Gravitational waves and neutron stars: Why this discovery is huge
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Gravitational waves have been in the news a lot lately.
Two weeks ago, the http://www.abc.net.au/news/2017-10-03/2017-nobel-physics-prize-awarded-scientists-gravitational-waves/9012738" target="_self" title="">Nobel Prize in Physics went to three of the leading scientists behind the 1,000-strong international project which first detected these miniscule ripples in the fabric of space-time.
That gravitational-waves-detected/7140750" target="_self" title="">first ever detection was back in 2015; the gravitational-waves-detected-for-fourth-time-with-ligo-and-virgo/8994006" target="_self" title="">fourth ever detection was announced last month.
Now an international team of scientists has detected gravitational waves gravitational-waves-and-light/9053750" target="_self" title="">from a new source: the cataclysmic collision of neutron stars, the smallest and densest stars in the universe.
And this is much more than just the fifth in that historic series of detections.
Scientists say this is a game-changer, which arguably rivals the original Nobel Prize-winning detection.
Not only has it produced an unprecedented amount of data and opened a new window on the universe, it will revolutionise science for decades to come.
Here are six reasons why.
The first hint of gravitational waves came from neutron stars
Albert Einstein predicted that if you had two stars orbiting each other they would give off an intensifying burst of gravitationalwaves, just before they collided.
A neutron star is the small, heavy core left behind after a supernova explodes.
Because it packs the mass of up to two Suns into a ball the size of Adelaide, a neutron star is also a perfect laboratory for studying the extremes of gravity.
We only know of their existence through the discovery of pulsars — regular pulses of radiation created when these stars spin, incredibly fast.
In 1974, astronomers Russell Alan Hulse and Joseph Hooton Taylor Jr discovered radio pulsars coming from a pair of neutron stars circling each other.
This discovery, which was awarded a Nobel Prize in Physics in 1993, was the first indirect hint of gravitational waves and kicked off a new era of astronomy, according to Associate Professor Matthew Bailes, director of the ARC Centre of Excellence for Gravitational Waves.
"About 30-odd years ago people conceived of building what's called an interferometer, that could detect neutron stars ripping each other apart in roughly the nearest million galaxies," he said.
That concept eventually became reality and the twin detectors of the Advanced Laser Interferometry Gravitational Waves Observatories (LIGO) finally detected gravitational waves in 2015 — but not from colliding stars.
"When they turned them on they discovered black holes merging instead [of neutron stars]. That was a bit of a surprise."
Since then, three other bursts of gravitational waves have been detected, all created by merging black holes.
Because they are so much less massive, neutron star collisions have proved elusive.
Now, after decades of research, the researchers have not only picked up gravitational waves from the stars spiralling to their death, they have also witnessed the fiery collision and aftermath of one of those mergers for the very first time.
We've located the source of the waves for the first time
That makes this discovery the first time any object in the universe has been detected using a combination of gravitational waves and electromagnetic waves.
It heralds a new type of physics called multi-messenger astronomy.
Like the previous discoveries, the initial signal was picked up by LIGO. The two observatories based 3,000 kilometres apart in the US use lasers to detect miniscule movements along perpendicular beams several kilometres long.
These instruments detected the faint "chirp" of a wave that was created as the stars spiralled in together. The signal, at 100 seconds, was much longer than four previous examples.
But while LIGO and its European counterpart Virgo can help narrow down part of the sky to look at, using a technique called triangulation, the detectors are not sensitive enough to pinpoint the exact location.
"So on their own they can't really do much extra science except, 'Oh look we saw something merge'," said Dr Bailes.
And when black holes destroy each other, there is nothing left over to see.
"It's just black. They're not very interesting to study after they merge because there's nothing to see."
Neutron stars, on the other hand, give off light and radio waves when they collide, which can be picked up by space and ground-based telescopes.
"Unlike black holes ... these guys put on a fireworks display that really brings the entire astronomical community together to monitor the fireball.
"If you can see a fireball in the sky it enables you to identify whether it's in an existing galaxy or not, and if you locate the galaxy then you know how far away it is."
It gives us a more accurate idea of the size of the universe
These findings also provide a new direct measurement of the size of the universe, according to Professor David Blair from the University of Western Australia.
Scientists have been trying to work out the precise size of the universe for decades, but it is hard to do this using traditional astronomy. They use a complicated formula called the "cosmic ladder", which has its limitations.
Gravitational waves, however, produce a characteristic sound that tells you how far away their source is — and now the researchers have also traced the source of those waves to a collision in a particular galaxy.
So they now have a much more accurate fix on the distance of that galaxy.
"For the first time we have a 'tape measure' to measure the size of the universe that is free of the uncertainties of traditional astronomy," said Professor Blair.
We've witnessed the creation of gold in the Universe
Scientists have long thought that the atomic explosions created by neutron star collisions created all the gold in the Universe, said Dr Christian Wolf from the Australian National University.
"If you look at rare Earth metals they have not been produced in normal stars, they have not been produced by the Big Bang.
"Instead we believe they would have formed in neutron star collisions that would have happened long before the Earth formed from a cosmic cloud."
Up to four days after the gravitational waves and the first flash of gamma rays, scientists were still monitoring the afterglow from this collision to work out its chemical composition, explained Professor Blair.
"It's not the minutes since the thing happened that counts, it's the fact that we were actually able to observe it and follow it very clearly."
The optical glow as the light decays is the result of gold, said Associate Professor David Coward, who observed the fireball — also called a kilonova — with the Zadko Telescope in Western Australia.
"We were able to monitor for four consecutive nights and then we were able to use the data to understand this very exotic process of producing gold in the distant universe."
We've solved the mystery behind gamma ray bursts
Nearly two seconds after the gravitational waves were detected, the FERMI space telescope registered a short burst of gamma-rays.
This blast of energy solves another mystery in itself, according to Associate Professor Tara Murray from the University of Sydney.
She said scientists have speculated for the past 50 years that neutron star collisions are behind short gamma ray bursts — rapid jets of high-energy light that can last up to two seconds.
"We've known about those for a long time but we've never been certain what causes them," Dr Murray said.
And Einstein was right.
Again.The combination of gravitational wave astronomy and traditional astronomy has once again proved Einstein right, said Professor Blair.
In 1915, Einstein predicted the speed of gravity would be the same as the speed of light, which according to his theory of special relativity is a constant at 300 million meters per second.
The different techniques used in this discovery gave scientists the opportunity to compare the two speeds directly.
They found that the measurements matched — within one part in a million billion.
"So this one little measurement has told us, to a precision that's almost unbelievably good, that these two types of waves travel at the same speed.
"That is absolutely astonishing and mind blowing that one measurement can get that."
(c) 2017, Australian Broadcasting Corporation. All rights reserved.
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