Physics

Gravitational waves roll in again, detected from both sides of Earth

Gravitational waves roll in ag...
Gravitational waves have been detected for the fourth time, now as a collaboration between the twin LIGO detectors in the US and the Virgo facility in Italy
Gravitational waves have been detected for the fourth time, now as a collaboration between the twin LIGO detectors in the US and the Virgo facility in Italy
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The LIGO facility in Hanford, Washington, one of two facilities operating in the US in the search for gravitational waves
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The LIGO facility in Hanford, Washington, one of two facilities operating in the US in the search for gravitational waves
Gravitational waves have been detected for the fourth time, now as a collaboration between the twin LIGO detectors in the US and the Virgo facility in Italy
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Gravitational waves have been detected for the fourth time, now as a collaboration between the twin LIGO detectors in the US and the Virgo facility in Italy
The Virgo facility near Pisa, Italy, which played a part in the new detection of gravitational waves
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The Virgo facility near Pisa, Italy, which played a part in the new detection of gravitational waves
The LIGO facility in Livingston, Louisiana, one of two facilities operating in the US in the search for gravitational waves
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The LIGO facility in Livingston, Louisiana, one of two facilities operating in the US in the search for gravitational waves

Another day, another detection of gravitational waves. Although it may seem like they're becoming mundane, it's worth remembering that observing these ripples in the very fabric of spacetime that are created by massive cataclysms in the very distant past, is one of the most important scientific finds in a century. The Laser Interferometer Gravitational Wave Observatory (LIGO) has just detected gravitational waves for the fourth time, but it wasn't alone this time: the signals were also measured by the Virgo detector in Italy, marking a new milestone in the observation of the Universe.

This latest event was observed on August 14 at 10:30:43 am UTC, and it was caused by a collision between two huge black holes about 1.8 billion light-years away. The two black holes involved in the mammoth merger had masses about 31 times and 25 times larger than the Sun, and the end result was a spinning black hole with about 53 times the mass of the Sun. That means about three solar masses were converted into the energy seen in the ripples.

Gravitational waves were first predicted by Albert Einstein over 100 years ago, but it took until September 2015 for them to be directly observed. LIGO went on to detect the phenomenon again in December that same year, and then again in January 2017.

Those events were all picked up by the twin LIGO detectors in Louisiana and Washington. Each of those facilities reflects laser beams down two 4-km (2.5 mi)-long tunnels, and by measuring the light as it exits, scientists can measure physical distortions as tiny as a fraction of a proton. This newest detection was backed up by the Virgo facility near Pisa, Italy, just two weeks after it joined the current observing run.

The Virgo facility near Pisa, Italy, which played a part in the new detection of gravitational waves
The Virgo facility near Pisa, Italy, which played a part in the new detection of gravitational waves

"Today, we are delighted to announce the first discovery made in partnership between the Virgo gravitational-wave observatory and the LIGO Scientific Collaboration, the first time a gravitational wave detection was observed by these observatories, located thousands of miles apart," says France Córdova, Director of the National Science Foundation. "This is an exciting milestone in the growing international scientific effort to unlock the extraordinary mysteries of our universe."

With Virgo joining the search from the other side of the planet, scientists can triangulate the source of the signals more precisely. The three-detector network shrinks 10-fold the section of sky that the waves originated from, allowing researchers to narrow down the search.

"Being able to identify a smaller search region is important, because many compact object mergers – for example those involving neutron stars – are expected to produce broadband electromagnetic emissions in addition to gravitational waves," says Laura Cadonati, deputy spokesperson for the LIGO Scientific Collaboration. "This precision pointing information enabled 25 partner facilities to perform follow-up observations based on the LIGO-Virgo detection, but no counterpart was identified – as expected for black holes."

The current observing run ended on August 25, but the next is slated to begin in mid-2018. During that time, the scientists expect to be able to detect gravitational waves on a weekly basis.

"With this first joint detection by the Advanced LIGO and Virgo detectors, we have taken one step further into the gravitational-wave cosmos," says David H. Reitze, executive director of the LIGO Laboratory. "Virgo brings a powerful new capability to detect and better locate gravitational-wave sources, one that will undoubtedly lead to exciting and unanticipated results in the future."

Source: National Science Foundation

7 comments
Bob
I don't doubt that gravity waves could exist but these articles leave me wondering. How do they know how far away the source was? How do they know how large the masses were? Since the force of gravity varies inversely with the square of the distance, you would have to precisely know the distance. Since they can't see the black holes and don't know exactly from what part of the sky they are located, this just has to be a guess. How do they know that somebody didn't just walk past the detector? How do you predict the masses to such accuracy if you can't see the source and don't know the exact distance? If the gravitational constant can only be measured accurately to 5 to 7 decimal places due to interference from closer objects, things can vary significantly when used for distances in the billions of light years especially things that can't be seen. When they assign such precise numbers as fact, they lose all credibility to me. They need a refresher course in significant digits and sources of error.
Kpar
Well said, Bob. Another question that I have is: What is the speed of gravity? There is some controversy about it- some scientists have indicated that the speed of gravity may significantly exceed the speed of light. That would certainly affect the accuracy of these observations.
eMacPaul
@Bob, they know somebody didn't just walk past because they have more than one detector, located far apart. And really, if you think "about 1.8 billion light-years" or "about 31 times" have a high number of significant digits, you should perhaps review significant digits, yourself; all the measurements presented in this article only have two significant digits.
wolfdoctor
Bob - sometimes you just have to assume that people who a lot smarter than you know what they are doing.
Bob
eMacPaul and wolfdoctor, I have degrees in chemistry and physics plus working for years on many types of test equipment that measure to the limits of detection. I have also worked in advanced physics labs measuring gravity. Any time you take measurements like this, electrical noise and even somebody walking by can affect the readings. Having a simple noise spike happen coincidentally at two locations at the same time is well within the realm of possibility. How they can get ANY significant digits from an unseen event at an unknown distance is beyond me. I don't claim to be smarter than they are. They may have detected a gravity wave but my point is that the detailed masses and distance claimed exceed any credibility. Wolfdoctor, unless you have a flying car in your garage and a cold fusion reactor in your basement, don't believe everything you read.
RobertEhresman
Ligo and Virgo comprise 3 (not two) globally diverse detectors. Dynamic gravity waves are believed to travel at the speed of light (relativity will be broken if they dont), a fact that will be confirmed shortly when these detectors find an event that can be confirmed through optical observation e.g. neutron star merge. Precise timing of the 3 (not two) globally diverse events give a vector to the target. The detectors in question are fully correlated with seismographic networks including locally. High correlation between the detectors of the candidate waveform is required before a candidate event is considered. The distance to the source and magnitude of the masses is obtained by signal processing. The signal produced by a large mass merge is a classic "chirp". People walking by do not produce chirps. The begin and end frequencies, the harmonic content, and envelope of the chirp indicates the the number of individual masses involved, the velocities relative to observer, angle of the plane of the rapidly decaying orbit and relative sizes of the masses. Pulling out 1 or 2 significant digits is not hard. The size of the error bars is perhaps another story. All predicated on a model assuming the usually paired bodies are bound in a rapidly decaying orbit before merge.
Bob
RobertEhresman, thank you for a little more information but it raises more questions than answers. Vectoring anything from a baseline of only 8,000 miles will only be accurate to a few light years. Even from the two opposite sides of earth's orbit vectoring is only accurate for a few hundred light years, not 1.8 billion light years. It appears that quite a few assumptions are being made. First, that the speed of a gravity wave will be the same as the speed of light. For instance, the speed of a sound wave through water is far faster than the speed of a surface wave. Signal processing to determine distance and mass also sounds like another group of assumptions since these signals are being observed for the first time. How long did the signal last for the merger of two black holes? Red shift can give an indication of speed in light but does this apply to a gravity wave? Then you mention the predictions are based on a model. Models are always based on preconceived assumptions and do not prove anything. After reading this explanation any claims of distance and masses involved sound even more dubious. Scientific method is not what it used to be. Today, assumptions seem to evolve into theories and theories are claimed as fact with insufficient data and evidence to the contrary.