Gravitational waves send text messages directly to Chad Hanna’s cellphone. His team’s computer codes—all 250,000 of them—constantly listen to the heavens for the telltale sound of a ripple through spacetime. When one burbles toward Earth and lands in the wave detector, and the supercomputer crunches the numbers and realizes what it is, Hanna’s phone thrums. That’s how he heard the second-ever gravitational wave detection at the Laser Interferometer Gravitational-Wave Observatory, or LIGO, which scientists will announce today.
The waves took about one second to crash into LIGO, and Hanna’s computer software picked them up about 70 seconds later. But they took about 1.4 billion years to reach Earth, after rippling out from the final cataclysmic 27 orbits of a pair of black holes before they merged.
The waves are ripples in the curvature of spacetime, and they were predicted by Einstein’s theory of general relativity, though he believed they would never be found. You can compare them to what happens when you drop a pebble in a pond. The mass of the pebble forces water to move out of the way, and it radiates outward, eventually reaching the shoreline as a little wave. LIGO was built to detect the faint lapping of these gravitational waves arriving at Earth.
The first discovery, announced in February, was an astounding achievement for physics and astronomy—it proved Einstein right, and opened a new window into the cosmos. Scientists say it is like gaining the sense of touch, after years of only being able to see and hear. But like with any scientific endeavor, you want more than one data point, and that’s what makes today’s announcement a big one.
“The excitement around the first of anything is obviously enormous, but this one is really on par,” Hanna told me. “The thing with having only one is, while we’re certain that was a gravitational wave, you don’t know if you happened to get lucky. When we got the second one, we’re all breathing a sigh of relief. Not that we had any doubts about the first one, but this establishes the idea that these are common enough that we hope to make regular observations.”
The second wave detection involved two smaller black holes whose consummation happened about 1.4 billion years ago. The black holes were about 14 and 8 times the mass of the sun, and their merger birthed a single, spinning black hole that was 21 times the mass of our sun. As they gradually approached each other, they lost energy in the form of gravitational waves. One sun’s worth of mass was left over, and this is what LIGO detected.
The black holes were smaller than the ones that produced LIGO’s first detection, which meant the signal lasted about a second, and this made them much harder to find. The first wave sounded kind of like a thud. The newer detection sounded more like a chirp, but its amplitude was significantly below the detectors’ noise level, Hanna says.
LIGO detects gravitational waves by looking for a slight disruption in light. It has two 2.5-mile-long arms arranged in an L, and a split beam of laser light pulses back and forth between mirrors at the end of the arms. The laser light measures the distance between the mirrors. An incoming wave would warp the arms just a bit, making one longer or shorter than the other, which would change the time it takes the light to arrive. By a bit, I mean the difference is exquisite — a ten-thousandth of the diameter of a single proton. The wave scientists are trying to detect is smaller than the atoms that make up the detector.
This insane sensitivity means background noises can easily interfere with the measurements. Things like washing machines in nearby homes, waves on distant shorelines, and airplanes flying thousands of feet overhead can all get in the way. That’s where Hanna and his codes come in.
Thanks to Einstein, scientists know what to listen for, so Hanna and his collaborators populated LIGO’s computers with software that can pick out the waves above the din. Hanna likens it to a Where’s Waldo painting.
“The scenes are full of crazy stuff. There are all kinds of things going on. But you know what you’re looking for. You are sure what Waldo looks like,” he says. “We’re in the same situation. We’re not throwing models around to make the data fit. We have a family of Waldos—maybe one has a red cap, one has an orange cap. They are a little bit different, but they are all Waldos. You have to look for something that really does look like a Waldo.”
In this scenario, there are 250,000 possible Waldos, and a supercomputer searches for all their telltale characteristics at the same time. A specific type of chirp triggers an alert to a database, which does some additional calculations, and if the signal crosses a set of thresholds, the computer notifies the humans. People have set up different alert systems, and if someone happens to be at his computer, she might get a popup or an email. Hanna set it up to send him text messages.
“It would be cool if we had a computer program, or something saying, ‘Good morning, Chad. You have detected a gravitational wave.’ We could do that, but we need a little more money,” he says with a laugh.
Now that physicists have seen two black hole mergers, we can start to make inferences about a lot of things—like how often we might be hearing the birth cries of black holes, and how many sun-sized black holes there are. The detector will be able to hear death, too, in the violent endings of massive stars. Scientists want to use it to study neutron stars, including centimeter-high “mountains” on their surfaces that could make them spin as pulsars. This second wave finally puts the word “observation” in LIGO, in other words.
And it is only going to get better. LIGO is continually being upgraded, and changes to the detector—and algorithms—will enable it to see 10 times farther into space. This means a 1,000-fold increase in the volume of space scientists like Hanna can search.
“What you’re seeing is basically the first round of things. We literally don’t have enough human power or money to search for everything we want,” he says.
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