The first-ever sighting, part of a large body of discoveries by more than 70 international observatories announced in Washington, D.C., on Monday, advances the exciting new field of gravitational waves, which won this year’s Nobel Prize in Physics and fulfills Einstein’s general theory of relativity.
“It was bright and very blue for a few days, fading to red,” said UC Santa Cruz astronomer Ryan Foley, who led the team that made the August sighting and took a photo of the light, emitted by material flung off during the collision. “It changed fast, and faded out of sight.”
Scientists at UC Berkeley and Lawrence Berkeley National Laboratory did the theoretical work that helped others recognize the flash. At Stanford University and SLAC National Accelerator Laboratory, researchers laid the foundation for the discovery by developing tools that were critical to the project’s early iterations.
These brief but violent collisions are thought to be the source of all our heaviest elements, ranging from the silver and gold in your jewelry to radioactive uranium.
They emit powerful gravitational waves, which hold clues to many old questions in astrophysics and cosmology, such as the characteristics of matter and the behaviors of black holes and pulsars.
That flash occurred during the time of the dinosaurs, and is just now being detected. While mergers are common among neutron stars, the collapsed cores of old stars, they had never before been seen. The sighting opens a new window into understanding the physics of neutron star merges, and offers a new way of looking at the universe.
It also helps resolve a hotly debated question about the origins of gold and other heavy elements, according to UCSC astrophysicist Enrico Ramirez-Ruiz. “We are seeing the heavy elements like gold and platinum being made in real time,” he said in a statement.
Foley was in Copenhagen, visiting Tivoli Gardens during a break in a scientific conference, when he got a text about the possible detection of gravitational waves by the Laser Interferometer Gravitational-Wave Observatory, or LIGO. LIGO’s antennae heard the signal, then triggered an automatic alarm, mobilizing a team led by physicist David Shoemaker of the Massachusetts Institute of Technology.
Foley quickly pedaled his bicycle back to the University of Copenhagen, where he planned a detailed search plan for his team.
“You feel every possible emotion” when spotting the light, said Foley, assistant professor of astronomy and astrophysics, who joined the international news conference.
“There’s euphoria and exhilaration, then despair, while you’re processing all the data” to prove the sighting, “feeling the weight of the world,” he said.
Foley’s team didn’t witness the actual merger, but saw the flash of materials emitted by it.
To recognize the flash, they needed to know what to look for. Scientists at UC Berkeley and Lawrence made the theoretical predictions that made it possible to recognize the flash.
“We have been working for years to predict what the light from a neutron merger would look like,” said Daniel Kasen, an associate professor of physics and of astronomy at UC Berkeley and a scientist at Berkeley Lab, in a statement. “Now that theoretical speculation has suddenly come to life.”
Scientists at Stanford University and SLAC National Accelerator Laboratory also contributed to the foundations of the discoveries. The chip for LIGO’s initial laser was designed by physicist Robert Byer; aeronauticist Daniel DeBra developed the observatories’ original platforms, so stable that they move no more than an atom relative to the movement of Earth’s surface. A SLAC team built the camera’s optic system.
More than a century ago, Einstein described gravity as bending and rippling, like waves, of space and time.
Ever since, we’ve sought proof of these gravitational waves. They’re so small that it required new and advanced detection devices, like LIGO, to find them.
“If everything we see in the universe were the brush strokes of a painting, a gravitational wave would be a quiver in the canvas itself,” wrote venture capitalist and physicist Yuri Milner of Los Altos Hills, in a 2016 New York Times essay.
To find the waves, scientists looked at the most violent cataclysms in the universe: the merging of neutron stars. Neutron stars are among the most exotic forms of matter in the universe, according to UC Santa Cruz. A sugar cube of neutron star material would weigh about a billion tons.
The actual collision — the first-ever detection of a neutron star merger — was reported on Aug. 17 by the LIGO detectors in the United States and the Virgo detector in Italy.
Only 1.7 seconds later, a space telescope recorded a short burst of gamma rays from the same region.
The neutron star merger, dubbed GW170817, was immediately telegraphed to observers around the world. The first visible light of materials from the merger was detected by Foley’s team 11 hours later.
The teams are publishing their research in the Oct. 16 issue of Science, as well as Nature and Astrophysical Journal Letters.
The search for Einstein’s waves has been underway for nearly half a century, ever since MIT’s Rainer Weiss and Caltech’s Kip S. Thorne and Barry C. Barish first designed a device to detect them.
LIGO’s two interferometers — one in Washington, the other in Louisiana — measure the stretching and squeezing of gravitational waves.
In 2016, scientists made headlines with the news that they had detected gravitational waves. In January 2017, they announced they heard them. That faint chirp is the first direct evidence of gravitational waves.
But we still hadn’t seen them, or witnessed the light emitted in the aftermath of a violent merger.
So scientists scanned the skies for colliding neutron stars, in a galaxy within the constellation Hydra.
The Berkeley scientists knew that the light from this collision — that is, the light emitted by its flotsam and jetsam — would be extraordinarily bright, about one thousand times brighter than normal nova explosions in our galaxy.
But basic questions remained as to what the flash would actually look like. So the team — Kasen, Eliot Quataert and Brian Metzger — turned to fundamental physics and math to figure out how the structure of heavy atoms determines how they emit light.
That is just what astronomers saw. The emitted light matched the Berkeley team’s theoretical predictions.
Peering through a large telescope at the Carnegie Institution’s Las Campanas Observatory in Chile, the UC Santa Cruz team focused on galaxies within the search field already indicated by the LIGO team, targeting those most likely to harbor binary pairs of neutron stars.
“As soon as the sun went down, we started looking,” said Foley, 37, a resident of Santa Cruz. Because they found it quickly — in the ninth field of view — “we were able to build up a really nice data set.”
“We didn’t see the car wreck” of the colliding stars, he explained. “We saw the aftermath of the collision.”
“It doesn’t look like anything we’ve ever seen before,” he said. “It got very bright very quickly, then started fading rapidly, changing from blue to red as it cooled down. It’s completely unprecedented.”