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LIGO upgrade
Detector engineers Hugh Radkins (foreground) and Betsy Weaver (background) take up positions inside the vacuum system of the detector at LIGO Hanford Observatory to perform the hardware upgrades required for Advanced LIGO’s third observing run. (LIGO / Caltech / MIT Photo / Jeff Kissel)

Physicists won’t be fooling around on April 1 at the Laser Interferometer Gravitational-Wave Observatory in Washington state and Louisiana, or at the Virgo gravitational-wave detector in Italy.

Instead, they’ll all be bearing down for the most serious search ever conducted for signs of merging black holes, colliding neutron stars — and perhaps the first detection of a mashup involving both those exotic phenomena.

Both experiments have been upgraded significantly since their last observational runs, resulting in a combined increase of about 40 percent in sensitivity. That means even more cosmic smashups should be detected, at distances farther out. There’s also a better chance of determining precisely where cosmic collisions occur, increasing the chances of following up with other types of observations.

“With our three detectors now operational at a significantly improved sensitivity, the global LIGO-Virgo detector network will allow more precise triangulation of the sources of gravitational waves,” Jo van den Brand, a Dutch astronomer who serves as the spokesperson for Europe’s Virgo Collaboration, said today in a news release. “This will be an important step toward our quest for multi-messenger astronomy.”

Multi-messenger astronomy involves looking at the same source with a wide variety of instruments, focusing on different electromagnetic wavelengths plus whole new ways of looking at the universe. That’s how the first observation of a neutron star merger was made in 2017.

LIGO’s detections of black holes have already won a Nobel Prize in physics for three of the project’s leaders, and who knows? There could well be future Nobel-worthy discoveries waiting to be made during the yearlong run that’s due to begin April 1. For example, physicists haven’t yet detected the gravitational-wave signature that should accompany the collision of a black hole and a neutron star.

This will be the third observing run for the Advanced LIGO program, and the first run since the LIGO detectors were shut down in August 2017 for major upgrades.

LIGO’s detectors in Hanford, Wash., and near Livingston, La., look for subatomic-scale ripples in the fabric of spacetime that are caused by gravitational-wave disturbances generated many millions of light-years away.

The ripples are measured by looking for interference patterns in laser beams bouncing back and forth between mirrors in an L-shaped network of 2.5-mile-long tunnels. LIGO’s two detectors are placed more than 1,500 miles apart to serve as a double-check for each detection. The Virgo detector in Italy provides a triple-check and makes it possible to figure out where in the sky a gravitational-wave burst is coming from.

For the upcoming run, the laser power has been doubled, and most of the mirrors in the detectors have been replaced with better-performing equipment.

“We had to break the fibers holding the mirrors and very carefully take out the optics and replace them,” said Calum Torrie, LIGO’s mechanical-optical engineering head at Caltech. “It was an enormous engineering undertaking.”

LIGO’s team also took advantage of quantum physics to improve the signal-to-noise ratio for gravitational waves. The upgrades employ a technique called “squeezing” to shift the uncertainty caused by random fluctuations of photons in the detector from the phase of the light waves to their amplitude. That’s a neat trick, because measuring the phase of the light waves is what’s key to detecting gravitational waves. Measuring the amplitude is less crucial.

As a result of the upgrades, LIGO should extend its range for detecting neutron star mergers from 360 million light-years to an average of 550 million light-years.

“One of the things that is satisfying to us engineers is knowing that all of our upgrades mean that LIGO can now see farther into space to find the most extreme events in our universe,” Torrie said.

LIGO is funded by the National Science Foundation and operated by Caltech and MIT, with nearly 1,300 scientists from around the world on the LIGO Scientific Collaboration. The Virgo detector is hosted at the European Gravitational Observatory in Pisa, Italy, and is funded by research centers in France, Italy and the Netherlands. The Virgo Collaboration has about 350 scientists, engineers and technicians from institutes in Belgium, France, Germany, Hungary, Italy, the Netherlands, Poland and Spain.

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