The Laser Interferometric Gravitational-Wave Observatory or LIGO is designed to detect gravitational waves—ripples of curvature in spacetime moving at the speed of light. It’s recently been upgraded, and it will either find gravitational waves soon or something really strange is going on.
Rumors are swirling that LIGO has seen gravitational waves produced by two black holes, of 29 and 36 solar masses, spiralling towards each other—and then colliding to form a single 62-solar-mass black hole!
You’ll notice that 29 + 36 is more than 62. So, it’s possible that three solar masses were turned into energy, mostly in the form of gravitational waves!
According to these rumors, the statistical significance of the signal is supposedly very high: better than 5 sigma! That means there’s at most a 0.000057% probability this event is a random fluke – assuming nobody made a mistake.
If these rumors are correct, we should soon see an official announcement. If the discovery holds up, someone will win a Nobel prize.
The discovery of gravitational waves is completely unsurprising, since they’re predicted by general relativity, a theory that’s passed many tests already. But it would open up a new window to the universe – and we’re likely to see interesting new things, once gravitational wave astronomy becomes a thing.
Here’s the tweet that launched the latest round of rumors:
For background on this story, try this:
• Tale of a doomed galaxy, Azimuth, 8 November 2015.
The first four sections of that long post discuss gravitational waves created by black hole collisions—but the last section is about LIGO and an earlier round of rumors, so I’ll quote it here!
LIGO stands for Laser Interferometer Gravitational Wave Observatory. The idea is simple. You shine a laser beam down two very long tubes and let it bounce back and forth between mirrors at the ends. You use this compare the length of these tubes. When a gravitational wave comes by, it stretches space in one direction and squashes it in another direction. So, we can detect it.
Sounds easy, eh? Not when you run the numbers! We’re trying to see gravitational waves that stretch space just a tiny bit: about one part in 1023. At LIGO, the tubes are 4 kilometers long. So, we need to see their length change by an absurdly small amount: one-thousandth the diameter of a proton!
It’s amazing to me that people can even contemplate doing this, much less succeed. They use lots of tricks:
• They bounce the light back and forth many times, effectively increasing the length of the tubes to 1800 kilometers.
• There’s no air in the tubes—just a very good vacuum.
• They hang the mirrors on quartz fibers, making each mirror part of a pendulum with very little friction. This means it vibrates very well at one particular frequency, and very badly at frequencies far from that. This damps out the shaking of the ground, which is a real problem.
• This pendulum is hung on another pendulum.
• That pendulum is hung on a third pendulum.
• That pendulum is hung on a fourth pendulum.
• The whole chain of pendulums is sitting on a device that detects vibrations and moves in a way to counteract them, sort of like noise-cancelling headphones.
• There are 2 of these facilities, one in Livingston, Louisiana and another in Hanford, Washington. Only if both detect a gravitational wave do we get excited.
I visited the LIGO facility in Louisiana in 2006. It was really cool! Back then, the sensitivity was good enough to see collisions of black holes and neutron stars up to 50 million light years away.
Here I’m not talking about the supermassive black holes that live in the centers of galaxies. I’m talking about the much more common black holes and neutron stars that form when stars go supernova. Sometimes a pair of stars orbiting each other will both blow up, and form two black holes—or two neutron stars, or a black hole and neutron star. And eventually these will spiral into each other and emit lots of gravitational waves right before they collide.
50 million light years is big enough that LIGO could see about half the galaxies in the Virgo Cluster. Unfortunately, with that many galaxies, we only expect to see one neutron star collision every 50 years or so.
They never saw anything. So they kept improving the machines, and now we’ve got Advanced LIGO! This should now be able to see collisions up to 225 million light years away… and after a while, three times further.
They turned it on September 18th. Soon we should see more than one gravitational wave burst each year.
In fact, there’s a rumor that they’ve already seen one! But they’re still testing the device, and there’s a team whose job is to inject fake signals, just to see if they’re detected. Davide Castelvecchi writes:
LIGO is almost unique among physics experiments in practising ‘blind injection’. A team of three collaboration members has the ability to simulate a detection by using actuators to move the mirrors. “Only they know if, and when, a certain type of signal has been injected,” says Laura Cadonati, a physicist at the Georgia Institute of Technology in Atlanta who leads the Advanced LIGO’s data-analysis team.
Two such exercises took place during earlier science runs of LIGO, one in 2007 and one in 2010. Harry Collins, a sociologist of science at Cardiff University, UK, was there to document them (and has written books about it). He says that the exercises can be valuable for rehearsing the analysis techniques that will be needed when a real event occurs. But the practice can also be a drain on the team’s energies. “Analysing one of these events can be enormously time consuming,” he says. “At some point, it damages their home life.”
The original blind-injection exercises took 18 months and 6 months respectively. The first one was discarded, but in the second case, the collaboration wrote a paper and held a vote to decide whether they would make an announcement. Only then did the blind-injection team ‘open the envelope’ and reveal that the events had been staged.
Aargh! The disappointment would be crushing.
But with luck, Advanced LIGO will soon detect real gravitational waves. And I hope life here in the Milky Way thrives for a long time – so that when the gravitational waves from the doomed galaxy PG 1302-102 reach us, hundreds of thousands of years in the future, we can study them in exquisite detail.
For Castelvecchi’s whole story, see:
• Davide Castelvecchi Has giant LIGO experiment seen gravitational waves?, Nature, 30 September 2015.
For pictures of my visit to LIGO, see:
• John Baez, This week’s finds in mathematical physics (week 241), 20 November 2006.
For how Advanced LIGO works, see:
• The LIGO Scientific Collaboration Advanced LIGO, 17 November 2014.