A facility in Italy will soon join two detectors in the USA and together they may be able to solve the mystery of the origin of gravitational waves
Gravitational waves were first identified using a detector in September 2015, when several of them crossed the Earth. Two highly sensitive detectors, one in the state of Washington and one in Louisiana, picked up distortions in space-time that resulted in this case from the merger of two black holes. When the scientists responsible for the detectors, called gravitational wave observers, use laser interferometry (LIGO), announced the finding five months later, in February 2016, it caused international excitement and was the most important news in the world of physics in 2016. Physicists have been searching for decades for direct evidence of gravitational waves, whose existence was first predicted by Albert Einstein in 1916.
While this is an impressive breakthrough, it leaves several key questions unanswered. And the most important of them: where was the source of the waves? If all goes as planned, scientists will soon be able to tackle this issue in their future discoveries.
In the spring of 2017, physicists plan to turn on a third gravitational wave detector, Virgo His name, located near Pisa in Italy. Virgo was down for an upgrade when the LIGO detectors picked up the first two signals in September 2015. With these three giant instruments working together, scientists hope to greatly improve the ability to determine where gravitational waves come from. A quick response to a "triple hit," in which the same gravitational waves would cause distortions in all three detectors, could allow ground-based telescopes to focus on a region of the sky that the LIGO detectors will define using Triangulation, and perhaps locate the collisions from which the waves originate.
A gravitational wave detector, which is shaped like a giant R with arms that are several kilometers long, picks up distortions in space-time when the waves change the length of one of the arms by a change that is smaller than the diameter of a proton. But when a detector with such a high sensitivity works alone, it is impossible to rule out vibrations originating from the Earth. Each detector watches over a huge chunk of the universe: its field of view covers more than 40% of the sky surrounding the Earth, about the piece of sky that a person standing in the heart of the desert sees as he rotates around himself. Try to isolate even one faint star in all this.
LIGO's pair approach is also crucial for another reason. A gravitational wave travels at the speed of light, but if it does not happen to hit the two detectors completely head-on, there is a difference of a few milliseconds in the times when the two detectors register the disturbance. By measuring this difference, the scientists can calculate the direction of the impact, trace a source in the sky and thus reduce the source of the waves to a relatively small area. In the 2015 discovery, this region was about 2% of the sky. That's still a huge chunk of the universe to be combed through in search of the source event.
And this is where Advanced Virgo comes into the picture. Before its upgrade, it was not sensitive enough to detect gravitational waves, even those of extremely high energy. New mirrors, vacuum pumps and lasers, used to measure minute changes in the length of the instrument's arms, were installed to sharpen its sensitivity. Its electronic systems have been completely overhauled. The installation of the new hardware has been completed, and it is now being adjusted to remove local fluctuations that are not related to gravitational waves and may obscure real signals that will reach it. Crew members are working day and night to prepare Virgo for operation before summer, when they plan to shut down the LIGO detectors to upgrade them as well. [Due to last minute glitches, There may be a delay of a year - the editors]
When advanced Virgo begins to operate, the area of the sky where the source of a gravitational wave might be located will shrink fivefold, says Fulvio Ricci, a Virgo speaker and physicist from the University of Spienza in Rome. ToIdo Berger, an astrophysicist at Harvard University who uses telescopes to study the events that LIGO and Virgo detect, has his own version of this improvement. "By adding a third detector to the network, location should improve considerably, reducing the problem of detecting [the source] from terrible to terrible," he says.
Still, the opportunities ahead for astrophysicists cannot be denied. Collisions of black holes are not the only events capable of warping space-time. And unlike black holes, some of the other events are also supposed to emit light and other electromagnetic radiation that can be picked up by telescopes. One can imagine watching the aftermath of a supernova or bursts of high-energy radiation emitted from the vicinity of the event horizon of merging black holes or perhaps any optical evidence of the collision of two neutron stars or of a neutron star captured in the gravitational mouth of a black hole. Gravitational wave detectors have not yet detected the ripples created by such events, so Berger and other physicists are ready for the moment when they do: then they will direct their telescopes to the area that the three detectors, not just two, will define for them. The exact position in the sky will also allow smaller telescopes to participate in the search and absorb the abundance of radiation emitted by these events.
The initial plan is to operate the three detectors together for at least a month. There is a good chance that a month is a long enough period of time to absorb waves from merging black holes, if not from other, rarer events. The collaboration might motivate LIGO operators and Virago operators to consider extending the run, he says B. S. Sathyaparkash, a physicist from Pennsylvania State University and a member of the LIGO team. "The plan might change if people get excited," he says. This bodes well for astrophysics.
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