LIGO detects gravitational waves by splitting a powerful infrared laser beam in two, then sending the beams at right angles through tunnels to mirrors 2.5 miles away. The beams are recombined upon return. A gravitational wave will warp space and briefly change the relative distance between the mirrors and photo detector situated near the LIGO control room. The difference is astonishingly small, just 1/10,000 of a proton’s diameter, but it can be detected if the mirrors are isolated from all external sources of vibration.
Discover photo editor Ernie Mastroianni visited the facility in November as physicists and engineers were calibrating equipment.
Massive stainless steel tubes, vacuum equipment, and seismic isolation gear are prepared for installation at the corner station of the Laser Interferometer Gravitational-wave Observatory (LIGO) detector in Hanford, Washington. The facility is a near twin of the Livingston detector.
To insure the beam’s integrity, the laser travels through sealed stainless steel tubes, 1.2 meters wide, that hold a vacuum to just one trillionth of earth’s atmosphere, eight times less than open space. This vacuum, says LIGO spokesman William Katzman, has been maintained since 1999. It is the largest sustained ultra-high vacuum in the world and is necessary to prevent any air currents from deflecting the laser’s path.
Astrophysicist Stuart Aston monitors external vibrations on the LIGO test mass mirrors during an engineering run in November 2016. Aston's job is to keep optical components isolated from external vibration, and he was not happy as he scanned the data from LIGO’s control room.
“The multi-stage suspensions provide incredible levels of isolation from ground motion,” said Aston, but it wasn’t happening on this day. Less than a half-mile away, a logging crew was plowing a path through the forest, creating massive ground vibrations and swamping the gear that normally nullifies the noise.
Aston was soon driving the 2.5-mile distance to the detector’s far end to fix the glitch.
Janeen Romie, the LIGO engineering group leader, coordinates the efforts of her colleagues as they calibrate instruments and attempt to lock the laser on the target mirrors. She troubleshoots problems with alignment lasers and the mirrors that direct their paths.
“There are actually 11 kinds of lasers that are part of the detector system,” she said, for such uses as optical levels, fiber welding, thermal compensation and photon calibration.
The X-arm of the LIGO detector stretches 2.5 miles into the southern Louisiana wilderness. When the detector is locked and running, researchers who drive this road are required to stay under 10 miles per hour to minimize vibrations.
“It takes an eternity,” said group leader Romie. A concrete tunnel protects the beam tube from the elements and occasional stray bullets from hunters.