Far from our galaxy, in the vast darkness of space, two massive black holes merged into a single, larger hole.
And now researchers say they have detected rumblings from that cataclysmic collision as ripples in the very fabric of space-time itself. The discovery comes a century after Albert Einstein first predicted such ripples should exist.
"It's a really big event," says Saul Teukolsky, a theoretical astrophysicist at Cornell University. "This is probably the most exciting episode of my professional career."
Einstein predicted the existence of such ripples, known officially as gravitational waves, in 1916, as part of his general theory of relativity. General relativity re-imagines the gravitational pull between heavy objects like Earth and the sun as a "warping" of space and time. When very heavy objects such as black holes are involved, the theory predicts that gravitational waves will emerge and ripple across the entire universe.
That's the idea. But in practice, seeing such gravitational waves has been nearly impossible. To make detectable waves, massive objects must be moving quickly. Researchers predicted a collision between two black holes would do the trick. But nobody knew how often that might happen.
Nevertheless, scientists built two massive detectors to take a look. Known collectively as the Laser Interferometer Gravity-Wave Observatory (LIGO), the detectors are located in Washington state and Louisiana. They are separated by thousands of miles in order to detect ripples coming in from deep space as they pass through Earth.
Each detector looks like a big L, made up of two tunnels 2.5 miles long. It's designed so that if a gravitational wave passes by, it will stretch space along one direction of the tunnel and squish space along the direction of the other. The stretching and squishing changes the tunnels' lengths by a tiny amount, and that change can be detected by lasers.
Although LIGO was completed in 1999, it took more than a decade for it to see anything. The detectors had to be made incredibly sensitive to pick up the tiny waves. But they were so touchy, they were set off by everything from minute shifts in Earth's core to traffic entering the parking lot. And even after researchers got rid of all the terrestrial jiggles, LIGO still wasn't quite good enough to see gravitational waves.
All that changed after a major upgrade in 2014. Better vibrational isolation and upgrades to lasers and mirrors dramatically boosted the instrument's power.
And the black hole collision was seen almost as soon as the team began observing again in the fall of 2015. On Sept. 14 at 5:51 a.m., the waves passed through both of the detectors.
According to a paper published in the journal Physical Review Letters, the two black holes were each roughly 30 times the mass of the sun. They merged some 1.3 billion light years from Earth. The waves were generated in the final moments before the black holes merged. The signal was brief but definitive.
The measurements are dramatic proof that gravitational waves exist. The signal in the detector matches well with what's predicted by Einstein's original theory, according to Teukolsky, who was briefed on the results. It matches predictions of the ripples produced by two large black holes, in the final moments before they merge, swirling together at an enormous speed.
This is, arguably, the most direct observation of black holes ever made. Because black holes are (as their name implies) "black", they can't be seen with ordinary telescopes. Up until now, their existence has been inferred by looking at the stars and gas swirling around them. This gravitational signal comes directly from the holes, and it is virtually incontrovertible proof that the holes are out there. "If black holes didn't really exist, you couldn't explain these waves," he says.
Other researchers believe that the gravitational waves could tell us even more about our cosmos. "It's like looking at the universe with new eyes — the amount of information that's there is going to be amazing," says Mina Arvanitaki, a theorist at the Perimeter Institute of Physics in Waterloo, Ontario. Arvanitaki will use LIGO's data to probe for undiscovered fundamental particles that might only exist in the warped space around black holes.
Teukolsky says the discovery shows just how extraordinary the natural world can be. "The universe is stranger than any kind of fiction we could imagine," he says. "I mean, it's preposterous."
KELLY MCEVERS, HOST:
Scientists today announced a huge discovery that many had been working toward for decades.
DAVID REITZE: Ladies and gentlemen, we have detected gravitational waves. We did it.
MCEVERS: That was David Reitze. He's director of something called the Laser Interferometer Gravitational-Wave Observatory. Did I get that right, Geoff?
GEOFF BRUMFIEL, BYLINE: Yeah, you nailed it. Laser Interferometer Gravitational-Wave Observatory.
MCEVERS: OK, that's NPR science correspondent Geoff Brumfiel who you can you hear is here with me in the studio. Tell us about this huge discovery.
BRUMFIEL: So, I mean, I can play you the actual sound of this discovery if you want to hear it.
BRUMFIEL: All right, let's do it.
(SOUNDBITE OF GRAVITATIONAL WAVE)
BRUMFIEL: So if you hear this bloop, bloop almost like dropping water...
(SOUNDBITE OF GRAVITATIONAL WAVE)
BRUMFIEL: ...That bloop is the gravitational wave, and it came from two black holes colliding, according to researchers. This collision happened around 1.3 billion light-years away. So it took 1.3 billion years for the signal to get here. But it reached us on September 14, 2015, at 5:51 a.m. Eastern time.
MCEVERS: And so what are gravitational waves and why is this such a big deal?
BRUMFIEL: So Einstein predicted this a century ago as part of his theory of gravity called general relativity. And this theory said that gravity is actually a warping of space and time itself. It's a warping of the very dimensions we live in. When heavy objects move through space and time, though, the theory also predicts that they create ripples. The best way to maybe think about it is like a rock being thrown into a pond. When the rock goes in, it'll generate these ripples that spread out. Those are the gravitational waves.
MCEVERS: And so how did they discover them?
BRUMFIEL: They used lasers like any good scientists would. And what they did was they basically have these two long tunnels at a right angle. It's kind of an L-shape. And they shoot lasers down each one. It's all a bit complicated. But basically, these are like rulers. And they can measure tiny stretching and squishing of space. They've actually got two of these rulers. One's in Washington state and one's in Louisiana. And so both of them saw the signal. The signal was about the size of one one-thousandth of a proton's diameter. So that gives you an idea of how tiny these waves are.
MCEVERS: I mean, if it was so small, how could they be sure that what they were saying was real?
BRUMFIEL: You know, I asked them that myself. And I was surprised to hear that many of the physicists involved were skeptical when they first saw the signal. Here's Rainer Weiss who's been working on this since the 1960s.
KEN WEISS: I felt disbelief. A lot of us felt that way until we really did all the checks. And then little by little, all of us began to believe.
BRUMFIEL: So they had to rule out all kinds of stuff. For example, in the Louisiana detector, they have, like, an exercise ball in the control room that they sit at sometimes. If they bounce that, that'll set off the detector. They ruled that out. The even thought about, like, Chinese hackers - you name it, they tried to think of it. And the signal that was left after they did all that checking is just textbook.
MCEVERS: OK, so where do they go from here?
BRUMFIEL: For one thing, we've never actually seen black holes directly before in this way. I mean, don't forget, they're black, so telescopes can't see them. These waves will tell us a lot about black holes. And I think that they're going to be very, very powerful tools for probing all sorts of things in the universe.
MCEVERS: That's NPR's science correspondent Geoff Brumfiel. Thank you.
BRUMFIEL: Thank you. Transcript provided by NPR, Copyright NPR.