Science has chosen as its 2017 Breakthrough of the Year the first observations of a neutron-star merger, a violent celestial event that transfixed physicists and astronomers and proved a theory first posed by Albert Einstein.
As the two neutron stars spiraled together 130 million light years away, they generated tiny ripples in the fabric of spacetime called gravitational waves, sensed by enormous gravitational wave detectors on Earth. This merger also triggered an explosion studied by hundreds of astronomers around the world.
Researchers first picked up on gravitational waves two years ago, when two massive black holes crashed into each other. This space tremor was detected by the Laser Interferometer Gravitational-Wave Observatory (LIGO), a discovery that was named Science's Breakthrough of the Year for 2016 and won the 2017 Nobel Prize in Physics. The discovery showed that gravitational waves offer a new way of observing the universe and are a major tool for astronomers.
"Gravitational waves are the gift that keeps on giving," explained Science News Editor Tim Appenzeller. "This year, observers not only detected gravitational waves from a collision of two neutron stars; they also saw the event at all wavelengths of light, from gamma rays all the way to radio. Being able to get the full picture of violent events like this promises to transform astrophysics and that made this year's observation the clear Breakthrough for 2017."
"Last year's big discovery is now the tool that made this year's Breakthrough possible," agreed Science staff writer Adrian Cho. "This is the first time in the 22 years that the same scientific team has been involved in the Breakthrough of the Year two for years in a row. But both discoveries were so compelling that there wasn't a lot of discussion of that issue."
On August 17, gamma-ray detectors and radio telescopes sensed the merging of the neutron stars. The observation "confirmed several key astrophysical models, revealed a birthplace of many heavy elements and tested general relativity as never before," said Cho. Gravitational waves emanating from the spiraling neutron stars triggered LIGO detectors in Hanford, Washington, and Livingston, Louisiana, and the French-Italian Virgo detector near Pisa, Italy.
Because the ripples were spotted by three widely spaced detectors, scientists were able to act quickly and triangulate on the stars' location in the sky. "Within just 11 hours, several teams had pinpointed a new source on the edge of the galaxy NGC4993. The explosion was easily the most-studied event in the history of astronomy: Some 3,674 researchers from 953 institutions collaborated on a single paper summarizing the merger and its aftermath," said Cho.
"Whether it's gamma rays or infrared radiation or radio waves, astronomers generally view the cosmos through some form of light. And when they see a violent astrophysical explosion, the glare can make it difficult to tell what's really going on inside," Cho said. "But gravitational waves are a different kind of radiation entirely and they let researchers see right through the glare."
Until now, astronomers had to watch these events as if they were watching a house burn and could only see the flames engulfing the building. Now, they can see through the flames to watch the beams within the house collapse. "In this case, the gravitational waves told scientists right away that this was a case of two neutron stars spiraling into each other. The waves revealed how much the neutron stars weighted and precisely when they collided," said Cho.
The observations supported a 25-year-old conjecture that neutron star mergers produce short gamma-ray bursts, and confirmed that gravitational waves travel at the same speed of light, ruling out some speculative alternatives to Einstein's theory of gravity and general relativity. "Astrophysicists say the neutron-star merger only whets their appetite for more data — they want to see more such mergers," said Cho.
Another key next step for astrophysicists is to detect the gravitational waves up until the moment the neutron stars smash together — a feat currently unachievable for the LIGO and Virgo detectors. In mid-August, the instruments followed the twirling stars at an increasing speed until the frequency of the gravitational waves surpassed the sensitivity range of the instruments, leaving them unable to monitor the last few revolutions before the merger.
According to Cho, "those final revolutions could provide insights into the nature of neutron stars. Astrophysicists want to know how stiff or squishy neutron star matter is. In principle, neutron-star mergers can reveal that information: The stiffer the matter is, the larger the neutron stars will be and the earlier they will start to tear each other apart as they spiral together."
Plans are already underway to improve LIGO's sensitivity at higher frequencies. Scientists will begin such efforts by manipulating the laser light circulating in the detectors, though such an endeavor might take a few years.
"Even with current detectors [researchers] hope to see new types of [rare] events such as mergers of a neutron star and a black hole. Supernova explosions of individual stars in our Milky Way galaxy should also produce detectable gravitational waves, which could help astrophysicists figure out exactly how the stars blow up. But perhaps the most intriguing observations would be signals theorists haven't even predicted," said Cho.
This year's special Breakthrough section of Science also discloses the results of a readers' choice poll in which the public voted on its favorite science breakthrough, declaring advances in gene therapy their winner.
The Breakthrough runners-up include a wide range of scientific topics. Among them is a major improvement in a nascent technique called base editing, which can alter just one letter of the DNA alphabet at a specific point in the genome. The upgraded method now also targets RNA, an advance that researchers are already exploiting, as it could lead to medical applications. Another contender reinvigorated the analysis of modern human origins, when a skull found in a Moroccan cave pushed back the fossil record of Homo sapiens. Scientists confirmed that it was 300,000 years old — 100,000 years older than fossils from Ethiopia that had held the record as the oldest widely accepted remains of H. sapiens.
Science also chose "Breakdowns" of the year. They included perilous falls in the populations of several whales and porpoises, the dysfunctional relationship between the science community and the administration of President Donald Trump and continuing revelations of sexual harassment in science.
[Credit for associated image: Illustration by Robin Dienel courtesy of The Carnegie Institution for Science]