On Monday, scientists announced the first observation of a cosmic event using gravitational wave detectors and conventional telescopes. They witnessed a kilonova, a violent, brilliant explosion that occurs when two neutron stars collide.
The discovery was a massive undertaking. Thousands of researchers from diverse fields in physics and astronomy played crucial roles. And there’s a lot of science to understand. Here are answers to basic questions about today’s news:
Remind me, what are gravitational waves?
Gravitational waves are ripples in space-time caused by cosmic events. They travel at the same speed as light. Their existence was first predicted by Albert Einstein as a byproduct of his theory of general relativity.
See, Einstein had figured out that gravity was a consequence of the way mass warps space-time. Heavy objects sit in the fabric of the universe like bowling balls on a trampoline, curving the space around them. That’s why moons orbit planets and planets orbit the sun — not because they’re attached by some cosmic leash, but because space-time is curling around the larger object, and they’re caught in the whirlpool.
Sticking with the trampoline metaphor, gravitational waves are what might happen if you smash two bowling balls together and caused them to explode. The cataclysm would vibrate the fabric of the trampoline, sending ripples out toward the edge. When two black holes collide, or when neutron stars merge, the gravitational waves from the event ripple through the universe.
What’s a neutron star?
Neutron stars are formed when an aging mid-sized star (about four to eight times bigger than the sun) goes supernova. That is, as the star’s outer layers are blown off, the remnants collapse in on themselves, forming a small, compacted core so dense that a single teaspoon of matter would weigh a billion tons.
The neutron stars that collided to create the kilonova observed by astronomers were small enough to fit inside the Beltway, but each contained as much matter as the sun.
“It’s the most tightly bound something can be and exist in our universe,” said Andy Howell, an astronomer at the California-based Las Cumbres Observatory and a professor at University of California, Santa Barbara.
The intense gravity of a neutron star crushes its atoms, compacting protons and electrons together until they combine to make neutrons — subatomic particles with no charge at all.
What happens during a kilonova?
“Nova is an ancient word. It basically means new star,” said Michael Siegel, a Penn State University astronomer who leads the ultraviolet instrumentation team for NASA’s Swift satellite. Long-ago stargazers observed new points of light in the night sky and assumed they'd witnessed the birth of infant stars.
But nova is a misnomer. These novas weren’t completely new objects — rather, these were existing suns that flared up and spat out bright light before dying down.
Scientists first coined the term kilonova about a decade ago, when they calculated that there should be events roughly 1,000 times brighter than the garden-variety cosmic nova. They’re also sometimes called “macronova” (a term, in Howell's estimation, that sounds “stupid”). “We’re so early we don't have the terminology sorted out,” Howell said.
This kilonova lived up to its 1,000-times-stronger namesake. “That was very satisfying,” said Columbia University theoretical astrophysicist Brian D. Metzger, whose work involves predicting the electromagnetic counterparts to gravitational waves.
Neutron star collisions shoot jets of radioactive matter into space. Their expelled guts beam out in a line: Metzger said it was almost like smashing your palm on a full tube of toothpaste with holes at both ends. “A lot of matter will come flying out,” he said. These are materials that the universe does not otherwise generate in bulk. The neutron star merger churned out the equivalent of 10,000 Earth masses in gold and tens of Earth masses in uranium.
The cataclysm in galaxy NGC 4993 suggests neutron star mergers are the dominant process by which the universe creates gold, platinum and other elements, Metzger said. “That’s been a mystery for something like 60 years.”
These stars were probably twin suns in a binary system. One after the other they became dead husks. They circled each other, shaking off gravitational waves, which in turn pulled them closer together. Imagine two large marbles rolling toward the bottom of a funnel, until they meet with a catastrophic thwack.
How exactly do gravitational wave detectors work?
There are three working gravitational wave detectors on the planet: one in Louisiana, Washington state and Italy, near Pisa. The American sensors are L-shaped tubes, 2.5 miles long, with all of the air sucked out. The Virgo detector in Italy is V-shaped, and is also a similarly lengthy vacuum.
Detector facilities emit a laser beam, which is split and shot down each tube. At the end of the each tube sits a mirror. Under normal conditions the lasers hit the mirrors and return to the detector at the same time. But if something warps the fabric of space, the lasers will no longer be synchronized. The difficulty is in sensing the asynchronous return: The ripples are unfathomably tiny — far less than the diameter of an atom.
Virgo, which came online this summer, helped researchers target the location of the collision. Massachusetts Institute of Technology research scientist David Shoemaker, spokesman for the LIGO Scientific Collaboration, has described the three detectors like the feet of a camera tripod. Travel up the legs, and where they meet is the cosmic object of interest — in this case, the neutron star merger.
Why were so many scientists involved?
To understand a collision requires general relativity (to understand why the stars merged), hydrodynamics (to understand how they collide) and nuclear physics (to understand what elements they produced), Metzger said. And that's just the theory of the thing. This detection and follow-up via telescope involved professors, scientists, engineers, technicians and students.
There were more than 3,500 names in the author list of the biggest paper to come out of this observation. “It's a monumental thing, a testimony to a lot of people working together,” Shoemaker said (even, if he added, it's likely that, given the sheer number of researchers, some of the author names are incorrect or missing).
Merging the two communities together — the physicists, used to large collaborations, with the astronomers, some of whom weren't — wasn't always easy. Shoemaker said if he would have done anything differently two months ago, when the researchers realized they had a neutron star collision on their hands, “it would have been to read about the sociology of astronomy before making any decision to put papers together. That would have saved a lot of sturm und drang.”