How LIGO Detected Gravitational Waves

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.”

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

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.

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 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.

What Kind of Noise Annoys an Interferometer?

But it gets more complicated. Everything from passing trucks to distant ocean waves can shake the mirrors, causing “noise” that muddies the measurements. That’s why LIGO isn’t just one machine, but a pair of facilities with identical-twin designs, one located in Livingston, Louisiana and the other Hanford, Washington. By putting almost 2,000 miles between the two instruments, physicists can discriminate between local jiggling, which will only be felt by one detector, and authentic cosmic gravitational waves, which should be felt by both.

LIGO's Hanford, Washington, facility

Still, picking out the tiny signal expected from a gravitational wave from mundane background vibrations is like trying to hear the crickets chirping at an AC/DC concert. The trick is to isolate LIGO’s mirrors from external shaking as perfectly as possible. To do that, Advanced LIGO has a completely revamped isolation system that exploits seven different layers of technology to effectively “float” the optics. For LIGO’s first run, the mirrors were hung from simple pendulums. This time around, the mirrors are heavier, and each one is suspended from welded glass fibers that hang from a quadruple pendulum—that is, a pendulum that hangs from a pendulum that hangs from a pendulum that hangs from a pendulum. This quadruple pendulum system is great at cushioning the mirrors from very high frequency vibrations.

“Though LIGO’s suspension system is much more complex than a car’s, it operates on a similar principle,” says MIT research scientist Fabrice Matichard, who leads design and testing of Advanced LIGO’s seismic isolation system. “When you go very fast, the wheels follow the bumps, but the wheels are decoupled from the frame,” so you feel a smooth ride. With LIGO, it’s like that, but times four.

To handle lower-frequency tremors, Advanced LIGO is using a new, “active” isolation system that senses and corrects for vibrations in real-time. Outside the LIGO vacuum chamber, a hydraulic system neutralizes the slow swaying coming from distant effects like tides. Inside the vacuum chamber, another set of seismometers triggers two stages of magnetic actuators that counteract mid-frequency vibrations. This active isolation system also keeps in check the Achilles’ heel of the pendulum system: its inconvenient tendency to resonate at certain frequencies, making the internal vibrations stronger instead of weaker.

A member of the pendulum suspension system

Even with all this noise-canceling technology, today, much of the day-to-day business of operating LIGO is trying to understand all the background noise that it picks up. And now, after eight years spent acclimating to the standard noise of the first LIGO run, physicists find themselves back at the beginning.

“The instrument is so different” after the upgrade, says LIGO spokesperson Gabriela González, a professor at Louisiana State University, that the “noise looks very different—in a good way.” In fact, the very same active seismic isolation system designed to keep the noise out can introduce new noise as it pushes against the mirrors, noise that in the past would have been imperceptible. Like hunters skilled at finding their prey against one landscape, LIGO scientists must now relearn how to pick out their quarry in a totally new environment.

The Thrill of the Chase

Which is where physicists like Rob Owen come in. “You have to know what you’re looking for before you can check whether it’s in the data,” says Owen, an assistant professor at Oberlin College who is also part of the Simulating Extreme Spacetimes (SXS) collaboration.

Owen and his colleagues use computer simulations to solve the equations of Einstein’s general relativity and predict what different sources of gravitational waves will look like to LIGO and other gravitational wave detectors. Computer codes like theirs and those being developed in other labs show how gravitational waves should ring out from a panoply of exotic cosmic collisions: black holes merging with black holes, neutron stars and white dwarfs corkscrewing into each other, and every combination thereof. Their work will help LIGO scientists find what they’re looking for in a forest of data.

These output mode cleaners help improve the quality of laser light.

Before today’s announcement the smart money was on neutron stars as the source of the first signals. Neutron stars are the “bread and butter” of LIGO, González says. Less camera-shy than black holes, neutron stars—ultra-dense, fiercely magnetized stellar corpses left behind by supernova explosions—have already been observed by radio telescopes in death-spiral orbits that will ultimately lead them to crash and coalesce, perhaps birthing a black hole in the process. In fact, some of the strongest indirect evidence for gravitational waves had come from observations of a pair of neutron stars known as the Hulse-Taylor pulsar, named for its discoverers, who also won a Nobel prize for their work.

Astrophysicists guesstimate that a single neutron star pair merger happens in our galaxy every 10,000 years or so. “That’s too long to wait,” González says, but by expanding LIGO’s reach to include hundreds of thousands or even millions of galaxies, we can tip the odds in our favor. Right now, LIGO should be able to pick up the signal of a garden-variety neutron star merger if it happens within about 200 million light years of Earth. (The Milky Way, for comparison, is about 100,000 light years across.) As LIGO scientists better understand the sources of noise in the LIGO system, they expect to be able to extend LIGO’s reach to some 650 million light years. At that point, González estimates, LIGO should be picking up tens of events each year.

Fortunately, the expanded search technique also applies to the short wavelength gravitational waves from binary black holes, which LIGO is most attuned to detecting. The observatory is most sensitive to small black holes, but there was a chance—which was proven correct—that it could sense in the range of tens or hundreds of solar-masses. Those have proved almost totally resistant to observation with other kinds of observatories. “We know next to nothing about these black holes,” González says.

The End of the Beginning

Ultimately, LIGO won’t be searching alone. Already, the LIGO group is working in partnership with the European GEO and VIRGO gravitational wave detectors, with plans for collaborations with new instruments, like LIGO-India and the Japanese KAGRA project, in development.

Furthermore, just as traditional telescopes are only sensitive to a certain range of light wavelengths—ultraviolet, say, or radio—LIGO and similarly sized detectors are only built to pick up high-frequency gravitational waves. That means that while LIGO can detect gravitational waves from merging stellar-mass black holes and possibly neutron stars, they are probably deaf to a thick slice of the gravitational wave spectrum, including the low-frequency waves physicists think should be thrumming out from colliding supermassive black holes.

LIGO's test mass compensation plates help control for thermal lensing, or the tendency for heat to distort key components.

Two decades from now, an orbiting gravitational wave observatory called eLISA may be able to pick up these low-frequency waves. But in the meantime, far from the big-ticket fanfare, radio astronomers have been developing a clever, bargain-basement technique to try to catch these waves using the “instruments” that nature has provided: millisecond pulsars, rapidly-rotating neutron stars that issue precisely-timed beams of radio waves. By clocking the arrival times of pulsar pulses from across the galaxy, radio astronomers can analyze how space-time is “waving” under the influence of gravitational waves.

The LIGO collaboration also has agreements with 30 different astronomy groups, each poised to swiftly follow up on any future gravitational wave detection with X-ray, ultraviolet, radio, and visible-light telescopes. They are playing a sort of celestial matching game, trying to spot a transitory light source that coincides with the gravitational wave signal. They want to see what LIGO hears.

But it isn’t easy. The problem: LIGO gives terrible directions. Together, the twin LIGO detectors can only localize a gravitational wave source to a patch of sky that’s many hundreds of square degrees. In a field that large, other telescopes are practically guaranteed to find some now-you-see-it-now-you-don’t object. Telling the difference between a true match and a coincidence may be impossible. A decade from now, González and her colleagues hope, LIGO will be just one part of a network of detectors that can provide the kind of localization electromagnetic astronomers need to make confident matches.

For now, though, LIGO—one of the most audacious astrophysical experiments—has proven that gravitational waves exist and that we can detect them. “We couldn’t make a baby step,” Larson says. “We had to do the big experiment to get the data at all.” Which is exactly what they did.