Science and engineering are inextricably linked.  Oftentimes scientists have an idea, experiment or goal in mind, but they turn to engineers to design and construct the necessary tools. The bigger and more complex the project, the greater the engineering challenges there are to overcome.

One example that cropped up in the news last year is LIGO and its successful detection of gravitational waves.  This is exciting science, but engineering was integral to the project’s success.

In this video from the STEM-education-focused USA Science and Engineering Festival in 2016, ENGINEERING.com had the opportunity to speak with Martin Hendry from the University of Glasgow about what LIGO is, how it works, and the engineering challenges involved in building a gravitational wave detector. These included building equipment sensitive enough to detect the right signals, while also being seismically isolated in a way that screens out signal noise from local disturbances.

What is LIGO?

LIGO stands for Laser Interferometer Gravitational-Wave Observatory. It is the world’s largest gravitational wave observatory, comprised of two giant laser interferometers located in Hanford, Washington and Livingston, Louisiana.

“Gravitational waves are ripples in space-time,” explained Hendry. “Einstein predicted that gravitational waves should exist in the universe, formed when violent events like exploding stars or colliding black holes occur.”

When these violent cosmic events happen, they send ripples out through space-time in the same manner as throwing a stone into a pond will create ripples across the water’s surface.

“What LIGO does is use lasers to measure the stretching of space caused by the passing gravitational wave.  But the amount that space will typically stretch is something like a million millionth the width of a human hair, which is why the LIGO detectors are so big.”

Both detectors are L-shaped, with four-kilometer-long arms.  Laser beams pass between mirrors back and forth along the arms, and this distance provides the necessary sensitivity to detect when a passing gravitational wave stretches and squashes space-time between the mirrors at either end.

Overcoming the Engineering Challenges of Sensitivity and Local Disturbances

With the arms of the detectors being four kilometers long, Hendry explained, “The major engineering challenge is to make sure that the laser beams are perfectly collimated, which essentially means going in a straight line. The laser beams bounce back and forth many times, so there are many things that can disturb the laser beams, such as scattering of the laser light by atoms and molecules in the air."  

“So the tubes, these four kilometer long tubes, are held under vacuum.  It’s about one billionth of an atmosphere in there, just a phenomenally low pressure environment,” Hendry said.

The other engineering challenge involved devising a way to minimize the effect of local ground disturbances on the detectors.  “Many of the local disturbances would naturally be much bigger than the signal we’re trying to detect.  So, what you’ve got to do is try and isolate the mirrors that the lasers are bouncing off of, as best you can, from all those local disturbances.”

The LIGO equipment for seismic isolation involves a series of weighted pendulums. “What we’ve got is that the mirror that the laser actually bounces from is suspended in a rig the involves four stages.  What that does is help to isolate you from any of the ground disturbances, especially at lower frequencies,” Hendry said.

“You can do this demonstration for yourself,” he continued. “If you take some yo-yos, and you tie them together. You can shake your hand at the top, and the first yo-yo will move a lot, but further down the chain they won’t move as much because they become isolated from the disturbance by the multiple stages of the pendulum.”

To learn more about the engineering of LIGO, watch the video above or visit the website for the LIGO Scientific Collaboration.