About "aLIGO"

When reading or hearing news about LIGO, you may have wondered why we often refer to the instruments as “Advanced LIGO”, or “aLIGO”. Well, LIGO has actually been around for over 20 years, and our interferometers have undergone many changes and upgrades in that time. In fact, the first round of LIGO data collection, which took place between 2002 and 2010, used what we call “Initial LIGO”. As the name suggests, initial LIGO refers to the first version of interferometers that were built at the beginning of LIGO’s quest to detect gravitational waves.

When LIGO was approved for funding in 1990, it was understood that it would likely take many years, or even decades for the observatory to reach its full potential. The first version of LIGO’s interferometers, “Initial LIGO” (iLIGO for short) would not only actively listen for gravitational waves, but would also serve as a pathfinder, a proof of concept instrument, used to test and spur the invention of new technologies required to make something like LIGO work as originally intended.

Overall, iLIGO operated for 9 years but never detected gravitational waves. Disappointing as it was, this was not entirely unexpected. The lessons learned about how to operate, maintain, and improve one of the world’s most highly technological measuring devices, were incalculable. Construction of Advanced LIGO’s (aLIGO) upgraded components began in 2008, two years before iLIGO was retired.

Initial LIGO’s duties came to an end in 2010, at which point it was disassembled to make way for the installation of the new-and-improved Advanced LIGO detectors. The construction, preparation, and installation of aLIGO took 7 years (from 2008 to 2015).

The image and table below illustrate the differences between Initial LIGO and Advanced LIGO. Ultimately, these changes will render Advanced LIGO 10 times more sensitive than Initial LIGO.

iLIGO vs aLIGO (with caption)

 

Changed component

Initial LIGO

Advanced LIGO

Impact of the change

Mirrors (aka Test Masses)

25cm (9.8in) across

10cm (3.9in) thick

11kg (22lb)

34cm (13.4in) across

20cm (7.8in) thick

40kg (88lb)

In very basic terms, LIGO is designed to measure, to the highest level of precision possible, how far apart its mirrors (we call them "test masses") are. We achieve this by using a laser. But as much as lasers are necessary, they are also problematic.

Laser photons striking the mirrors can actually move them (called, "recoil"). Unfortunately, any movement other than that caused by a gravitational wave is unwanted. Thankfully, the principle of inertia gives us a solution ot this problem: increase the size and mass of the mirrors. The heavier the mirror, the harder it is for anything (like photons) to move it.

Lasers also heat up the mirrors, which can change their shape. In LIGO, even the tiniest change in the mirror’s shape could render our ability to detect gravitational waves impossible. Here again, the solution is ‘bigger is better’: a large mirror can also absorb more heat without deforming than a small mirror.

By making LIGO’s mirrors larger, these two troublesome sources of noise are greatly reduced.

Suspension

Single pendulum

Quadruple pendulum

Initial LIGO’s test masses were suspended as a single pendulum. Advanced LIGO’s test masses reside at the bottom of a 4-segment pendulum. Starting at the top, each pendulum in the chain reduces the motion transmitted to the one below, and ultimately to the mirror.

Metal wires

Glass fibers

Initial LIGO used metal wires to hang the mirrors in their suspensions. Unfortunately, molecules in metal happen to jiggle around a lot, so much so that they can cause unwanted jiggling in the mirrors themselves. To greatly reduce this problem, LIGO engineers opted to use silica fibers to suspend aLIGO’s mirrors since the molecules in silica are much less energetic.

Seismic Isolation

Passive only

Passive + active

The term “seismic isolation” refers to the mechanisms designed to shield LIGO’s mirrors from physical vibrations caused by everything from trucks driving on nearby roads, to ocean movements, to earthquakes anywhere on Earth.

Initial LIGO used a ‘passive’ isolation system only; basically fancy shock absorbers that would absorb vibrations from the environment and prevent them from reaching the mirrors.

Advanced LIGO’s seismic isolation mechanisms were greatly enhanced. In addition to a passive system, aLIGO also uses “active” isolation systems. Here, multiple sensors monitor movement in hundreds of LIGO components and send signals to ‘actuators’ that deliberately and with great precision counteract the detected movements. This is called 'feedback'.

The resulting combination of ‘passive’ and ‘active’ seismic isolation greatly reduces the vibrations reaching LIGO’s mirrors further improving its sensitivity to the mindbogglingly small vibrations caused by gravitational waves. You can learn more about LIGO’s seismic isolation system here, and about LIGO’s feedback systems, here.