How LIGO Detected Gravitational Waves

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A visualization of two black holes merging based on general relativity’s predictions.

By Kate Becker

Kate Becker is a Boston-based science writer who was previously senior researcher for NOVA and NOVA scienceNOW.

For 13 years, the scientists of LIGO—the most ambitious, and expensive, project in the history of the National Science Foundation—had been waiting.

LIGO, the Laser Interferometer Gravitational-Wave Observatory, has been twenty-five years and more than half a billion dollars in the making. It involves 900 scientists and engineers, including many whose entire careers have been spent designing, building, and preparing to analyze data streaming in to LIGO. Their goal: To confirm, once and for all, Einstein’s century-old idea that gravity travels across space-time in the form of gravitational waves.

Well, They did.

This morning, the LIGO team announced that they had picked up gravitational waves from the collision of two orbiting black holes. “We have detected gravitational waves from a binary black hole system,” says Matthew Evans, an assistant professor of physics at MIT and a LIGO collaborator. “They’re the largest stellar mass black holes we’ve ever observed, and the first time we’ve observed such things merging together.”

The collision landed on the detectors like a half-second thump—somewhat literally. “I would say heard more than saw in the sense that gravitational waves signal is much more like audio signal than a video signal,” Evans says. “We can literally put the signal on a speaker and listen to it.”

It sounds like a happy ending, but in reality, this first detection is just the beginning of a brand new way of looking at the universe, one that will pull back the covers on some of the most extreme events in the universe, like black hole collisions and supernova explosions. We’ve been blind—but we’re about to see a lot more.

A New Way of Seeing
For the entire history of civilization, we’ve explored the universe in just one way, using light—that is, electromagnetic waves. “Everything that you and I know about the universe—every time we see a picture on the web, or in the news—it has been something that’s been learned with light,” says Shane Larson, a gravitational wave researcher at Northwestern University’s Center for Interdisciplinary Exploration and Research in Astrophysics.

For millennia, humans have used visible light, the kind that you can see with your naked eyes, to learn about the cosmos. In the last century or so, we’ve expanded that view, building telescopes that can see infrared, ultraviolet, radio, X-rays and gamma rays. Each kind of light reveals something different about the cosmos. Objects that are invisible to the eye may be glaringly bright in radio waves, or vice versa; infrared light can pierce right through dust clouds that obscure visible light.

But gravitational waves are something else entirely: they are gravity’s messengers, ripples in the fabric of space that reverberate out from the source of a gravitational disturbance. You can’t see them, but you can feel them, like you feel the wake of a passing speedboat on the water. The bigger the boat, the bigger the waves, and so it goes with gravitational waves. Really hefty objects—supermassive black holes containing as much matter as hundreds of millions of suns—make big billows in space-time. Littler things—a moon, a mouse—leave barely a quiver.

Big or small, if an object is just sitting still, its gravitational field is static and unchanging. But if the object’s motion is changing, or if its mass is being dramatically rearranged, like in an explosion or implosion, the gravitational field changes, too, and that change produces gravitational waves. A black hole sitting peacefully in isolation is silent and invisible to gravitational wave detectors, but a black hole on a collision-course spiral around another black hole should be positively shrieking. Which is exactly what the LIGO team heard.

Gravitational Wave tube

Tubes connect the LIGO optics so the laser beam can pass between them.

LIGO started listening for gravitational waves back in 2002, but, after eight years, it was shut down without recording even one unambiguous gravitational wave detection. But that was okay: Initial LIGO, as the first phase of operations was called, was, in a sense, just for practice. Last September, the real games began. Freshly upgraded with technology that didn’t even exist when the LIGO first came online, LIGO, now rebranded Advanced LIGO, is able to detect gravitational waves coming from sources some ten times farther from Earth than before.

LIGO is called an observatory, but it isn’t like any astronomical observatory you might be imagining. Each site—there are two currently—is actually a big, L-shaped tube called an interferometer. Each arm is a little more than a meter wide and extends for two and a half miles. Where the two arms meet, a 180 Watt laser beam is split in two; the twin beams travel down the arms, where they hit ultra-precise mirrors and bounce back and forth some 400 times before before meeting again at a light sensor.

Because light is a wave, the two laser beams combine to make an interference pattern of bright and dark areas, or “fringes,” at that light sensor. Analyze the fringes, and you can tell how far each light wave traveled to cover the distance between the mirror and the sensor. More importantly, you can tell if that distance is changing over time. If a gravitational wave passes by, one arm of the observatory’s “L” should be stretched while the other gets squeezed, producing a telltale change in the fringe pattern.

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