Last week, a new era of astronomy began as Laser Interferometer Gravitational-Wave Observatory executive director David Reitze stood before a packed room at the National Press Club in Washington, DC, uttering the words we’ve been waiting a century to hear: “We’ve detected gravitational waves.” Applause erupted from the room and all across the globe as universities, individuals, and institutions were watching via a live feed.
LIGO is a $1 billion international collaboration with nearly 1,000 scientists working together, hoping to catch a glimpse of the enigmatic cosmic phenomenon known as gravitational waves. 100 years ago, Einstein first predicted the existence of gravitational waves as part of his theory of general relativity, which says that space and time are not two separate entities, but part of a dynamic, interwoven fabric called space-time.
Like ripples on a galactic pond, gravitational waves are pulsating perturbations or ripples in the very fabric of space and time, caused by the motion of massive objects. As they propagate through the Universe, they stretch and squeeze objects on a subatomic scale. What makes this so incredible is it allows us to view the Universe in an entirely new way. By opening this new door to the Universe, we may be able to solve some of its biggest mysteries.
"The information carried on the gravitational wave is exactly the same as when the system sent it out; and that is unusual in astronomy. We can't see light from whole regions of our own galaxy because of the dust that is in the way, and we can't see the early part of the Big Bang because the Universe was opaque to light earlier than a certain time," Professor Bernard Schutz of Cardiff University explained. "With gravitational waves, we do expect eventually to see the Big Bang itself."
We’ve had very good circumstantial evidence for the existence of these elusive waves since the 1970s, but astronomers weren’t satisfied. They wanted to see the waves for themselves. Last autumn, their wish came true. After decades of searching, a pair of interferometers picked up the first direct signal of gravitational waves.
Here’s how it happened: At 5:51 AM on September 14, 2015, LIGO’s twin detectors—one in Livingston, Louisiana and one in Hanford, Washington—both picked up a minute signal, or chirp, within milliseconds of each other. The recorded sound waves very closely matched what models predicted. In fact, the signals were so good that scientists thought it might be too good to be true. They spent months checking and rechecking the data, before finally concluding that the signal was real.
“This is why there are two detectors and why they are so far apart. So if you see the same thing in the two detectors, you know it's not an artifact,” explained Gabriela González, LIGO spokesperson and astronomy professor at Louisiana State University. “You know it's not a local disturbance. It’s a gravitational wave.”
The tiny chirp is like an audio fingerprint, providing scientists with a lot of information about what created these galactic ripples. The shock wave heard today is the result of a massive collision between two black holes. These cosmic heavyweights, each with a mass about 30 times that of the Sun, were locked into an orbital dance. The duo spiraled around each other, until eventually merging into one, even more massive black hole.
LIGO researchers explained during a press conference that in the final seconds of the merger, pure energy about 50 times more powerful than the output of all the stars in the Universe was emitted in the form of gravitational waves. This powerful shockwave went rippling through the Universe at nearly light speed, where it was picked up by LIGO 1.3 billion years later.
“Until now, we scientists have only seen warped space-time when it’s calm,” Dr. Kip Thorne explained yesterday. “It’s like only seeing the ocean’s surface on a calm day but had never seen it in the midst of a storm, with crashing waves. That changed on Sept. 14, as the colliding black holes responsible for these waves, created a violent storm in the very fabric of space-time.”
Two black holes collide in a computer simulation from LIGO. Image: LIGO
“Scientists have been looking for gravitational waves for decades, but we’ve only now been able to achieve the incredibly precise technologies needed to pick up these very, very faint echoes from across the universe,” he said. “If Einstein had access to this type of technology, I’m certain he would have made the same detection.”
This discovery is huge, as it not only represents another test that Einstein’s famous theory has passed, but it’s also the first proof scientists have that binary black holes exist. Until now they had been purely theoretical: meaning no one knew for sure that when two black holes combine, they form an even larger black hole. What’s even more incredible is this discovery came about during an engineering run of the newly updated LIGO observatories. The Advanced LIGO — which works by bouncing lasers back and forth in two 4-kilometer-long L-shaped arms, allowing scientists to measure incredibly tiny changes in space-time — is not yet at full sensitivity, and when it is, scientists expect to see many more events like this.
With this discovery, LIGO is transforming how we view the Universe. 400 years ago, Galileo changed the field of astronomy when he turned a spyglass up to the heavens. By observing planets and moons, he kicked off the era of modern optical astronomy. Then in the 1960s, the field of astronomy was changed again with the discovery of radio waves and the big bang.
“This detection ushers in a new era of astronomy,” González said during a talk on the discovery. “The field of gravitational wave astronomy is now a reality.”
Gravitational wave astronomy is fundamentally different from traditional astronomy, which is predominantly based on light. The light we see is an electromagnetic wave, meaning a vibration or ripple in electric and magnetic fields. On the other hand, gravitational waves are ripples in the very structure of space-time. Traveling at the speed of light, these vibrations can produce sounds we can actually hear.
“We can hear gravitational waves,” González said. “We can hear the universe. That’s one of the beautiful things about this. We are not only going to be seeing the universe. We are going to be listening to it.”
Just like with light and the electromagnetic spectrum, gravitational waves and the tones they produce vary across an entirely different spectrum. LIGO is tuned to listen for specific frequencies of gravitational waves, mostly higher pitched waves in the kilohertz range. Different types of observatories are in the works to detect the full range of gravitational waves.
“First detection is very important in terms of fundamental physics because of what it says about gravity, but it also opens a window onto what has previously been the dark universe,” said Avery Broderick, a physicist on the LIGO team. “For centuries, astronomers have been looking up at the night sky and thinking about the light side of the Universe. Now we’re going to get our first look at the dark side. There’s every expectation that it will be just as rich and exciting.”
This probably won’t change your day to day life unless you’re an astrophysicist, but it could help us answer one of the Universe’s biggest mysteries: where did we come from? By studying gravitational waves, we’ll be able to study things that we would never be able to see in any kind of detail or maybe even at all. With this type of technology, scientists will be able to observe gravitational waves from the early Universe, and in turn, better understand how our Universe began.