Upgrades Hoped to Bring Better Results on LIGO’s Third Run

LIGO, the world’s largest gravitational-wave observatory, once considered a “total waste of money,” has nearly doubled its capability to detect gravitational waves and is now more sensitive than ever to the tiny ripples in spacetime generated when black holes and dead stars collide.

The broader capability is a result of numerous upgrades to the lasers, mirrors, and quantum noise filters at LIGO’s facilities in Washington state and Louisiana, making it possible to scan about twice the volume of space for smashups between two black holes or two neutron stars.

Engineers install hardware upgrades inside the vacuum system of the detector at LIGO’s Washington site. Courtesy of Jeff Kissel/LIGO/Caltech/MIT.

Robert Byer, a professor at Stanford University whose group developed LIGO’s laser source, and Brian Lantz, a senior research scientist who led the development of the isolation mount for LIGO’s mirrors, discussed the changes with Ker Than of Stanford and said the first gravitational wave detection in 2015 was met with widespread skepticism, but there have been 11 successful findings since then.

“It was not fun to deal with a broader astronomy community who were absolutely convinced this was a total waste of money,” Byer said. “It took an enormous amount of hard work and very good luck along the way for LIGO to advance to the point where it was sensitive enough to see gravitational waves.”

The first gravitational wave detected, he said, was caused by two black holes colliding with one another. When they merged, 10% of their mass was converted instantly into gravitational radiation.

“That one merger of two black holes emitted in a fraction of a second more power than all the stars in the universe combined,” Byer said.

Lantz said that would never have been discovered if LIGO had not been running.

“We get this one tiny little clue from watching the distance between two mirrors change just a little bit as the space between them gets stretched by this huge amount of power passing by,” he said.

In the early days, LIGO faced challenges from natural elements such as the moon, wind, and earthquakes, as well as from cars driving by the facilities and trains near the Louisiana site. Upgrades in Advanced LIGO addressed those issues. LIGO and its Italian counterpart, Virgo, are now on their third run, which began April 3.

“When the moon goes overhead, the ground goes up and down by about plus or minus six inches,” Lantz said. “When this bulge travels past the LIGO sites, the arms of the detectors get stretched. The isolation tables have to compensate for that motion so that the distances between the optics doesn’t change.”

Earthquakes posed an even bigger problem, Lantz said, because any time one occurs anywhere on the planet that is bigger than magnitude six, the machine becomes inoperable for several hours. It is an issue on which scientists are currently working to resolve.

“It shakes the mirrors so much, they fall out of lock,” he said.

Upgrades currently in place include the replacement of the main mirrors that were damaged and the addition of baffles to catch light that was scattered around in the machine.

“If light scatters off of the main optic and bounces around in the vacuum chamber, and then makes its way back to the detector somehow, that looks like noise for us, and that’s a problem,” Lantz said.

Byer said LIGO’s laser power has also been increased in an effort to reduce noise at a high frequency. There is a limit, however, to how much it can be increased without distorting the beam and creating quantum noise.

“To counteract that, we employ a technique called ‘squeezing’ to push the noise from one part of the measurement into another place that you don’t care as much about,” Byer said. “The squeezing of light has been an object of research since the 1980s, but it never found a practical use until LIGO came along.”