Good things come in threes, even in the world of astrophysics. The Laser Interferometer Gravitational-wave Observatory (LIGO), responsible for finding gravitational waves twice over already, has detected those ripples of spacetime for the third time. The new results, though quite similar to the first two detections, are an ebullient affirmation that the millions of dollars poured into these ultra-sensitive instruments is helping to open up a profound new window in the world of astrophysics.
“This allows us to learn things about the characteristics of the black holes that we could have never known until now,” David Shoemaker, a physicist from MIT and the lead scientist for the Advanced LIGO program, told reporters on Wednesday. “I think the key thing to take away from this third highly-confident event, is that we’re really moving from novelty into new observational science, and the astronomy of gravitational waves.”
Gravitational waves are ripples of spacetime first hypothesized by Albert Einstein. They are essentially signals created by any object in the universe that has mass. But they are extremely faint, capable of being detected by only the most sensitive instruments. In this instance we’re talking about interferometers — L-shaped vacuum tubes stretching for four kilometers in length housing lasers being bounced back and forth by mirrors. These babies are able to pick up on signals that are as slight as one-thousandth the width of a proton.
And even those instruments need a helping hand, in the form of signals produced by the most powerful of astrophysical phenomena. Like, you know, two black holes smashing into one another, which produce more energy than all the light emitted by every single star in the galaxy put together at a given moment.
The new gravitational waves, first detected by LIGO’s pair of interferometers on January 4 of this year, were created by a collision of two black holes to form a singular, gargantuan black hole. The resulting body, about 3 billion light-years away, possesses a mass 49 times that of the sun.
The first set of gravitational waves was were observed in September 2015 and announced in February 2016. The second set was detected in December 2015, and announced the following July. Both of those findings were the result of binary black hole mergers as well. Located about 1.3 and 1.4 billion light-years away, respectively, those resulting black hole masses varied dramatically, between 62 solar masses and 21 solar masses, respectively.
Thus, the newest findings, described in a new paper published Thursday in Physical Review Letters, fills in a mass gap left between the first two detections. “We now have a second candidate in the ‘heavy black hole’ category,” said physicist and LIGO researcher Bangalore Sathyaprakash of Penn State and Cardiff University. “It really establishes a new population of black holes that were not known before LIGO discovered them.”
The third detection isn’t just a validation of LIGO’s ability to successfully observe gravitational waves which are produced from much farther distances in the universe. The repeated measurements of gravitational waves from black hole mergers means those events are much more common than scientists ever dreamed. “We should expect to see one binary black hole merger event per day” once LIGO reaches its ideal capabilities, said Sathyaprakash.
They also reveal more about the way in which black holes spin and move. See, when two black holes begin the dance that leads to their merger, they sort of really are dancing — they are spiraling around one another and inching closer. All the while, they are also spinning on their own axes, either in the same direction (aligned) or a different direction.
Gravitational waves provide a kind of “fingerprint” for this spin, explained physicist and LIGO scientist Laura Cadonati of Georgia Tech. If the spins are aligned, they take longer to merge due to the need to shed rotational energy.
The new data doesn’t actually provide evidence as to whether these specific black holes were moving in a particular direction, but the data does provide the first suggestion that black hole mergers can be non-aligned.
Furthermore, the implication is that “this finding lightly favors the theory that these two black holes formed separately in a dense stellar cluster, [traveled] to the core of the cluster and then paired up, rather than being formed together from the collapse of two already-paired stars,” said Cadonati. These binary pairs seem to sync up with one another after they are born of a deceased star, rather than as the afterlife of two stars which were already paired up.
All of this directly puts Einstein’s theory of relativity — first proposed nearly a century ago — to the test. One of the ideas under the theory of relativity forbids the notion that gravitational waves are affected by dispersion — in which the signals will travel at different speeds as they move through certain mediums, the way light does. LIGO’s new findings found no such dispersion effect.
“In Einstein’s theory, no such dispersion is expected,” said Sathyaprakash. “We did not discover any such dispersion — once again failing to prove Einstein was wrong.”
Moving forward, LIGO is extremely encouraged that the new upgrades to the Livingston, Louisiana interferometer, which sought to improve the sensitivity of the detectors, seem to have paid off. Mike Landry from Caltech, who heads LIGO’s Hanford, Washington interferometer, says both detectors should be receiving more upgrades after the current test run ends in August.
In addition, LIGO’s team is looking forward to the completed construction and operation of the Virgo interferometer near Pisa, Italy. The three instruments combined should help dramatically improve the search for gravitational waves. Virgo should go online sometime this summer.
In addition, LIGO’s team is looking forward to the completed construction and operation of the Virgo interferometer near Pisa, Italy. The three instruments combined should help dramatically improve the search for gravitational waves.
“We’re extremely excited to have this detection of binary black hole mergers,” Landry said. He and his colleagues hope not just to find more gravitational waves emanating from binary black hole mergers, but also from binary neutron stars — another phenomena of massive celestial bodies smashing into one another. “That will tell us about the extreme state of nuclear matter.”