About aLIGO

Some LIGO scientists talk about “Advanced LIGO”, or “aLIGO” for short. When they do, they’re referring to the current design of the interferometers themselves. As a research facility, LIGO has actually been around for over 20 years, but the interferometers that make up the observatory have undergone many changes and upgrades. In fact, the first round of LIGO data collection, which took place between 2001 and 2010, was performed by what is sometimes called, “Initial LIGO”. 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 be a ‘pathfinder’, used to test and spur the invention of new technologies required to make something like LIGO work as originally intended.

iLIGO operated for 9 years without detecting a gravitational wave. Disappointing as it was, this outcome was not entirely unexpected. Where iLIGO was a resounding success was in the lessons learned about how to operate, maintain, and improve one of the world’s most highly technological measuring devices. With each new breakthrough in understanding and engineering, the next generation of LIGO detectors was being designed in anticipation of the conclusion of Initial LIGO operations. 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 redesign, construction, preparation and installation of aLIGO took 7 years (from 2008 to 2015).

The image below dramatically illustrates the differences between iLIGO and aLIGO. Below the figure is a table that describes in greater detail the changes that were made, the reasons for the changes, and the impact they had on LIGO's ability to detect gravitational waves.

iLIGO vs aLIGO (with caption)

 

Changed component

Initial LIGO

Advanced LIGO

Impact of the change

Mirrors

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 within the interferometer. It achieves this by using a laser. But as much as lasers are necessary, they are also problematic.

For one, laser light hitting the mirrors actually moves the mirrors (recoil)! (This is because of the quantum nature of light). And any movement other than that caused by a gravitational wave is unwanted. Thankfully, the principle of inertia gives us a solution. Greatly increase the mass of the mirrors. Why? Because the heavier the mirror, the harder it is for anything to move it.

Lasers also heat up the mirror, which can change its shape. For LIGO, even the tiniest change in the mirror’s shape could affect its sensitivity to gravitational waves. Here again, the solution is ‘bigger is better’: a larger piece of glass can take the heat better and deform less than a smaller piece of glass.

By making LIGO’s mirrors larger, two troublesome sources of noise are greatly reduced, which means that aLIGO doesn’t have to strain as much as iLIGO did to hear the faint whisper of a passing gravitational wave. To learn more about LIGO optics, click here.

Suspension

Single pendulum

Quadruple pendulum

Initial LIGO’s test mass/mirror was suspended as a single pendulum. Advanced LIGO’s test mass is suspended as the 4th in a 4-segment pendulum. Each pendulum further reduces the motion transmitted to the mirror/test mass. For more on how this works, click here.

Metal fibers

Glass fibers

Initial LIGO used metal fibers to hang the mirrors in their suspensions. Unfortunately, molecules in metal happen to jiggle around a lot; so much so that they could cause unwanted jiggling in the mirrors themselves! To reduce or eliminate 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 and active isolation

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

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. This means that multiple devices 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 a gravitational wave. To learn more about LIGO’s seismic isolation system, click here. And to learn more about LIGO’s feedback systems, click here.

 

Combined, these improvements were designed to further reduce the vibrations reaching the interferometers’ mirrors: unwanted vibrations that could drown out the exceedingly delicate signals from a gravitational wave. Overall, the improvements inherent in aLIGO’s interferometers make the observatory 10 times more sensitive than its predecessor.

We now know that the advanced detectors are so much more sensitive that within days of it operating as a fully-functional gravitational wave observatory, LIGO successfully made the world's first direct detection of gravitational waves. In other words, with the advanced detectors in place, LIGO achieved in just a few days what the initial LIGO detectors did not achieve in 9 years of operation! This speaks highly of the efforts of those who worked on initial LIGO and made critical decisions about where the instruments could best be improved.

Even with this first detection confirming that LIGO can detect gravitational waves, the current interferometers are not the final versions of LIGO instruments. LIGO engineers at MIT, Caltech, and numerous other technical partners are continuously looking for and inventing new ways to improve the interferometer’s performance. Over the next several years, the Advanced LIGO detectors will undergo further changes until the instrument reaches its expected ‘design sensitivity’ (expected around  2020). Who knows what discoveries await!