LIGO Has Detected Gravitational Waves for the Second Time
About 1.4 billion years ago, the universe gave scientists a Christmas present. Two black holes spiraled toward one another, approaching closer and closer until they finally collided. Ripples in spacetime spread from the collision at the speed of light until, for about a second on December 25, 2015 (if you were in the United States), the space holding the atoms that make you up buckled and then relaxed.
You didn’t notice, but the Laser Interferometer Gravitational-Wave Observatory did. Today, the LIGO team announced its second detection of gravitational waves—the flexing of space and time caused by the black hole collision. The waves first hit the observatory in Livingston, Louisiana, and then 1.1 milliseconds later passed through the one in Hanford, Washington.
By now, those waves are 2.8 trillion or so miles away, momentarily reshaping every bit of space they pass through.
It was hard to miss the fanfare back in February when LIGO announced its first gravitational wave observation. The discovery was published in Physical Review Letters, one of the top physics journals in the world, and has since been picked up by more news outlets than any other paper the journal has ever published.
The gravitational waves themselves were also pretty huge, relatively speaking. They were the fingerprint of two black holes spiraling into each other and colliding to form one bigger black hole—62 times the mass of the Sun. Such a big collision is pretty uncommon. “The first signal was so loud,” says Gabriela González, spokesperson for the LIGO Science Collaboration and a physicist who has been working on LIGO since its beginnings in the 1990s. “We didn’t expect such a loud signal, especially not to be the first.”
They expected signals more like the one that came in on Christmas. The two LIGO detectors measured gravitational waves from the inspiral, as the decaying mutual orbits of two bodies are called, and merger of two black holes. One was eight and the other 14 times the Sun’s mass, merging to form a black hole with 21 solar masses.
Many of you will have noticed that 8+14=22, not 21. The missing mass didn’t go into the final black hole; it went into the energy that generated the gravitational waves LIGO saw in December. Remember that E=mc2, which means that mass and energy are different names for the same thing. It takes energy to distort spacetime and create gravitational waves, and that energy came straight out of the mass of the final black hole.
The best part about watching smaller black holes collide is that they stay in LIGO’s sights for longer. When two objects are orbiting around each other, they move faster as they get closer together. This is the conservation of angular momentum, and it applies to everything in the universe: Sit on a well-lubricated barstool and set it spinning, and you’ll find that you, too, spin faster when your arms (the black holes) are close to your chest than when they’re spread out.
LIGO can only detect gravitational waves created by objects orbiting faster than about 30 times per second. González says that once big black holes are close enough to be orbiting this quickly, they’ve already just about run into each other. The signal from the first collision LIGO detected, for instance, only lasted for about .2 seconds. Smaller black holes can get much closer before they collide, so they can spend much longer orbiting faster than LIGO’s minimum threshold. The signal announced today lasted for about a full second—five times longer than the first one!
Checks on Checks on Checks
The only problem with a smaller collision is that it produces a smaller signal, all other things being equal. But LIGO has that part of things covered, explained Anamaria Effler, who has spent years as part of the huge team at LIGO that checks everything in the book to eliminate sources of error.
Both LIGO detectors are shaped like a giant letter “L”, built out of two 2.5 mile-long vacuums. Light travels down each leg before bouncing off an 88 pound mirror dangling from a system of pulleys held by pulleys held by pulleys held by pulleys connected to equipment that actively measures and counteracts the seismic motion of the Earth. Each pulley naturally dissipates any motion that gets through, and the quadruple system magnifies the effect. “So there are a lot of mechanical considerations,” Effler says. “And this is just one! Just one mirror, of so many that we have.”
The light recombines in a detector at the crux of the L. If the mirrors are exactly where they should be, the crests of one ray line up with the troughs of the other, and no light hits the detector. If the mirrors move at all, the rays line up imperfectly and some light ekes through—alerting the system to look for sources of error. Everything from magnets in the sensors to acoustic noise around the vacuum tubes to passing trucks can create noise in the measurements.
The computers analyzing the data automatically notified the project leaders within within minutes of the detection, but there was still a lot of work left to do. They double-, triple-, and quadruple-checked the results, and then they had to compare them to models and actually see what they learned from the gravitational waves themselves. There are a ton of steps and checks to follow, says Sarah Caudill, a postdoctoral researcher who works on LIGO. “All of that has to be reviewed very thoroughly. So that takes months.”
And months after Christmas 2015 brings us to, well, today, and the announcement of LIGO’s second direct detection of gravitational waves. For this event, space buckled enough to move the mirrors of both detectors by .7 attometers. That’s 7 tenths of a millionth of a trillionth of a meter.
Whatever you’re looking for, LIGO seems to have it. “Personally, I am interested in the precision instruments that are the gravitational wave detectors,” González admits. “But as the spokesperson for the collaboration, and in general as a physicist, I think this is a monumental discovery because this is confirming a new field of gravitational wave astronomy.”
It might be a second detection, but it’s no less influential than the first. “This is important because it does confirm that the first detection that we announced was not a fluke,” Caudill says, “and that we really are entering the era of gravitational wave astronomy.”
“I think the coverage of [gravitational waves] has been slanted a little bit the wrong way,” says Maura McLaughlin, a professor at West Virginia University who helps run NANOGrav, a project looking for gravitational waves that are too slow for LIGO to detect. Back in February, a ton of stories said that gravitational waves are the final confirmation of general relativity. First of all, McLaughlin says, that isn’t true. There’s still the polarization of gravitational waves out there to be discovered, for example.
But more importantly, it wouldn’t matter if it were. Gravitational waves are important not because they point backward but because they point forwards. They’re a new aperture on the universe that science is just starting to access. “For me, that’s the awesome thing,” says McLaughlin. “That we actually can see things that have only been theorized or indirectly observed before.”