In February, scientists announced a historic discovery: For the first time — after years of waiting — they’d successfully detected gravitational waves, the ripples in space-time produced by massive objects like black holes. On Wednesday at the annual meeting of the American Astronomical Society, they announced that they’re ready to confirm a second detection, one that occurred before the first was even made public.
Late on Christmas of 2015, the thousand-odd scientists who work with the Laser Interferometer Gravitational-wave Observatory (LIGO) had their holidays interrupted by text alerts.
“I was in Argentina with my family, in the mountains, but I still had some cell signal if I got in the right place,” LIGO spokeswoman Gabriela González, a professor of physics and astronomy at Louisiana State University, told The Washington Post. “And I got this text message saying, ‘There’s another one, and we have to make some decisions!’”
Gravitational waves are the ripples in the pond of space-time. The gravity of large objects warps space and time, or “space-time” as physicists call it, the way a bowling ball changes the shape of a trampoline as it rolls around on it. Smaller objects will move differently as a result — like marbles spiraling toward a bowling-ball-size dent in a trampoline instead of sitting on a flat surface.
Unlike light waves, gravitational waves pass through solid objects without being changed, so scientists hope they can use them to study distant, ancient events such as the collisions of black holes and neutron stars. They’re also emitted by “dark” objects — like black holes — that don’t emit light and are therefore difficult to study with optical telescopes.
The second detection event, like the first, caught the gravitational waves produced when two black holes collided and merged. But this event was much smaller: The first event involved black holes that were 29 and 36 times as massive as the sun, while this new collision, which took place 1.4 billion years ago, brought together black holes of 8 and 14 solar masses. Although the resulting signal was weaker, scientists from LIGO and Virgo — a similar facility in Europe — have confirmed the signal’s validity with a confidence level of over 99.99 percent. Their results have been accepted for publication in the journal Physical Review Letters.
LIGO has been operating since 2002, but a recent upgrade to the detector's sensitivity led its first detection almost immediately. The facility wasn't even officially back online when it came through.
“That first one kind of shook our expectations,” LIGO physicist Peter Shawhan of the University of Maryland told The Post. “We’d been operating for over a decade assuming we just had to patiently wait for the detection of gravitational waves, and suddenly here was this one.”
That made the scientists hope that these kind of detections would become common place, but they couldn’t be sure.
“We were wondering how lucky we got,” Shawhan said. “Was it out of the blue? Would it be years before we saw another one? Or were these events more common than we ever dreamed?”
The team is thrilled to have confirmed a second observation so quickly. While the first detection “jumped out” from the data, the researchers say, the second one was only noticed thanks to the analysis of specially developed software. Being able to detect a weaker signal is useful for several reasons: Smaller black holes spend more time merging before their massive gravity pulls them together, and it’s during this brief window that LIGO can detect the waves they produce. The signal of the second event lasted for an entire second — almost 10 times longer than the first — which allowed scientists to glean more information about the behavior of the two black holes.
The detection of smaller black holes makes some LIGO researchers hope that they’ll soon detect the interactions of ultra-dense neutron stars, which are even less massive and very poorly understood.
“Our experience with this new black-hole binary gives us confidence that we can detect — in near real-time with our present techniques — systems with very small amplitudes as we expect for binary neutron stars,” Pennsylvania State University’s Chad Hanna, who led the group that developed the software, said in a statement. “We now have good reason to believe that the future for gravitational-wave astronomy and multi-messenger astrophysics is bright.”
While Hanna isn’t the only team member who’s optimistic about detecting neutron stars, González is a little more cautious. LIGO is going to do another round of data collection starting this fall, but she suspects that the signals produced by these mysterious stars will still be too weak to detect — unless they happen to catch one that’s very close by.
That doesn’t mean they won’t be trying. González hopes to see the detection of strange new systems with the help of improved detectors and analytical software. And even if it takes years — or even a totally new kind of detector — to find something new, the LIGO teem is now confident that they’ll be seeing plenty more black hole collisions.
“We like to say that we now have put the ‘O’ in LIGO,” González said. The facility isn’t a theoretical instrument anymore: It’s an observatory, and it’s going to be kept very, very busy.