Gravitational waves detected 100 years after Einstein's prediction — LIGO
On February 11, 2016, the LIGO (Laser Interferometric Gravitational-wave Observatory) Collaboration announced the detection of gravitational waves. For the first time, scientists have observed ripples in the fabric of spacetime called gravitational waves, arriving at the Earth from a cataclysmic event in the distant universe. This confirms a major prediction of Albert Einstein's 1915 general theory of relativity and opens an unprecedented new window onto the cosmos. 1)
Gravitational waves carry information about their dramatic origins and about the nature of gravity that cannot otherwise be obtained. Physicists have concluded that the detected gravitational waves were produced during the final fraction of a second of the merger of two black holes to produce a single, more massive spinning black hole. This collision of two black holes had been predicted but never observed.
The gravitational waves were detected on September 14, 2015 at 09:51 UTC by both of the twin LIGO detectors, located in Livingston, Louisiana, and Hanford, Washington, USA. Each LIGO detector is 4 km long, the observatories are separated by a distance of 3,002 km. As a gravitational wave passes through a detector, it distorts spacetime such that one arm lengthens, and the other shortens. By comparing the disturbances at the two detectors, the scientists can confirm the direct detection of a gravitational wave. - The LIGO Observatories are funded by the NSF (National Science Foundation), and were conceived, built, and are operated by Caltech and MIT. The discovery, accepted for publication in the journal Physical Review Letters, was made by the LIGO Scientific Collaboration (which includes the GEO Collaboration and the Australian Consortium for Interferometric Gravitational Astronomy) and the Virgo Collaboration using data from the two LIGO detectors. 2)
Based on the observed signals, LIGO scientists estimate that the black holes for this event were about 29 and 36 times the mass of the sun, and the event took place 1.3 billion years ago. About 3 times the mass of the sun was converted into gravitational waves in a fraction of a second—with a peak power output about 50 times that of the whole visible universe. By looking at the time of arrival of the signals—the detector in Livingston recorded the event 7 milliseconds before the detector in Hanford—scientists can say that the source was located in the Southern Hemisphere.
According to general relativity, a pair of black holes orbiting around each other lose energy through the emission of gravitational waves, causing them to gradually approach each other over billions of years, and then much more quickly in the final minutes. During the final fraction of a second, the two black holes collide into each other at nearly one-half the speed of light and form a single more massive black hole, converting a portion of the combined black holes' mass to energy, according to Einstein's formula E=mc2. This energy is emitted as a final strong burst of gravitational waves. It is these gravitational waves that LIGO has observed.
The new LIGO discovery is the first observation of gravitational waves themselves, made by measuring the tiny disturbances the waves make to space and time as they pass through the Earth.
The discovery was made possible by the enhanced capabilities of Advanced LIGO, a major upgrade that increases the sensitivity of the instruments compared to the first generation LIGO detectors, enabling a large increase in the volume of the universe probed—and the discovery of gravitational waves during its first observation run. The US National Science Foundation leads in financial support for Advanced LIGO. Funding organizations in Germany (Max Planck Society), the STFC (Science and Technology Facilities Council) of the UK, and Australia (Australian Research Council) also have made significant commitments to the project. Several of the key technologies that made Advanced LIGO so much more sensitive have been developed and tested by the German UK GEO collaboration. Significant computer resources have been contributed by the AEI Hannover Atlas Cluster, the LIGO Laboratory, Syracuse University, and the University of Wisconsin- Milwaukee. Several universities designed, built, and tested key components for Advanced LIGO: The Australian National University, the University of Adelaide, the University of Florida, Stanford University, Columbia University of the City of New York, and Louisiana State University.
LIGO research is carried out by the LIGO Scientific Collaboration (LSC), a group of more than 1000 scientists from universities around the United States and in 14 other countries. More than 90 universities and research institutes in the LSC develop detector technology and analyze data; approximately 250 students are strong contributing members of the collaboration. The LSC detector network includes the LIGO interferometers and the GEO600 detector. The GEO team includes scientists at the Max Planck Institute for Gravitational Physics (Albert Einstein Institute, AEI), Leibniz Universität Hannover, along with partners at the University of Glasgow, Cardiff University, the University of Birmingham, other universities in the United Kingdom, and the University of the Balearic Islands in Spain.
Figure 1: Photo of the LIGO facility on Livingston, Louisiana (image credit: LIGO)
Light is—or has been up until now—the only way to study objects in the universe (actually the entire electromagnetic spectrum). This includes everything from the Moon, all the way out to the most distant objects ever observed. Astronomers and astrophysicists use observatories that can see in not only visible light, but in all other parts of the electromagnetic spectrum, to study objects in the universe. And we've learned an awful lot. But things will change with this announcement. 3) 4)
Gravitational waves are a new way to study notoriously difficult things to observe like black holes and neutron stars. Black holes emit no light at all, and their characteristics and properties are inferred from cause and effect relationships with objects near them. But the detection of gravitational waves holds the promise of answering questions about black holes, neutron stars, and even the early days of our universe, including the Big Bang.
It's almost impossible to overstate the magnitude of this discovery. Once we understand how to better detect and observe gravitational waves, we may come to a whole new understanding of the universe, and we may look back on this day as truly ground-breaking and revolutionary.
Some background: Experiments to detect gravitational waves began with Weber and his resonant mass detectors in the 1960s, followed by an international network of cryogenic resonant detectors. Interferometric detectors were first suggested in the early 1960s and the 1970s (Ref. 2). A study of the noise and performance of such detectors, and further concepts to improve them, led to proposals for long-baseline broadband laser interferometers with the potential for significantly increased sensitivity.
By the early 2000s, a set of initial detectors was completed, including TAMA 300 in Japan, GEO600 in Germany, the LIGO (Laser Interferometer Gravitational-Wave Observatory) in the United States, and Virgo in Italy. Combinations of these detectors made joint observations from 2002 through 2011, setting upper limits on a variety of gravitational-wave sources while evolving into a global network. In 2015, Advanced LIGO became the first of a significantly more sensitive network of advanced detectors to begin observations.
On September 14, 2015 at 09:50:45 UTC, the LIGO Hanford, WA, and Livingston, LA, observatories detected the coincident signal GW150914 shown in Figure 2. The initial detection was made by low-latency searches for generic gravitational-wave transients and was reported within three minutes of data acquisition.
Figure 2: The gravitational-wave event GW150914 observed by the LIGO Hanford (H1, left column panels) and Livingston (L1, right column panels) detectors. Times are shown relative to September 14, 2015 at 09:50:45 UTC (image credit: LIGO consortium).
In 2016, gravitational-wave astronomy is going international, as LIGO India (sometimes referred to as INDIGO) received the green light recently in the wake of the detection announcement. Set to begin science operations around 2019, the third LIGO detector will be constructed in India. This will give LIGO the ‘third vector' it was initially envisioned with, allowing researchers to pin down the source direction in the sky. Other detectors are on the hunt as well, including VIRGO near Pisa, Italy, GEO600 in Germany, and KAGRA (Kamioka Gravitational Wave Detector), University of Tokyo, Japan. 5)
The LISA Pathfinder mission of ESA also started science operations in late February 2016. Launched on December 3rd, 2015 from Kourou, French Guiana, LISA Pathfinder won't detect gravitational waves. It will, however, pave the way for a full-up space based gravitational wave detector, eLISA (evolved Laser Interferometer Space Antenna), set to launch sometime in the 2030s.
• A second gravitational wave source was detected by LIGO as reported on June 15, 2016. The LSC (LIGO Scientific Collaboration)and the Virgo Collaboration used data from the twin LIGO detectors — located in Livingston, Louisiana, and Hanford, Washington — to make the discovery, which is accepted for publication in the journal Physical Review Letters. 6) 7)
From the data of the gravitational wave event, named GW151226, the researchers concluded the second set of gravitational waves were produced during the final moments of the merger of two black holes that were 14 and 8 times the mass of the Sun, and the collision produced a single, more massive spinning black hole 21 times the mass of the Sun. In comparison, the black holes detected in September 2015 were 36 and 29 times the Sun's mass, merging into a black hole of 62 solar masses.
The inferred component masses are consistent with values dynamically measured in X-ray binaries, but are obtained through the independent measurement process of gravitational- wave detection. Although it is challenging to constrain the spins of the initial black holes, we can conclude that at least one black hole had spin greater than 0.2. These recent detections in Advanced LIGO's first observing period have revealed a population of binary black holes that heralds the opening of the field of gravitational-wave astronomy.
The merger occurred approximately 1.4 billion years ago. The detected signal comes from the last 27 orbits of the black holes before their merger. Based on the arrival time of the signals—the Livingston detector measured the waves 1.1 milliseconds before the Hanford detector—researchers can roughly determine the position of the source in the sky.
"GW151226 perfectly matches our theoretical predictions for how two black holes move around each other for several tens of orbits and ultimately merge," said Alessandra Buonanno of UMD (University of Maryland). "Remarkably, we could also infer that at least one of the two black holes in the binary was spinning."
"It is very significant that these black holes were much less massive than those observed in the first detection," said Gabriela Gonzalez, LSC spokesperson and professor of physics and astronomy at Louisiana State University. "Because of their lighter masses compared to the first detection, they spent more time—about one second—in the sensitive band of the detectors. It is a promising start to mapping the populations of black holes in our universe."
• June 1, 2017: Another gravitational wave has been detected by the LIGO (Laser Interferometer Gravitational-wave Observatory). An international team announced the detection today, while the event itself was detected on January 4, 2017. 8) 9)
The team, including engineers and scientists from Northwestern University in Illinois, published their results in the journal Physical Review Letters. 10)
Like the previous two detections, this one was created by the merging of two black holes. These two were different sizes from each other; one was about 31.2 solar masses, and the other was about 19.4 solar masses. The combined 50 solar mass event caused the third wave, which is named GW170104. The black holes were about 3 billion light years away.
LIGO is showing us that their is a population of binary black holes out there. "Our handful of detections so far is revealing an intriguing black hole population we did not know existed until now," said Northwestern's Vicky Kalogera, a senior astrophysicist with the LSC (LIGO Scientific Collaboration), which conducts research related to the twin LIGO detectors, located in the U.S.
"Now we have three pairs of black holes, each pair ending their death spiral dance over millions or billions of years in some of the most powerful explosions in the universe. In astronomy, we say with three objects of the same type you have a class. We have a population, and we can do analysis."
This third finding strengthens the case for the existence of a new class of black holes: binary black holes that are locked in relationship with each other. It also shows that these objects can be larger than thought before LIGO detected them. "It is remarkable that humans can put together a story and test it, for such strange and extreme events that took place billions of years ago and billions of light-years distant from us." – David Shoemaker, MIT.
"We have further confirmation of the existence of black holes that are heavier than 20 solar masses, objects we didn't know existed before LIGO detected them," said David Shoemaker of MIT, spokesperson for the LIGO Scientific Collaboration . "It is remarkable that humans can put together a story and test it, for such strange and extreme events that took place billions of years ago and billions of light-years distant from us."
"With the third confirmed detection of gravitational waves from the collision of two black holes, LIGO is establishing itself as a powerful observatory for revealing the dark side of the universe," said David Reitze of Caltech, executive director of the LIGO Laboratory and a Northwestern alumnus. "While LIGO is uniquely suited to observing these types of events, we hope to see other types of astrophysical events soon, such as the violent collision of two neutron stars."
Further detections of gravitational waves
• February 21, 2019: An international research team including astronomers from the Max Planck Institute for Radio Astronomy in Bonn, Germany, has combined radio telescopes from five continents to prove the existence of a narrow stream of material, a so-called jet, emerging from the only gravitational wave event involving two neutron stars observed so far. With its high sensitivity and excellent performance, the 100 m radio telescope in Effelsberg played an important role in the observations. 11)
- The results are published in this week's issue of „Science". 12)
- In August 2017, two neutron stars were observed colliding, producing gravitational waves that were detected by the American LIGO and European Virgo detectors. Neutron stars are ultra-dense stars, roughly the same mass as the Sun, but similar in size to a city like Cologne. This event is the first and only one of this type that has been observed so far, and it happened in a galaxy 130 million light years away from Earth, in the constellation of Hydra.
- Astronomers observed the event and the subsequent evolution across the entire electromagnetic spectrum, from gamma-rays, X-rays to visible light and radio waves. Two hundred days after the merger, observations combining radio telescopes in Europe, Africa, Asia, Oceania, and North America proved the existence of a jet emerging from this violent collision. These findings are now published in the scientific journal Science by an international team of astronomers, led by Giancarlo Ghirlanda from the Italian National Institute for Astrophysics (INAF).
- This neutron star merger represented the first case where it was possible to associate a detection of gravitational waves to an object emitting light. The event has confirmed scientific theories that have been under discussion for tens of years, and the association of neutron star mergers with one of the most powerful explosions in the Universe: gamma-ray bursts. After the merger, a huge amount of material was expelled into space, forming a shell around the object. Astronomers have been tracing its evolution at different wavelengths. However, there were still some remaining questions concerning this event that could not be clarified by any previous observations.
- "We expected part of the material to be ejected through a collimated jet, but it was unclear whether this material could successfully pierce through the surrounding shell." explains Ghirlanda. "There were two competing scenarios: In one case, the jet cannot break through the shell, instead generating an expanding bubble around the object. In the other, the jet is successful in penetrating the shell and then propagates further into space", expands Tiziana Venturi (INAF). Only the acquisition of very sensitive radio images with very high resolution would discard one scenario or the other. This required the use of a technique known as very long baseline interferometry (VLBI) that allows astronomers to combine radio telescopes all around the Earth.
- The authors of this publication conducted global observations in the direction of the merger on 12 March 2018 using thirty-three radio telescopes from the European VLBI Network (that connects telescopes from Spain, the United Kingdom, The Netherlands, Germany, Italy, Sweden, Poland, Latvia, South Africa, Russia, and China), e-MERLIN in the UK, the Australian Long Baseline Array in Australia and New Zealand, and the Very Long Baseline Array in the USA.
Figure 3: Artist's impression of the jet of material launched after the merger of the two neutron stars (image credit: JIVE, Katharina Immer)
- „Our 100 m radio telescope in Effelsberg participated in the observations and was a key element, due to its high sensitivity and excellent performance", says Carolina Casadio, a member of the research team from the Max Planck Institute for Radio Astronomy (MPIfR).
- The data from all telescopes were sent to JIVE (Joint Institute for VLBI in Europe), The Netherlands, where the most advanced processing techniques were used to produce an image with a resolution comparable to resolving a person on the surface of the Moon. In the same analogy, the expanding bubble would appear with an apparent size equivalent to a truck on the Moon, whereas a successful jet would be detected as a much more compact object.
- "Comparing the theoretical images with the real ones, we find that only a jet could appear sufficiently compact to be compatible with the observed size.", explains Om Sharan Salafia from INAF in Italy. The team determined that this jet contained as much energy as produced by all the stars in our Galaxy during one year. "And all that energy was contained in a size smaller than one light year." says Zsolt Paragi, also from JIVE.
- "Within Europe we utilize the RadioNet consortium for an efficient use of our members' radio telescopes. The observations described here combine radio observatories all over Europe and world wide. They require a well-coordinated effort of the collaborating observatories and institutions to achieve such exciting results", explains Anton Zensus, Director at MPIfR and coordinator of the RadioNet consortium.
- In the coming years, many more of these neutron star binary mergers will be discovered. "The obtained results also suggest that more than 10% of all these mergers should exhibit a successful jet.", explains Benito Marcote from JIVE. "These types of observations will allow us to unveil the processes that take place during and after some of the most powerful events in the Universe.", concludes Sándor Frey from the Konkoly Observatory in Hungary.
Figure 4: Image of the source obtained from the combination of thirty-three radio telescopes from five continents. The source can be seen in the center of the image as a red spot (false color image made entirely for illustration), image credit: Giancarlo Ghirlanda/Science
• December 4, 2018: Scientists with the LIGO and Virgo gravitational wave observatories report four new sets of these ripples in spacetime. Those additions bring the total count to 11, the researchers say in a study published December 3 at arXiv.org, marking major progress since the first gravitational wave detection in 2015 (SN: 3/5/16, p. 6). 13)
- All but one of the 11 sets of waves were stirred up in violent collisions of two black holes. The one remaining detection, reported in October 2017, instead came from the smashup of two stellar corpses called neutron stars (SN: 11/11/17, p. 6).
• December 4, 2018: Researchers from the University of Portsmouth (UoP) have made vital contributions to the observations of four new gravitational waves, which were announced this weekend (1 December). 14)
- The new results are from the National Science Foundation's LIGO (Laser Interferometer Gravitational-Wave Observatory) and the European-based VIRGO gravitational-wave detector. The results were announced at the Gravitational Wave Physics and Astronomy Workshop in College Park, Maryland, USA.
- Three years ago LIGO made the first observation of a binary black hole merger. Today, there have been observations of 11 gravitational-wave signals (10 stellar-mass binary black hole mergers and one merger of two neutron stars, which are the dense, spherical remains of stellar explosions).
- These observations are revolutionizing our understanding of the processes by which high mass stars (10 – 100 times as heavy as our sun) are formed, how they evolve, and the method by which black holes are produced.
- The new events are known as GW170729, GW170809, GW170818, and GW170823, in reference to the dates they were detected. All of the events are included in a new catalogue, also released Saturday, with some of the events breaking records.
- Researchers from the newly-formed Gravitational-Wave Physics Group in the University's ICG (Institute of Cosmology and Gravitation) in the University of Portsmouth have played a significant role in the observation of the first 11 gravitational-wave events.
- Dr Laura Nuttall, a senior lecturer in the ICG, made the initial observation of GW170729. This event, detected in the second observing run on July 29 2017, is the most massive and distant gravitational-wave source ever observed. Any theoretical work trying to understand the mechanisms by which black holes form, now must allow for black holes as massive as this one to be produced.
- Dr Andrew Lundgren, a reader at the ICG, was one of the main developers of the noise subtraction scheme, which was necessary to increase the sensitivity of the LIGO observatories to be able to confirm that GW170729, GW170809 and GW170818 were genuine gravitational-wave signals. Without this work, we would only be talking about eight observed gravitational-wave signals today. 15)
- Dr Ian Harry, a senior lecturer at the ICG, is one of the two main developers of the PyCBC algorithm, which is responsible for searching for merging black holes and neutron stars in Advanced LIGO and Advanced Virgo data. This code made the first observation of many of the 11 gravitational-wave signals seen so far.
- Dr Harry said: "I'm happy to see the vital contributions our researchers have made to the observation of the first 11 gravitational-wave events. Gravitational-wave observations offer us a way to observe astrophysical sources that have never been seen before, including the collision of two black holes. These observations allow us to begin to understand the processes by which black holes are produced and explore the environments in which they are formed.
- "However, this is only the beginning of gravitational-wave astronomy, and as our observatories become more sensitive we expect to observe hundreds of sources in the coming years. My personal hope is that we observe something truly unexpected in the next years, which would help us to better understand the Universe that we live in. Gravitational-wave astronomy is one of the fastest growing fields in astronomy and collaboration between the new Gravitational-Wave Physics Group and existing ICG researchers offers tremendous possibilities for future world-leading research."
Figure 5: These observations allow us to begin to understand the processes by which black holes are produced and explore the environments in which they are formed (image credit: UoP Team)
• October 16, 2017: Another LIGO gravitational wave detection has spawned an explosion of new science across the global astronomical community. On August 17, 2017, the two LIGO instruments (funded by the National Science Foundation) and its sister facility, Virgo, near Pisa, Italy, sensed tell-tale signs of the remnant cores of two massive stars spiraling toward and then smashing into each other some 130 million light years away. The objects were quickly identified as neutron stars, the collapsed cores of stars that were once much more massive than our Sun. They are called "neutron stars" because their matter is so densely packed it is composed primarily of neutrons. One such star containing as much matter as our Sun would be just 10 to 15 km in diameter, and a teaspoon of its material would weigh about one-billion tons on Earth. Using the signals received in LIGO's detectors, the masses of the neutron stars were determined to 1.1 to 1.6 times as massive as our Sun. 16)
- LIGO Hanford Observatory (LHO) Head, Michael Landry explained what LIGO saw when it made this landmark discovery: "LIGO and Virgo detected 100 seconds of gravitational waves as these two neutron stars spiraled together in a massive and fiery collision," he said. "In a sprawling follow-up campaign involving about one-quarter of the world's professional astronomers, observatories in space and on the Earth have detected radiation in all wavelengths from gamma rays to radio waves. But the LIGO and Virgo detectors were absolutely essential in identifying and pinpointing the event in the sky, allowing this campaign to proceed", Landry added.
- This discovery adds a new way of learning about the universe through "multi-messenger astronomy", where data from traditional telescopes, neutrino detectors, and now gravitational wave observatories are shared and compared to glean even deeper insights into the nature of the universe.
- This historic detection came just three days after another historic detection, LIGO's fourth, which was also detected by the Virgo interferometer in Pisa Italy, making it the first detection by Virgo, and the first three-detector observation of a gravitational wave. Reveling after that event, LIGO scientists were astonished to learn of yet another detection, this one completely different from anything LIGO had seen before.
- Salvatore (Salvo) Vitale, assistant professor of physics at MIT, was attending a conference in Amsterdam along with other LIGO scientists, when he first got word of this second detection in 3 days. The first alert he received included a ‘false alarm rate' (FAR), a measure of how likely it is that the event was not real. In this case, the FAR was reported as 3 x 10-12, which is, according to Vitale, "ridiculously low!"
- How ridiculously low? This figure suggests that the chance that some random but nearly identical bits of ‘noise' that happened to look like gravitational waves appeared in the instruments at essentially the same time was less than 1 in 80,000 years.
- Two minutes after that first alert, the first scan of the event, automatically generated from the Hanford data, was distributed, and it was distinctly different from anything LIGO had seen before. Signals of black hole mergers last just fractions of a second. This signal lasted well in excess of 30 seconds (in the end, it was shown to have lasted nearly 2 minutes, 500 times longer than black hole mergers). This was a clear indicator that the objects that created the signal were much less massive than black holes. To Vitale and everyone else, the unique properties of the signal could mean only one thing: LIGO had caught its first gravitational wave from merging neutron stars.
- This was in itself a surprise, as Vitale explained. "I saw the omega scan from Hanford, and saw that there was a clear chirp signal, which I remember thinking is ridiculous, because we never thought we'd see anything in an omega scan from a binary neutron star merger .... But this [one] was so loud that we saw it too!"
Figure 6: Top: Thirty-seconds worth of binary neutron star inspiral as it appeared in the LIGO detectors. The entire signal lasted 100 seconds. Bottom: LIGO's first black hole merger detection. The duration of the "chirp" was just 0.2 seconds; 500 times shorter than the signal generated by the neutron stars (image credit: Caltech/MIT/LIGO Lab)
- At the same time that all this was happening, LIGO scientists were alerted to another remarkable astronomical event, which occurred within 2 seconds of LIGO's detection. The Fermi gamma ray space telescope had recorded a "short" gamma ray burst (sGRB) just 1.7 seconds after the arrival of the gravitational waves.
- Gamma ray bursts are seen quite frequently, but what causes them has remained a mystery. Knowing that neutron star mergers were expected to generate electromagnetic radiation, likely of very high energy, excitement among LIGO scientists began to grow as it became more and more plausible that the first electromagnetic counterpart to a gravitational wave (GW) had been observed. The time of arrival of the sGRB and GW signals was especially telling, and important to validating the relationship between them.
- Vitale explained, "You want the gamma ray burst to come after the gravitational waves because first you have to smash the objects together, then the material is warmed up, and then you get the radiation. So you would expect to see the gravitational waves first."
- As the pieces began to fall into place, the magnitude of LIGO's detection became all the more weighty.
- "Then it was, like .... ‘Okay. Oookay .... let's take a chair .... and sit down ....." said Vitale, laughing as he recalled his feelings at that moment.
- The only way to confirm a correlation between the GRB and the GW, however, would be to find the source object on the sky; but there was a problem. At that point, only the LIGO Hanford data had been processed and distributed; without the Livingston data, no such localization of a source would be possible.
- Matt Evans (Assistant Professor of Physics at MIT) recalls the flurry of communication he was receiving in those early moments.
- "There was this hubbub by Salvo talking about a signal at LHO that looked like a binary neutron star coincident with the Fermi alert. But there hadn't been anything from Livingston, so there was a moment of doubt of the validity of the signal."
- The missing data from Livingston was puzzling. Reed Essick (Postdoctoral Fellow, UChicago Kavli Institute for Cosmological Physics) explained: "On the search side, everything looked good, and a sanity check of the detectors told us that LLO (LIGO Livingston Observatory) was in science-mode. So why didn't the event ‘trigger' in Livingston?"
- Essick decided to check Livingston data for ‘glitches', random bits of loud, sudden noise that can drown out other signals in the detectors. Running an algorithm designed specifically for this task, Essick saw that a glitch had in fact occurred at LLO at the same time that the signal appeared in the Hanford interferometer. Sifting through the files manually, Essick found the data stamped with the time of the glitch (and the detection), and there it was.
- This was why LLO didn't automatically send out a trigger alert. The glitch caused LLO's computers to disregard, or ‘veto' that part of the data stream. Looking at it, it's no wonder! At first glance, it looked ugly. However, Evans explained that it really wasn't as bad as it seemed.
- "The glitch looks really terrible on the scan. But the truth is, it's large in amplitude and short in time, so it wouldn't ruin our ability to do any science on it."
- Evans added, "Glitches happen every few hours, so the probability of one landing on top of a signal is very low. Nevertheless, people have been working on this sort of possibility for a while, so we were prepared."
- Most remarkably, despite the size of the glitch, the gravitational wave signal itself was still clearly visible (see image of Figure 7). Seeing that was a moment that Vitale remembered vividly.
- "It was a mix of happiness, tension, and disbelief. We saw that beautiful image of the chirp going through the glitch and coming out the other side. And at that point, it was pretty incredible."
- In a mathematical equivalent of a game of Operation, at least three teams of people began working on separating the glitch from the signal. As with everything LIGO-related, even cleaning a fraction of a second of data required a group effort! Ultimately, the work paid off. After a few hours, the glitch had been cleanly removed, and some extraordinary science was about to begin.
Spreading the Word
- In anticipation of this kind of event, over the years, LIGO had signed agreements with 90 astronomical observatories around the world to hunt for signs of electromagnetic (EM) radiation escaping from a gravitational wave event; LIGO would share sky-coordinates with its partners, who would then start searching. Until 17 August 2017, no one had found any such counterparts, but the lure of being among the first ones to detect some familiar radiation from a gravitational wave event has kept LIGO's astronomy partners engaged for over 10 years. This event, combined with a coincident gamma ray detection, represented the best chance yet for astronomers to find something, but to do so, LIGO had to tell them where to look. To that end, LLO data (cleaned of the glitch) were combined with Hanford and Virgo data, and a sky-map narrowing down the possible location of the source of the gravitational waves was generated. The GW map was then merged with Fermi's GRB map and another region calculated using the INTEGRAL gamma ray space telescope. The result was nothing short of amazing.
Figure 7: The glitch that prevented LIGO Livingston's automated system from distributing the signal from the merging neutron stars. Despite the glitch, the curved GW signal is clearly visible. It looked bad at first, but only 10ms of data needed to be cleaned up (image credit: Caltech/MIT/LIGO Lab)
- The location of the source of the gravitational waves as predicted by LIGO-Virgo data sat beautifully inside the regions of the sky estimated to contain the source of the gamma ray burst as determined by Fermi and INTEGRAL. The resulting search area was small enough that within 12 hours LIGO's optical astronomy partners had successfully tracked down and imaged a residual fireball at the edge of a galaxy (NGC4993) some 130 million light years distant–most of that time was spent waiting for dusk in Chile, where the first observations could be made. The long-awaited discovery of an EM counterpart to a gravitational-wave detection had been confirmed! Furthermore, the answer to another long-sought-after question was solved: Astronomers could now say with certainty, that at least one source of short gamma ray bursts in the universe is merging neutron stars.
Figure 8: The skymap created by LIGO-Virgo (green) showing the possible location of the source of gravitational waves, compared with regions containing the location of the gamma ray burst source from Fermi (purple) and INTEGRAL (grey). The inset shows the actual position of the galaxy (orange star) containing the "optical transient" that resulted from the merger of two neutron stars (image credit: NASA/ESO)
Figure 9: "The six happiest people in the world" on August 17, 2017. Salvatore Vitale (left) snapped this photo moments after first seeing the LLO scan. The tell-tale trace of gravitational waves generated by merging neutron stars was clearly visible. Clockwise from top: Chris van den Broeck, Jo van den Brand, Peter Pang, Ka Wa Tsang, and Michalis Agathos (image credit: Salvo Vitale)
Figure 10: The first image of an 'optical transient' resulting from the merger of two neutron stars, and the first image of an optical counterpart to a gravitational wave detection. The box at left shows the host galaxy NGC4993, 130 million light years distant, as it appeared in a Hubble Space Telescope image taken April 28, 2017. At right is the same galaxy imaged by the Swope Telescope just a few hours after the gravitational wave and gamma ray detections on August 17, 2017. The arrow points to the short-lived visible fireball that resulted from the merger of two neutron stars in that galaxy (image credit: Swope Supernova Survey via UC Santa Cruz)
- For LIGO, this optical observation was important for another reason. The distance to the galaxy, as determined by astronomers, was wholly consistent with LIGO's estimated distance of the source of the gravitational waves. Thus, the astronomers provided a completely independent verification that LIGO's methods for determining the distances to gravitational wave sources are sound.
- From that moment on, all eyes were, and continue to be (now two months after the initial detection), on the skies. LIGOs astronomy partners immediately began observing this object in every wavelength possible, from gamma rays to visible radiation to radio waves, as the remnant ‘object' changes over time. To date, some 70 of LIGO's optical astronomy partners have observed this extraordinary event.
- Today, few would disagree with the statement that the level of interest in and study of this event is unprecedented. Within 8 weeks of the detection, over 100 scientific papers describing the results of follow-up studies were written by scientists around the world. Dozens of these papers were published on Monday, October 16th alone, with many more certain to follow in the months and possibly years to come, making this the most broadly and intensely studied astronomical event in human history.
Beyond the obvious scientific importance of this discovery, the importance of this event for the LIGO Laboratory and the wider collaboration goes much deeper. For many, this single detection represents the apex of careers, the culmination of decades of hard work and dedication to LIGO and gravitational wave science.
Dr. Fred Raab, Associate Director for Observatory Operations at the LIGO Hanford Observatory, shared what this latest discovery means to him: "After nearly 30 years of working toward this discovery, I knew that observing the last minute of a binary neutron star system would give unprecedented precision in its parameters. Yet I was unable to continue reading an early paper draft past where I first saw the number for the chirp mass, a key parameter of the system. I stared in wonder for minutes at that number, measured to 4 significant figures for a pair of stars more than 100 million light years from Earth."
Raab continued, "This observation means that LIGO is transitioning now from studying extreme regions of space-time to extreme states of matter."
Mike Zucker, LIGO Systems Scientist, had a similar reaction in those first days after the detection:
"I literally stayed awake for days after GW170817 watching the [astronomy notices] roll in, marveling at all the extraordinary implications as the revelations topped each other one by one. I'm just a detector mechanic, but I consider this to be the most significant achievement of my career."
Janeen Romie, LLO's Detector Engineering Group Lead, was in her office in Livingston talking to her husband on the phone when she got the first alert: "I noticed that it was a binary neutron star merger and I was like, ‘I've got to get off the phone! I've got to run down the hall! I've got to find out what's going on!"
Unable to share the news with anyone outside of LIGO, the only thing Romie could do is run and talk to her colleagues.
"That's why it was so funny for me," she laughed, "I hung up on my husband!"
Matt Evans shared what he believes is most meaningful to the LIGO laboratory in light of this discovery: "This detection, and especially the triple binary black hole detection with Virgo are important because they demonstrate that we (LIGO) are not the only ones claiming to detect gravitational waves", he said. "This event solidifies our position in astronomy, not just physics. Other projects around the world will benefit greatly. We really can do multi-messenger astronomy, and that is really meaningful and useful."
This sentiment was echoed by Essick and Vitale: "What's most important is represented by the ‘O' in LIGO", said Essick, referring to the fact that the "O" in LIGO stands for Observatory. "We've been selling the idea that we will detect binary neutron stars for decades, and now we've finally done it. We've delivered on that promise."
Vitale would agree, "I think this event brings us a step closer to astronomy", he said. "The detection of these events ...it's not just ‘collecting stamps' anymore. Now we can do lots of cool stuff."
Salvatore's feelings ran a bit deeper still: "Those few days were among the most beautiful days of my life", he said. "We kept receiving the circulars from the astronomers. They'd say, ‘oh, we have a GRB', ‘oh, we have found an EM counterpart in optical', ‘oh, we found the galaxy', ‘we found the X-rays', etc. It was ... incredible."
He paused for a moment; then continued: "It's also been sad", he said. "I don't know if I'll ever live a moment like that again in my life." - It's doubtful that anyone at LIGO will.
Status of LIGO
• February 18, 2019: The National Science Foundation (NSF) is awarding Caltech and MIT $20.4 million to upgrade the Laser Interferometer Gravitational-wave Observatory (LIGO), an NSF-funded project that made history in 2015 after making the first direct detection of ripples in space and time, called gravitational waves. 17) 18)
- The investment is part of a joint international effort in collaboration with UK Research and Innovation and the Australian Research Council, which are contributing additional funds. While LIGO is scheduled to turn back on this spring, in its third run of the "Advanced LIGO" phase, the new funding will go toward "Advanced LIGO Plus."
- Advanced LIGO Plus is expected to commence operations in 2024 and to increase the volume of deep space the observatory can survey by as much as seven times.
- "I'm extremely excited about the future prospects that the Advanced LIGO Plus upgrade affords gravitational-wave astrophysics," said Caltech's David Reitze, executive director of LIGO.
- "With it we expect to detect gravitational waves from black hole mergers on a daily basis, greatly increasing our understanding of this dark sector of the universe. Gravitational-wave observations of neutron star collisions, now very rare, will become much more frequent, allowing us to more deeply probe the structure of their exotic interiors."
- Since LIGO's first detection of gravitational waves from the violent collision of two black holes, it has observed nine additional black hole mergers and one collision of two dense, dead stars called neutron stars.
- The neutron star merger gave off not just gravitational waves but light waves, detected by dozens of telescopes in space and on the ground. The observations confirmed that heavy elements in our universe, such as platinum and gold, are created in neutron star smashups like this one.
- "This award ensures that NSF's LIGO, which made the first historic detection of gravitational waves in 2015, will continue to lead in gravitational-wave science for the next decade," said Anne Kinney, assistant director for NSF's Mathematical and Physical Sciences Directorate, in a statement.
- "With improvements to the detectors - which include techniques from quantum mechanics that refine laser light and new mirror coating technology - the twin LIGO observatories will significantly increase the number and strength of their detections. Advanced LIGO Plus will reveal gravity at its strongest and matter at its densest in some of the most extreme environments in the cosmos. These detections may reveal secrets from inside supernovae and teach us about extreme physics from the first seconds after the universe's birth."
- Michael Zucker, the Advanced LIGO Plus leader and co-principal investigator, and a scientist at the LIGO Laboratory, operated by Caltech and MIT, said, "I'm thrilled that NSF, UK Research, and Innovation and the Australian Research Council are joining forces to make this key investment possible. Advanced LIGO has altered the course of astrophysics with 11 confirmed gravitational-wave events over the last three years. Advanced LIGO Plus can expand LIGO's horizons enough to capture this many events each week, and it will enable powerful new probes of extreme nuclear matter as well as Albert Einstein's general theory of relativity."
Figure 11: Aerial view of LIGO Hanford Observatory (image credit: LIGO)
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The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (email@example.com)