LIGO's Dual Detectors
General location of LLO and LHO, separated by 3002 km
Why Two Detectors?
The U.S. National Science Foundation Laser Interferometer Gravitational-wave Observatory is made up of two identical and widely separated interferometers situated in somewhat out-of-the-way places. The detector in Hanford, Washington is located in an arid, shrub-steppe region crisscrossed by hundreds of layers of ancient lava flows covered by rolling sand dunes created by Ice Age floods. The Livingston detector is situated in a completely opposite environment of a warm, humid, loblolly pine forest east of Baton Rouge, Louisiana. The instruments are 3002km apart.
There are three main reasons for the wide separation between the interferometers: Local vibrations, gravitational wave travel time, and source localization.
First, NSF LIGO’s detectors are so sensitive that they can sense the tiniest vibrations on the Earth. Earthquakes (waves arriving from around the globe), trucks driving on nearby roads, farmers plowing fields, and even local winds, can cause disturbances that can mask or mimic a gravitational wave signal in each interferometer. If the instruments were located close to each other, they would sense the same vibrations (local, Earth-based and actual gravitational waves) essentially at the same time, making it exceedingly diffcult to distinguish a gravitational wave signal from a non-gravitational wave signal in the data. Separating the detectors by 3000km ensures that each one experiences unique local vibrations. When data from the sites are compared, computers ignore vibrations that differ, and pay close attention only to signals that look the same.
Second, and equally as important, since gravitational waves travel at the speed of light, with detectors 3000km apart, the longest span of time that can elapse between a wave's arrival at LLO and LHO is about 10 milliseconds. So, any similar signal that appears in both detectors more than 10ms apart is also ignored, since it could not possibly have been caused by a passing gravitational wave.
Another advantage to placing the detectors so far apart is to aid in localizing the source of the GW on the sky. While at least 3 detectors are needed to 'triangulate' the signal (like a cellphone's location can be triangulated by 3 or more cellphone towers), two is enough to begin to narrow-down the possible direction-of-arrival of GW signals. LIGO then shares skymaps with electromagnetic (EM) astronomers who can scan the skies with their telescopes in case some light was emitted by the GW events. Of course, the more GW detectors that exist around the world, the narrower any source can be localized, a factor which was fundamental to the LIGO-Virgo historic detection of GW from colliding neutron stars in August, 2017. In that case, data from LIGO's two detectors, the Virgo GW detector in Italy, and two orbiting gamma ray observatories were combined to generate a skymap showing the region of the sky where the event had occurred. Astronomers in Chile were the first to find and image the galaxy that hosted the source of the GW and EM signals. That event became the most studied astronomical event in human history (so far!)
Logistical Challenges
The sheer size of the interferometers, their extreme sensitivity to vibrations, and the need to separate the detectors by thousands of kilometers presented significant challenges when deciding where to place the observatories. The sites weren't selected individually, but in pairs. Finding one site large enough to build the instrument and its facilities in a reasonably remote location would be challenging enough; finding two such sites at the same time was an entirely different prospect.
With population growth and urban sprawl, there are few places left where:
- a huge plot of land can be reserved for a massive science experiment requiring a lot of empty space around it,
- the local population density is reasonably low to minimize anthropogenic (human-generated) noise like traffic and farming activities, and
- the infrastructure (e.g., electricity, water) required to run the facility is within reasonable reach.
Overall, just as astronomical telescopes are built far from city lights that pollute the night sky with an obscuring fog ("light pollution"), gravitational-wave observatories need to be kept as far away as possible from the vibrations caused by human activity. Such vibrations can drown out the telltale signals of gravitational waves in a sea of noise, just as light pollution drowns out the fragile light of distant stars. In the end, the desert of eastern Washington, and the forests of Louisiana were chosen as the locations of LIGO's two detectors.
Aerial photograph (taken in 2023) of LIGO Hanford Observatory showing the scale of the instrument and the locations of the "Corner Station" (where the laser is generated) and one arm's "End-Station", where the all-important test-mass mirror resides. Note that the arm is so long that the perspective distorts the distance between the Mid- and End-Station. (Credit: Caltech/MIT/LIGO Lab)
