LIGO's Dual Detectors
Why Two Detectors?
LIGO is made up of two identical and widely separated interferometers situated in somewhat out-of-the-way places. LIGO Hanford in southeastern Washington State is in an arid, shrub-steppe region crisscrossed by hundreds of layers of ancient lava flows covered by rolling sand dunes left behind from the Ice Age floods. LIGO Livingston, 3002 km away, is situated in the completely opposite environment of a warm, humid, loblolly pine forest east of Baton Rouge, Louisiana.
There are two main reasons for the wide separation between the interferometers: Local vibrations, and gravitational wave travel time.
First, LIGO’s detectors are so sensitive that they can sense the tiniest vibrations on the Earth from local, nearby sources to big events occurring hundreds or thousands of miles away (in the case of large earthquakes). Earthquakes, 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, and it would be nearly impossible to distinguish a gravitational wave signal from a non-gravitational wave signal in the data. Being separated by 3000 km means that each detector experiences its own unique local vibrations; what LLO senses from its local environment will be completely different from what LHO senses. 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 3000 km 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 farther apart in time than this is also ignored, since it could not possibly have been caused by a passing gravitational wave.
In these ways, different signals and those occurring more than 10 ms apart are automatically filtered out, leaving only 'coincident' signals from gravitational waves as the stand-outs.
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 to LIGO designers when deciding where to place the observatories. The sites weren't selected individually, rather they were selected 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.