ut in two steps. First, the team built initial interferometers, which operated from 2002 to 2010, at a sensitivity that Barish characterized as being at a level where detections were "possible." This first step demonstrated the observatory's basic concepts and solved many technical obstacles. The development and approval of the next phase of LIGO, called Advanced LIGO, was also led by Barish and then-LIGO Laboratory deputy director Gary Sanders, and was designed to be sensitive to a level at which detections were "probable." Advanced LIGO was commissioned and built between 2010 and 2015. Though Barish left LIGO in 2006 to become director of the Global Design Effort for the International Linear Collider, he would rejoin the LIGO team in 2012, in time for the project's historic discovery in 2015. After Barish left, LIGO was led by Jay Marx of Caltech, followed by current executive director, Caltech's David H. Reitze.
"LIGO had to make the change from tabletop science to a real science facility," says Whitcomb. "Barry understood what was needed, and he guided that transformation without ever losing sight of the scientific goals."
Under Barish's leadership, several key technologies were developed that ultimately led to the detection of gravitational waves. For the first phase of LIGO, now referred to as Initial LIGO, he chose to use solid-state lasers rather than the gas lasers that were more commonly in use at that time. These solid-state lasers were the basis of more powerful versions developed for Advanced LIGO. He also oversaw the development of technologies for reducing unwanted movements in LIGO's mirrors, caused by earthquakes, passing trucks, and other ground vibrations.
"In the initial phase of LIGO, in order to isolate the detectors from the earth's motion, we used a suspension system that consisted of test-mass mirrors hung by piano wire and used a multiple-stage set of passive shock absorbers, similar to those in your car. We knew this probably would not be good enough to detect gravitational waves, so we, in the LIGO Laboratory, developed an ambitious program for Advanced LIGO that incorporated a new suspension system to stabilize the mirrors and an active seismic isolation system to sense and correct for ground motions," says Barish.
The active seismic isolation system developed for Advanced LIGO works in a similar fashion to noise-canceling headphones, except it can measure and cancel out ground vibrations coming from many directions. In conjunction with this system, a new "quieter" way to suspend LIGO's mirrors was developed with the help of the Glasgow group, which involved hanging the mirrors with a four-stage pendulum. The combination of these two advances gave LIGO a huge improvement in sensitivity to lower frequencies of gravitational waves, which was ultimately what was needed to detect the crashing of two black holes.
Barish also created the LIGO of today: a collaboration of approximately 1,200 scientists and engineers at about 100 institutions in 19 nations called the LIGO Scientific Collaboration (LSC).
"In addition to picking the right technologies and developing them, and securing funding, we needed to build a collaboration of the absolute best people possible for this almost impossible project," says Barish. "Forming an international collaboration, the LSC, enabled this. We attracted the best people from other universities and countries, creating an 'equal opportunity' collaboration, where there was no advantage to being at Caltech or MIT." The LSC conducted the scientific searches and analysis that led to the LIGO discovery.
While this experimental work was taking place, theorists outside Caltech, MIT, and the LIGO project were developing computer codes to simulate the massive collisions of black holes and other sources of gravitational waves that LIGO might detect. These simulations are essential to LIGO; by comparing the shapes of the waves that LIGO observes with the simulations' predicted wave shapes, LIGO scientists can figure out what produces the observed waves. In the early 2000s, Thorne 