Scientists detect gravitational waves from merging neutron stars
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E-mail Xinhua, October 17, 2017
Scientists announced Monday that they have for the first time detected the ripples in space and time known as gravitational waves as well as light from a spectacular collision of two neutron stars.
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Illustration of two merging neutron stars. [Photo/NSF/LIGO/Sonoma State University/A. Simonnet] |
The detection of the gravitational wave signal, called GW170817, was made at 8:41 a.m. EDT (1241 GMT) on August 17 by twin detectors of the Laser Interferometer Gravitational-wave Observatory (LIGO),located in Livingston, Louisiana, and Hanford, Washington.
"Groundbreaking" detection
It was touted as "groundbreaking" and "unprecedented" because all the four gravitational waves detected before came from two black holes orbiting each other and merging.
"The detection of gravitational waves from a binary neutron star merger is something that we have spent decades preparing for," Alan Weinstein, head of Caltech (California Institute of Technology) astrophysical data analysis group for LIGO, said in a statement.
"On that morning, all of our dreams came true."
About two seconds after the latest gravitational wave incident ended, a bright flash of light, in the form of gamma rays, was detected by U.S. space agency NASA's Fermi space telescope.
In the hours, days and weeks following the smashup, other forms of light or electromagnetic radiation -- including X-ray, ultraviolet, optical, infrared, and radio waves -- were detected.
"We quickly established that the two stars were each less than around twice the mass of the sun, putting them in the typical mass range of neutron stars," Weinstein said.
The coincident gamma-ray burst suggested that the stars, unlike black holes, emit light.
Later studies by Caltech and LIGO Scientific Collaboration, a group of more than 1,200 scientists worldwide, found that the two stars were located in NGC 4993, a galaxy about 130 million light years away in the constellation Hydra.
Neutron stars, formed when massive stars explode in supernovas, are the smallest, densest stars known to exist, with a teaspoon of neutron star material having a mass of about one billion tons.
Cosmic mine
The light-based detections that followed showed that the collision of the neutron stars released newly synthesized heavy elements into the surrounding universe.
That's "the first concrete proof that such smashups are the birthplace of half of the universe's elements heavier than iron, including gold and platinum," the LIGO team said in a statement.
Previously, scientists knew where the lighter elements in the periodic table were synthesized. Most of the hydrogen and helium came from the Big Bang, and elements up to iron are fused in the cores of stars.
"But the origin of half the elements heavier than iron has been uncertain," the team said. "Astronomers have long suspected that they are synthesized in neutron-star collisions through rapid capture of neutrons ... but had no proof."
This time, continued observations by an array of ground-based telescopes found features indicative of chemical elements produced by the neutron capture process
"For the very first time, we see unequivocal evidence of a cosmic mine that is forging about 10,000 earth-masses of heavy elements, such as gold, platinum, and neodymium," said Mansi Kasliwal, leader of a worldwide telescope network called GROWTH taking part in the discovery.
In other words, neutron stars are the primary factories for gold in the cosmos. The results were described in a series of papers appearing in various journals such as Science and Nature.
Big deal
Interestingly, the August 17 event was nearly missed because the gravitational wave signal was not immediately evident in Livingston data due to a burst of noise. However, the Hanford signal was good enough to trigger a deeper analysis of the data that quickly located the signature.
"Fermi's observation of a gamma-ray burst at nearly the same time added to the excitement and urgency of the moment," Weinstein said.
At around 10 a.m. EDT (1400 GMT), a little more than an hour after the gravitational wave was detected at the LIGO observatories, the astronomical community across the world was notified.
Originally predicted in the early 20th century by Albert Einstein, gravitational waves caused by cataclysmic cosmic events result in ripples that propagate through spacetime, just like the movement of waves away from a stone thrown into a pond.
The first three such events were detected by LIGO, and the fourth was detected simultaneously by both LIGO and Italy-based Virgo detectors.
Michael Guidry, professor of physics and astronomy at the University of Tennessee, who was not directly involved in the new discovery, called the first detection of gravitational waves produced by colliding neutron stars "quite a big deal" because the physics is "fundamentally different" from merging black holes.
"The primary difference is that for black hole mergers essentially no matter is involved, while for neutron star mergers a large amount of very dense matter is involved," Guidry told Xinhua.
"This has implications for broad issues in stellar evolution such as how the heavy elements are produced, the mechanism for gamma-ray bursts, detailed internal properties and maximum mass for neutron stars before they must collapse to black holes," he said.
"Such a detection would open entirely new avenues of inquiry in many fields of astronomy and astrophysics, so it would have a huge impact on those communities and on the future of those fields."
Just the beginning
For example, observations of gravity from the collisions of high mass objects, like GW170817, are considered one of the few ways to test Einstein's theory.
Gregory Harry, associate professor at the American University, explained that all five gravitational wave detections agree very well with Einstein's General Theory of Relativity.
"So far we haven't gotten any new hints, and it is possible that Einstein's theory does correctly describe our universe without any changes or additions," Harry, a member of LIGO Scientific Collaboration, told Xinhua.p "But even though our detections have been very clear and strong compared to what we expected, there is still plenty of room for stronger, clearer detections that might show us what is beyond Einstein's description of gravity," he said.
The discovery of gravitational waves was so important that three U.S scientists behind it were awarded the Nobel Prize in physics this month.
"I would suggest that the Nobel Prize in physics this year wasn't given so much for the first detection of gravitational waves, but for opening up the field of gravitational astronomy," Harry said.
"Gravitational astronomy is just the beginning," he said. "We can expect many more insights into cosmology, astronomy, astrophysics, nuclear physics, gravity, and other fields from gravitational wave observations as the 21st century progresses."
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