Learning about LIGO at Astronomy on Tap

The most recent gathering of Astronomy on Tap Seattle brought to town two scientists working in one of the most groundbreaking areas of astronomy: detection of gravitational waves.

Nature was kind to us

Jeff Kissel, a control systems engineer at the LIGO Hanford Observatory, talked about how exciting it was when they switched on advanced LIGO back in September 2015.

“Boom! Right out of the gate we saw this whopper of an event,” Kissel said, detecting gravitational waves from the merger of a pair of stellar-mass black holes. “Nature was very kind to us.”
What they spotted at Hanford and at LIGO in Livingston, Louisiana was a match.

“Inside our data, which is almost always noise, we saw this very characteristic wave form that was predicted by general relativity,” Kissel recalled. They found gravitational waves from a couple of other black-hole mergers in the following months.

“This is the beginning of gravitational wave astronomy,” Kissel said.

Gravitational waves oscillate through spacetime in a way
demonstrated by this animation. Credit: ESA–C.Carreau

Kissel pointed out that LIGO only detects a small part of the gravitational wave spectrum. As with light, gravitational waves can come in a wide range of wavelengths with periods ranging from milliseconds to billions of years. Longer-length waves might come from the mergers of galactic nuclei, or even from quantum fluctuations from the early universe.

“There’s a whole zoo of things to find out there,” Kissel said. He anticipates more ground-based observatories as well as some space LIGOs that could have detector arms millions of kilometers long.

How LIGO works

LIGO sounds awfully complicated, but, broken down, the idea is pretty simple. Jenne Driggers
is a Caltech postdoctoral scholar stationed at the LIGO Hanford Observatory, where her gig is improving the sensitivity of the interferometers. Driggers explained that, essentially, they shoot a laser beam into a splitter that sends beams down two equal arms four kilometers long. The beams reflect from mirrors and return to be put back together.

A simplified look at how LIGO works. A laser beam is split and sent down two equal
arms four kilometers long, then reflected back by mirrors. When they return to be
recombined, they will usually cancel each other out and no light will get to the detector.
But if a gravitational wave distorts the system, the light will be spotted by the detector.Credit: T. Pyle, Caltech/MIT/LIGO Lab

“When they recombine they can be exactly out of phase, and then there’s no laser light (at the detector),” Driggers said. “They cancel each other out totally. Or the lengths will change and these two electromagnetic waves can add up, and so we do get some light.”

When that happens it means that a gravitational wave has distorted the LIGO arms ever so slightly. They measure the light received at the detector to learn more about the wave.

In practice it’s a lot more complicated. It all happens in a total vacuum to avoid any distortion from air. The mirrors are suspended from a system of four pendulums, which helps to eliminate vibration. The mirrors are highly reflective pieces that each weigh around 100 pounds and cost half a million dollars. The laser is about the best there is.

“The laser wavelength itself is our ruler that we’re using to measure the distance between those two mirrors,” Driggers said, “and we need to be able to measure that distance to 10^-19 meters.”

“This is one of the highest-power, frequency stable, power-stable lasers on the planet,” she added.
Driggers invited people to tour LIGO Hanford. Public tours are held twice each month, and groups of 15 or more can arrange for a private tour.

Up next: LSST

Astronomy on Tap Seattle is presented and organized by astronomy graduates students at the University of Washington. Their next event is planned for Friday, October 28 at Peddler Brewing Company in Ballard and will feature UW scientists Dr. John Parejko and Dr. David Reiss, who are working on the Large Synoptic Survey Telescope project. The events are free. Enjoy beer and astronomy!