Gravitational Waves From Merging Black Holes Detected by New Joint Network

A three-way network of gravitational-wave detectors on different continents shows potential for learning more about Einstein's ripples in space-time.

Scientists have detected elusive gravitational waves once again. The announcement on Wednesday, which sent ripples of excitement throughout the scientific community, marks the fourth such detection in less than two years, but for the first time ever two observatories on two different continents worked together to make the observations, using three “ears” for listening for the faint signal emitted by the merger of two massive black holes.

“Today marks an exciting milestone in the growing international effort to unlock the extraordinary mysteries of the universe,” said France Córdova, director of the National Science Foundation, speaking at a press briefing from the G7 Science Conference in Turin, Italy. “We are delighted to announce the first discovery made in partnership between the Virgo gravitational-wave observatory in Italy and the LIGO Scientific Collaboration, the first time a gravitational wave detection was observed by these observatories, located thousands of miles apart.”

The first detection of gravitational waves, which was announced in early 2016, confirmed Einstein’s general theory of relativity, as he predicted the waves as part of his theory that proposed space-time as a concept.

Gravitational waves are undulations in space and time created when two massive, compact objects such as black holes merge. Their detection allows for a new way to look at the universe because, until recently, astronomers could only study objects in space by observing the different wavelengths of light. Being able to observe gravitational waves provides a new way to study objects that are notoriously difficult to observe, such as black holes and neutron stars.

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The recent detection of gravitational waves took place on August 14 at 6:30 am EDT.  They were produced by merger of two black holes with masses about 31 and 25 times the mass of the sun, located about 1.8 billion light-years away.

The newly produced single, spinning black hole has about 53 times the mass of our sun, which means that about three solar masses were converted into gravitational-wave energy during the merger.

LIGO — the twin Laser Interferometer Gravitational-Wave Observatory (LIGO) detectors, located in Livingston, Louisiana, and Hanford, Washington — has made the three previous detections of gravitational waves. The Virgo detector, located near Pisa, Italy, has been in operation since 2011, but recently had an upgrade to match the upgraded LIGO detectors. The two observatories started working in concert at the beginning of August this year.

Just 14 days later, the gravitational wave signal arrived first at the LIGO Livingston detector. Six milliseconds later, it arrived at the Hanford detector. Then it arrived at the Virgo detector another six milliseconds later. Using three detectors has allowed the research team to identify the source of the waves with greater accuracy.

Jo van den Brand, spokesperson of the Virgo collaboration, explained at the press briefing that combining the signals from all three detectors allows the researchers to locate the source of the waves with greater accuracy.

“The time differences allow for very accurate triangulation,” he said, “and with this we can locate the source that is emitting these gravitational waves with a precision that is more than 20 times higher than we could do before. This is important since we expect that many of such merger events will also emit other messengers, such as light, x-rays, radio waves, neutrinos, or other subatomic particles. So these events can be studied by both astronomers and astroparticle scientists. This opens a new field of multinational astronomy and I think we have now take the first step in that process.”

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Van den Brand added that the detection also highlights the scientific potential of a three-detector network. Astronomers hope the combined detectors can eventually help them determine the sources of gravitational waves with even greater accuracy, and also detect them more often.

David Shoemaker of MIT said in a statement that he expects detections “weekly or even more often” starting in the next observing run, planned for the fall of 2018.

While gravitational waves are not a form of sound, like sound waves, they cause vibrations in the material they pass through. Astronomers have called these vibrations “chirps” because it turns out the frequencies (or the number of vibrations per second) of gravitational waves are the same as the frequencies of sound waves audible to humans.

The incredibly powerful event, called GW170814, lasted only fractions of a second. While the waves are powerful at the source, after traveling 1.8 billion lightyears through space, they are quite weak by the time they arrive at these detectors. 

Astronomers can’t yet pinpoint exactly where in the universe the collision occurred — they need better technology and more practice in their detections — but they use mathematical models to estimate the masses of the objects involved, as well as their distance.

Scientists are excited by the new detection and the promise of more to come. They say we will gain a deeper understanding of astrophysical phenomena by combining gravitational wave astronomy with traditional methods using the electromagnetic spectrum.

“As with Galileo, humankind has a new instrument with which to look towards the skies,” said Giovanni Losurdo, Advanced Virgo Project Leader, Virgo Collaboration, speaking at the briefing. “Every time we use a new instrument, there is a new prospect for knowledge, and we provide knowledge, knowledge that can serve as inspiration for the imagination and intuition of the young, and can bring about change in thousands of minds, and therefore science as a whole.”

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