In deep space, two black holes spiraled toward each other, their tremendous mass warping spacetime and propagating gravitational waves across the fabric of the universe at light-speed. The two black holes eventually crashed into one another and merged into one even bigger black hole, emitting a crescendo of waves.
That quiet tremolo on the catgut of reality made it to Earth, where the Laser Interferometer Gravitational Wave Observatory was listening. For 13 years LIGO heard, it seemed, every vibration but the one it was supposed to. But on September 14, 2015 it detected those black-hole-crashing swells as they washed over the planet. “Ladies and gentlemen, we have detected gravitational waves,” David Reitze, LIGO’s Executive Director, declared today at a press conference. “We did it.” This is big-deal physics, a long-awaited bit of evidence that vindicates the work of Albert Einstein, opens a new scientific field, and gives astronomers a peek at a side of the universe they’ve never seen.
Decoding the data gave it more specificity. The waves came from black holes with 26 and 39 times the mass of the sun, respectively. Merged, the newly created black hole had 62 times the mass of the sun. Right, that arithmetic doesn’t exactly work, but don’t worry about it. The newly-formed body emitted energy to stabilize, a process called “ringdown.” That energy, emitted as gravitational waves, came from three suns’ worth of stuff. (Energy can turn into mass and vice versa—thanks to an equation you might have heard of: E=mc2.)
Now, it’s true, LIGO has had a few false alarms. But this time, the astronomers are pretty sure—with a 99.99994 percent confidence. “This marks the inauguration of the era of gravitational wave astronomy,” says Xavier Siemens, a LIGO team member from the University of Wisconsin-Milwaukee.
It’s an era that has been a long time coming. A century ago, Einstein’s theory of general relativity predicted that when anything with mass accelerates, it should make waves in spacetime, like a boat doing donuts sends waves lapping toward shore. But Einstein famously believed that humans would never be able to detect gravitational waves. They were too weak, he thought, Earth-made instruments too feeble.
Well, Einstein, welcome to the future. You’re welcome.
Besides making Einstein look more right than ever, detecting gravitational waves ought to let astronomers get a better look at dark, distant, supermassive, fast-moving, cataclysmic stuff using mass and movement instead of mere light. “We’ve been studying the light side of the universe for the past 10,000 years,” says Avery Broderick, an astrophysicist at the University of Waterloo and Perimeter Institute. “LIGO is going to begin the process of studying the dark side.” (Broderick wasn’t part of the LIGO collaboration.) What that might mean in practice is seeing what the universe was like in its infancy, what gravity is like at its most extreme, and how hyper dense matter behaves.
Because of how much it will change our view of the universe, theoretical physicist Lawrence Krauss of Arizona State University in Tempe compares LIGO’s detection to the invention of the telescope. “It will certainly be a Nobel-prize-winning discovery,” he says. “But more important is that they opened a new window on the universe.” Krauss has already shown himself to be more than a little excited about this. In September, he sparked rumors of this very discovery with a tweet:
Rumor of a gravitational wave detection at LIGO detector. Amazing if true. Will post details if it survives.
— Lawrence M. Krauss (@LKrauss1) September 25, 2015
Which he followed up in January with:
My earlier rumor about LIGO has been confirmed by independent sources. Stay tuned! Gravitational waves may have been discovered!! Exciting.
— Lawrence M. Krauss (@LKrauss1) January 11, 2016
People did indeed stay tuned, and the buzz increased. As the date of the announcement—which was itself supposed to stay secret—approached, leaks of varying merit began to spring. Some said a paper was going to come out in Physical Review Letters, others in Nature. Theoretical physicist Clifford Burgess of McMaster University in Hamilton, Canada, sent an email to his entire department—a screenshot showed up on Twitter—proclaiming that “spies” had seen the detection paper and told him the discovery’s specifics, which he then broadcast.
But during these heady days, an anonymous source warned TechInsider that some of rumored details were “laughable.” And Chiara Mingarelli, an astrophysicist at the California Institute of Technology, unaffiliated with LIGO, told me that some circulating specifics were “dead wrong.” No one knew who, or what, to believe. Most scientists who had seen the paper held their secrets and identities close, like true spies. Broderick’s ahead-of-time information came as if out of a Cold-War espionage movie. “Somebody saw a paper lying out and took photos, and I’ve seen photos of the paper,” he says. “I’m not sure if he used his matchbox camera or what.”
But even as excitement grew among the astrophysics community and its fans—what? We’re fans—the LIGO team, more than a thousand people, made no waves themselves. No official news, or hints of it, came. The silence, says Vallisneri, was simply because the LIGO team wanted to be sure about their results before they threw a public party. After all, nobody wants to claim they have “opened a new window on the universe” only to have to slam it shut on their own fingers.
Now it seems the window is open for good. “It really does look like they saw the perfect, ideal situation—so bright, a nice simple system,” says Broderick. “It really makes it a very strong case. There’s no wiggle room.” The signal from the merging black holes showed up at both of LIGO’s sites, in Livingston, Louisiana, and Hanford, Washington. The arrival times were ever-so-slightly different, with a tiny delay that matched the light-speed travel time between the two locations. Both of the detectors have two 4-kilometer-long arms that meet at 90 degrees, like a giant L. Mirrors are mounted at the far side of each arm and at the corner where the arms meet. Laser beams blast down the arms, pinball back and forth between the mirrors 400 times, and finally meet up at a detector that sits at the elbow.
If nothing interesting is happening, the lasers’ light waves exactly cancel each other out when they reach the detector. But when gravitational waves pass by, spacetime gets bent. One of the arms gets shorter and the other gets longer. That shift shows up as an “interference pattern” in the lasers’ light waves.
Plucking that pattern out of noise was no simple feat, but LIGO scientists welcomed the challenge. “It is not unlike going to the Moon,” says Vallisneri. “Part of it is that you want to see what’s where, and part is to push the limits what we can do.”
While this signal is the first direct detection of gravitational waves, it’s not the first evidence of their existence. That came in 1974, when astronomers Russell Hulse and Joseph Taylor discovered two pulsars in a binary orbit and watched as the pulsars lost energy and spiraled toward each other. The drop in energy precisely matched the amount they would shed if they were emitting gravitational waves. This indirect detection earned Hulse and Taylor the Nobel Prize in 1993. “But the direct detection of gravitational waves offers so much more to the community,” Mingarelli says.
Now astrophysicists can actually use the waves to do science. “You can observe the Hulse-Taylor binary all you like and find it to be decaying all you like, but you can’t do anything else with it,” Siemens says. By recording the way gravitational waves interact with a physical system, LIGO lets astronomers probe into the details and “paint a portrait of the sky using gravitational waves.
That portrait is probably more detailed than even today’s announcement suggests. “I expect if they’ve seen an event that is bright enough to claim detection, they’ve probably seen lots and lots of events that they just weren’t sure enough about,” Broderick says. Those detections either already have or will soon change the gravitational game. Astronomers who study them are about to change from hunter-gatherers to explorers.