The waves came from two black holes colliding MPI for Gravitational Physics/Institute for Theoretical Physics, Frankfurt/Zuse Institute Berlin

We just turned the volume up on the sky. Gravitational waves, the booming echoes of massive objects moving all over the universe, have been detected for the first time by LIGO, the Laser Interferometer Gravitational-Wave Observatory, which was recently upgraded.

Gravitational waves are predicted by Einstein’s theory of general relativity, which says that massive objects warp space-time around them. When these objects accelerate, they make gravitational waves: ripples in the fabric of space-time that spread outward, like the wake left behind a boat.

We have been pretty sure they exist for a while – their presence was inferred indirectly as far back as 1974 – but none had been observed directly.


Gravitational waves: Your cheat sheet on the find of the decade

In a press conference today at the National Press Club in Washington DC, which was simultaneously broadcast to the media and other members of the team that made the discovery, the LIGO collaboration announced that they had finally caught a wave.

“Ladies and gentlemen, we have detected gravitational waves,” said David Reitze, the executive director of the LIGO Laboratory, at the press conference. “We did it!” The announcement accompanies a paper published in Physical Review Letters.

Double lucky

This historic signal was produced by a pair of black holes roughly 1.3 billion light years away, one 29 times the mass of the sun and the other 36 times, orbiting each other and then merging into a single black hole.

LIGO’s dual detectors, based in Hanford, Washington, and Livingston, Louisiana, felt the tremors on 14 September 2015 at almost the same instant. Their sensors registered space-time expanding and contracting by as much as a thousandth of the size of a proton – a tiny distance, but 10 times larger than the smallest unit LIGO can measure.

This was a doubly lucky find: officially, the experiment wasn’t scheduled to begin taking data until four days later, on 18 September, in a run that continued until 12 January 2016. The signal arrived while the detectors were in “engineering mode”, making sure the instruments were running smoothly.

Black holes bumping

A second stroke of luck was the nature of the signal: it seems that black hole mergers happen more often than we expected.

All objects emit gravitational waves when they orbit each other, including Earth orbiting the sun. But as these two black holes circled each other, the energy they lost to gravitational waves was enough to bring them much closer together – causing them to distort space-time further and emit even more gravitational waves.

That set them on track to collide and merge into one bigger black hole. “It’s a runaway process,” says Frans Pretorius, of Princeton University in New Jersey. “The closer they get, the faster they spin.” Near the end, they were whirling so fast that each orbit lasted just a few milliseconds.

When they eventually merged, the single black hole that remained was 62 times the mass of the sun – three solar masses lighter than the two original black holes combined. That missing mass all went into creating gravitational waves that fluttered space-time like a sheet.

“The total power output of gravitational waves during the brief collision was 50 times greater than all of the power put out by all the of the stars in the universe put together,” said Kip Thorne of Caltech, one of LIGO’s founders. “It’s unbelievable.”

At first, the resulting bigger black hole was lumpy instead of round, and getting rid of the lumps caused it to emit more gravitational waves. It then settled into a sphere and grew quiet.

By translating the frequency of the gravitational waves into sound waves, you can actually hear the signal. Physicists call it a “chirp“: a rise in pitch and volume as the black holes circle each other faster and faster.

The chirp from this new signal was very short – “just a thump”, said LIGO spokesperson Gabriela Gonzalez at the press conference.

Listen to the pair of black holes colliding – as detected by LIGO:

Not a drill

There were lingering worries that this signal could have been a deliberate fake. The LIGO team is infamous for secretly introducing ersatz gravitational wave signals into the data stream to test the experiment’s analysis procedures.

A previous “detection” in 2010 was sunk this way, but experienced team members knew something comforting this time around: fake signals aren’t inserted into engineering runs.

Other false positives could come from the accidental insertion of a pattern that looks like a signal, or even from malicious tampering. But by following procedures to check each instrument and each step in the data analysis, the team ruled these out, too.

The big reveal, which team members call “opening the box”, was on a conference call on 5 October. At the agreed-upon moment, a graph showing how likely it was that this signal was due to chance went live. The event was overwhelmingly likely to have been real.

Hello, gravitational sky

The groundbreaking discovery opens several doors, and has the potential to win a Nobel prize.

Since gravitational waves were predicted by general relativity, they offer a chance to verify that Einstein’s theory really is the correct account of gravity. So far, general relativity is passing with flying colours: the observed signal is perfectly explained by Einstein’s equations.

But the real excitement is that gravitational waves can show us a side of the night sky we’ve never seen before. Until now, there had been no sign of black holes in this size range – much less two of them.

Now that the first event has been detected, the era of gravitational wave astronomy is under way, says Avi Loeb of Harvard University in Massachusetts.

“I used to say, as they are building the instrument, they can be thought of as a physics experiment,” he says. “But as soon they detect a single source, they will be thought of as an astronomical observatory.” LIGO will be able to spot signals from massive objects that we are unable to observe with other techniques.

“It’s been a very long road, but this is just the beginning,” Gonzalez said. “This is the first of many to come. Now that we have detectors able to detect these systems, now that we know that binary black holes are out there, we begin listening to the universe.”

All this time, the gravitational wave sky has been waiting for us patiently. We’re about to tune in.

Want to know more? Learn about gravitational waves from one of the experts using LIGO data at our upcoming London event