or so to help them keep their shape.
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 and ensure that when the laser beam leaves the ‘corner station’ (traveling in a straight line) it strikes the test mass/mirror at the end of each arm, and not a meter above it.