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, 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 testbed used to test and spur the invention of better technologies required to make LIGO achieve its full potential.

Overall, iLIGO operated for 9 years but never detected gravitational waves. This was not unexpected, and 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 new-and-improved Advanced LIGO detectors. The construction, preparation, installation, and testing 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 ("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 move them resulting in a "recoil" effect. Unfortunately, any movement not caused by a gravitational wave is bad for LIGO. Thankfully, the principle of inertia helps reduce this problem: By increasing the size and mass of the mirrors, photon impacts are much less noticeable.

Lasers also heat the mirrors, which can change their shape, which would affect the path of the laser and renderi our ability to detect gravitational waves impossible. Here again, ‘bigger is better’: a large mirror can absorb more heat without deforming than a small mirror.

Simply 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 hang at the bottom of a 4-segment pendulum. Taking advantage of the principles of pendulums, each link in the pendulum absorbs motion from above and prevents it from reaching the link below. This is so effective in aLIGO that any vibration that is felt at the top is 100-million times smaller by the time it reaches the bottom!

Metal wires

Glass fibers

Initial LIGO used metal wires to hang the mirrors in their suspensions. Unfortunately, molecules in metal jiggle around a lot, introducing 'noise' into the mirrors themselves. To reduce this problem, aLIGO's mirrors are suspended by silica fibers since the molecules in silica are much less energetic than metal.

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.

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

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

The resulting combination of ‘passive’ and ‘active’ seismic isolation greatly reduces the vibrations reaching LIGO’s mirrors, essentially making them 'feel' like they are floating in space. You can learn more about LIGO’s seismic isolation system here, and about LIGO’s feedback systems, here.