Scientists have detected gravitational waves and light formed from the collision of two neutron stars. It's the first time a cosmic event has been seen from Earth in both gravitational waves and in light, and the first time gravitational waves created by neutron stars have been observed at all.
"What's really special about this is that not only have we seen the event in gravitational waves, but because you have matter involved – these very, very dense objects slamming into each other at nearly the speed of light – it produces a tremendous amount of light and high-energy radiation too," said Patrick Sutton, head of the gravitational physics group at Cardiff University and a member of the LIGO collaboration, which is dedicated to the search for gravitational waves.
The discovery was announced at a press conference at the Royal Society in London on Monday, alongside two simultaneous events in the US and Germany. The results are detailed in several scientific papers published today, in journals including Physical Review Letters, Nature, Nature Astronomy, and Science.
Neutron stars are born from the explosive death of stars bigger than our own sun. When these stars grow old they blow off their outer layers in a spectacular supernova, leaving behind a super-dense, fast-rotating core – the neutron star – just a few miles wide. A lump of neutron star the size of a sugar cube would weigh as much as Mount Everest.
Gravitational waves are wrinkles in space-time created when massive objects accelerate really quickly, like when two black holes or neutron stars collide. The waves travel through space at the speed of light, and we detect them on Earth by looking for tiny variations in the distance between two objects. When we spot one of these tiny variations it means that space itself has been stretched out by a passing gravitational wave.
At 1:41pm UK time on 17 August, gravitational waves created during the collision of two neutron stars 130 million light years away reached the two Laser Interferometer Gravitational-Wave Observatory (LIGO) detectors in Hanford, Washington, and Livingstone, Louisiana, and a third run by the Virgo collaboration in Pisa, Italy. Within a couple of seconds, NASA's Fermi space telescope detected a bright burst of high-energy radiation called gamma rays.
The scientists involved quickly figured out that these two events were related, and astronomers around the world pointed their telescopes at the tiny patch of sky where the gamma ray burst had been spotted.
"We have an agreement with astronomers where we send out an alert when anything interesting happens," Andreas Freise, professor of experimental physics at the University of Birmingham and member of the LIGO collaboration, told BuzzFeed News. "These people saw our alert, and the public Fermi alert, and got excited and started looking. Ten hours later, optical telescopes spotted what astronomers historically call a 'nova' – a bright new spot where it used to be dark.
"In the end, it was almost fair to say all of the important telescopes on the planet were looking at the same thing."
"It’s probably one of the most actively observed astronomical events ever," Graham Woan, a professor of astrophysics at the University of Glasgow who works on LIGO, told BuzzFeed News. "There's fantastic excitement. Astronomers are hopping about with happy glee."
Carole Mundell, a professor of physics at the University of Bath who was also involved in the detection, told BuzzFeed News: "When LIGO detected the first black hole merger, we heard the thunder. In this case, we’ve heard the thunder and seen the lightning." It's a triumph of collaborative science, she said. "I think one paper has 3,000 authors. It's world science, lots of autonomous groups coming together with a science goal. It's transcended world politics; no one country could have done this alone."
Astronomers have suspected for the last 50 years that these sorts of gamma ray bursts are made when neutron stars collide, but before gravitational wave detectors like LIGO they had no way to prove it. "It's the smoking gun sign that these gamma ray bursts really are produced by neutron star collisions, which we'd always suspected but never been able to prove," said Sutton.
The discovery is the latest milestone for the LIGO-Virgo collaboration, three members of which last week won the Nobel Prize in Physics for building the detector and observing gravitational waves 100 years after Einstein first predicted that they exist. The first gravitational waves – created by two merging black holes – were detected by LIGO in September 2015 and announced in February last year.
This new detection opens up a whole new avenue of research into neutron stars themselves. "They’re really extreme objects; they’re probably the most hostile environments in the universe today," said Sutton. "They’re incredibly dense – a neutron star would typically weigh about the same as the Earth, but they're only about 30km across, about the size of Greater London. They're very hot – hundreds of thousands of millions of degrees – and they're highly radioactive."
"Neutron stars are the most remarkable objects in the universe, as far as I'm concerned," said Woan. "Imagine a perfect sphere the size of London, rotating as fast as a blender." Unlike black holes, neutron stars are made of normal matter, the same stuff that we are made of, albeit under fantastic pressures. "The pressures in the centre are thought to be sufficient to crush something the size of an aircraft carrier, 100,000 tons, down to the size of a grain of sand."
By looking at the signal picked up by gravitational wave detectors, we should be able to learn much more about these weird stars. As two neutron stars orbit each other closely just before they collide, the gravitational pull from each star distorts the other one.
"That can leave a telltale imprint in the gravitational wave data, and by careful analysis we can start to figure out some of the mechanics of the structure of these objects," said Sutton. "You can think of it as an astrophysics lab where we can start to do experiments that aren't possible in any laboratory on Earth."
Woan agrees: "Encoded in that waveform is information about how ordinary particles interact under enormously high energies."
All of this may sound far removed from life here on Earth, but all elements that come after iron on the periodic table actually formed when neutron stars collided. "So if you have a wedding ring, that gold probably came from a neutron star merger somewhere in our galaxy about 5 billion years ago," said Sutton. Dr Kate Maguire, an astrophysicist at Queen's University Belfast who worked on this aspect of the research, told a briefing at the Royal Society that the LIGO detection confirmed this theory: "Heavier elements, such as gold and platinum, are definitively formed in a neutron star merger. We can now say that conclusively."
Gravitational waves can also provide a way to measure distances in the universe – a traditionally tricky problem for astronomers.
The Hubble Constant, for example, is a measure of how fast the universe is expanding. But scientists can't settle on an exact number for it – using data from different telescopes gives different results. Using this neutron star collision, scientists have now been able to work out the Hubble Constant using gravitational waves. "In just one measurement we've got as good an estimate as astronomers have got over the last 100 years of trying," said Freise. "It's not yet good enough to decide which one is right, but it's a start."
As they continue observing, and ramp up sensitivity, the LIGO collaboration researchers expect to see more, similar events, and possibly even more exotic ones. As well as being made in colliding black holes and neutron stars, it's thought gravitational waves are created during supernova explosions. The detectors should be sensitive enough to pick up gravitational waves from a supernova that happens in our galaxy.
More sensitive detectors could even one day detect the gravitational wave echo of the big bang itself, which would have been created a fraction of a second after the beginning of the universe. "That's probably the most extreme, but exciting, prospect for things to look at in gravitational waves," said Sutton.
"We all say it's a new era in astronomy," said Freise. "It's easy to say that, but what it means is not so clear because there are so many unknown things we're hoping to discover, and that is really the exciting part."
