In 2015, a century-old prediction by Einstein was finally proven correct, as gravitational waves were detected for the first time. Now, the facilities behind this discovery – the Laser Interferometer Gravitational-Wave Observatory (LIGO) – are back up and running after a year-long upgrade, with a few new tricks up their sleeves.
Gravitational waves are ripples through the very fabric of spacetime, set in motion by cataclysmic events like the collision of two black holes or neutron stars. But by the time they wash over us here on Earth, they're extremely tiny, distorting reality on the scale of a thousandth the width of a single proton.
To detect them at LIGO, lasers are beamed back and forth down two 4-km (2.5 mi) long arms arranged in an L-shape. Incredibly precise sensors are installed to detect even the slightest distortion in the beams, which indicate that a gravitational wave has rolled through. Collaborating with other facilities can help triangulate where the signals originated from.
The two LIGO detectors, located in Washington and Louisiana, shut down about a year ago to undergo some much-needed upgrades, and as of April 1 they're back up and running. The laser power has been doubled, and five of the eight mirrors in each facility have been replaced with better-performing ones. And finally, new "quantum noise filters" were applied to reduce the background noise that could be masking small wave readings.
All up, the new and improved LIGO is about 40 percent more sensitive than during its last run, allowing it to survey a larger volume of space than ever before. The team says the detector can also now see neutron star collisions out to an average distance of 550 million light-years, which is more than 190 million further than previously possible.
Better yet, LIGO won't be alone. The European version Virgo has also received an upgrade that essentially doubled its sensitivity, and with the two of them working together new discoveries should come rolling in.
"For this third observational run, we achieved significantly greater improvements to the detectors' sensitivity than we did for the last run," says Peter Fritschel, LIGO's chief detector scientist. "And with LIGO and Virgo observing together for the next year, we will surely detect many more gravitational waves from the types of sources we've seen so far. We're eager to see new events too, such as a merger of a black hole and a neutron star."
