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LIGO (Laser Interferometric Gravitational-wave Observatory)

Interferometer    LIGO Facility    Detection Status    Facility Status    References

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.

LIGO Interferometer

LIGO is a national US facility for gravitational-wave research providing opportunities for the broader scientific community to participate in detector development, observation, and data analysis. The design and construction of LIGO was carried out by a team of scientists, engineers, and staff at the California Institute of Technology (Caltech) and the Massachusetts Institute of Technology (MIT), and collaborators from over 80 scientific institutions world-wide that are members of the LSC (LIGO Scientific Collaboration).

LIGO is the world's largest gravitational wave observatory and a marvel of engineering. Comprising two enormous laser interferometers located thousands of kilometers apart, LIGO exploits the physical properties of light and of space itself to detect and understand the origins of gravitational waves. 3)

LIGO's interferometers are the largest ever built. With arms 4 km long, they are 360 times larger than the one used in the Michelson-Morley experiment (which had arms 11 m long).


Figure 1: Basic Michelson interferometer with Fabry Perot cavities. Mirrors placed near the beam splitter keep the laser contained within the arms. This increases the distance traveled by the beams, greatly improving LIGO's sensitivity to changes in arm length like those caused by gravitational waves (image credit: Caltech, MIT)

This is particularly important in the search for gravitational waves because the longer the arms of an interferometer, the farther the laser travels, and the more sensitive the instrument becomes. Attempting to measure a change in arm length 1,000 times smaller than a proton means that LIGO has to be more sensitive than any scientific instrument ever built, so the longer the better. But there are obvious limitations to how long one can build an interferometer. Even with arms 4 km long, if LIGO's interferometers were basic Michelsons they would still not be long enough to detect gravitational waves...and yet they are. How is this possible?

This dilemma was fixed by adding something called "Fabry Perot cavities" to the basic Michelson design. The Figure 1 shows how a basic Michelson interferometer is modified to include Fabry Perot cavities. It is created by adding mirrors near the beam splitter that continually reflect parts of each laser beam back and forth within the 4 km long arms about 280 times before they are merged together again.

With Fabry Perot cavities, LIGO's interferometer arms are effectively 1120 km long, making them 144,000 times bigger than Michelson's original instrument! This bit of 'mirror magic' greatly increases LIGO's sensitivity and makes it capable of detecting changes in arm-length thousands of times smaller than a proton, all while keeping the physical size of the interferometer manageable.

Those familiar with telescopes will recognize this effect. Increasing a telescope's focal length doesn't just increase the magnification of any given eyepiece, it also magnifies the tiniest vibrations making them visible in the eyepiece; the longer the focal length, the smaller the vibration you see in the eyepiece. In a telescope, these vibrations are unwelcome, but LIGO is designed to feel them. And at effectively 1120 km long, LIGO's arms can readily magnify the smallest conceivable vibrations enough that they are measurable.

Power Boosted Laser: Length isn't the only design factor important to LIGO's sensitivity; laser power is too. While increasing length increases the interferometer's sensitivity to vibrations, increasing laser power improves the interferometer's resolution. The more photons that merge at the beam splitter, the sharper the resulting interference pattern becomes, making it 'easier' to recognize a gravitational wave signature.

But there's a problem here too. For LIGO to operate at full sensitivity, its laser has to shine at 750 kW, but LIGO's laser enters the interferometer at most at 200 W. And just as it is impossible to build a 1120 km-long interferometer, building a 750 kW laser is also a practical impossibility. So how does LIGO boost the power of its laser 3750 times without actually using more power?


Figure 2: Basic Michelson Interferometer with Fabry Perot cavities and Power Recycling mirror. LIGO's interferometers use multiple power recycling mirrors, but for simplicity only one is shown in the diagram (image credit: Caltech, MIT)

More mirrors! Specifically, "power recycling" mirrors placed between the laser source and the beam splitter. Like the beam splitter itself, the power recycling mirror is only partly reflective (a 'one-way mirror'). Figure 2 shows schematically where such a mirror is located.

In a power recycling mirror, light from the laser first passes through the mirror to reach the beam splitter where it is split and directed down the arms of the interferometer. The instrument is aligned so well that nearly all of the reflected laser light from the arms follows a path back to the recycling mirrors rather than to the photodetector. Laser light coming from the ends of the arms is thereby reflected back into the interferometer (hence 'recycling') where those photons add to the ones just entering; more photons equals more power. This process greatly boosts the power of the beam without needing to generate a 750 kW beam at the outset.

The boost in power generated by this recycling process enhances the interference pattern that results when the two beams are superimposed after their long journey through the interferometer. Since we expect to see particular interference patterns when a gravitational wave passes by, the more prominent the pattern, the easier it is for us to recognize and confirm that we have, in fact, detected gravitational waves.

LIGO Technology

Designing instruments like LIGO's interferometers, capable of measuring a distance on the order of 10-19 m required inventing and refining innovative technology. Most of LIGO’s most impressive technology resides in its seismic isolation systems (which remove unwanted vibrations), vacuum systems (to make sure the laser light is kept pure), optics components (to preserve laser light and laser power), and computing infrastructure (to handle the mindboggling amount of data that LIGO collects). These systems are like LIGO's internal organs. If any one fails, the whole instrument suffers (Ref. 3).

While each of these components is a feat of engineering in itself, without working seamlessly together, LIGO, as a single multifaceted instrument could never achieve its scientific goals. A basic overview of each of LIGO's critical systems is provided.

Seismic Isolation: LIGO’s greatest strength is also its greatest weakness. Since LIGO is designed to sense the smallest conceivable motions of mirrors caused by the passage of a fleeting gravitational wave, it is also extremely sensitive to all vibrations near (such as trucks driving on nearby roads) and far (earthquakes on the other side of the world). Without taking extraordinary measures, any number of Earthly vibrations could move LIGO’s primary mirrors (LIGO scientists call them “test masses”) enough to hide a gravitational wave signal. Isolating LIGO from as much environmental vibration as possible is the linchpin in LIGO’s quest to feel gravitational waves. To that end, LIGO uses multiple means of eliminating vibration falling into two broad categories: “active” and “passive” damping systems.

Active Damping: The first line of defense against unwanted vibration is LIGO’s “active” damping system. The Internal Seismic Isolation (ISI) system consists of devices that sense ground movements and then deliberately perform counter movements to eliminate them, keeping the instrument motion-free.

Obviously, the local environment is always creating vibrations of one form or another. LIGO’s ISI system contains sensors designed to feel different frequencies caused by different environmental vibrations. These sensors work side-by side and send their signals to a computer that combines the effects of all of these motions and then generates a net counter-motion to cancel all of the vibrations simultaneously. It is very similar to how noise-canceling headphones work!

Passive Damping: LIGO’s passive damping system holds the all-important mirrors perfectly still through a 4-stage pendulum called a "quad". In the quad, LIGO’s test masses (its mirrors) are suspended at the end of four pendulums by 0.4 mm thick fused-silica (glass) fibers. The "Main Chain" side faces the laser beam, while the "Reaction mass" side helps to keep the test mass steady from noise not associated with sources from space. This configuration absorbs any movement not completely canceled out by the active (ISI) system. The sheer mass of the suspension components (each mirror weighs 40 kg) also helps to prevent motion of the mirrors thanks to the Law of Inertia.

Working together, these active and passive vibration damping systems ensure that LIGO's lasers and mirrors are isolated from as much external noise and vibration as is physically possible.

Vacuum: LIGO contains one of the largest and purest sustained vacuums on Earth. In volume, it is surpassed only by the Large Hadron Collider in Switzerland. The atmospheric pressure inside LIGO's vacuum tubes is one-trillionth that of air pressure at sea level. LIGO needs such a good vacuum for two reasons:

1) Air – or even just a few molecules of air – can create noise which masks the tiny changes in distance between mirrors we seek to detect. One way this can happen is due to Brownian motion, or the fact that everything with a temperature above absolute zero is moving with heat energy. Molecules of air hitting the mirrors can cause them to move, masking the gravitational waves. Another way residual air in the laser light path causes problems is similar to the shimmering one sees over a hot road – air has an ‘index of refraction’, and like a glass lens, can change the path of light. Even just a few molecules passing through the laser light beam can affect the apparent distance between the mirrors, once again masking the subtle effects of a passing gravitational wave.

2) The second critical reason for operating in a vacuum is to eliminate the chances that dust will drift into the path of the laser, or worse, onto a mirror causing some of the light to scatter (i.e., be reflected in some random direction away from its path).

Without operating in such a high quality vacuum, LIGO’s lasers would be absorbed and deflected enough to create unwanted interference patterns that might be mistaken for or drown out a signal from a gravitational wave. Losing any laser photons could cripple LIGO’s ability to detect gravitational waves.

Creating such a large volume of empty space on Earth was no easy task. Many techniques were used to remove all the air and other molecules from LIGO’s vacuum tubes:

• The tubes were heated to between 150ºC and 170ºC for 30 days to drive out residual gas molecules.

• Turbo-pump vacuums (like little jet engines that create suction instead of thrust) sucked out the bulk of the air contained in the tubes

• Ion pumps then extract individual remaining gas molecules by electrically charging them and then attracting them away with opposite charge, like a magnet. Since the metal inside the vacuum chamber is always emitting some gaseous molecules ("outgassing"), these pumps operate continuously in order to maintain the pristine vacuum inside the tubes.

It took 40 days (1100 hours) to remove all 10,000 m3 of air and other residual gases from each of LIGO’s vacuum tubes to reach an air pressure one-trillionth that at sea level.

Optics System: LIGO's optics system consists of lasers, a series of mirrors, and a photodetector (a device that measures varying light levels). In order to measure a movement thousands of times smaller than a proton, LIGO's optical components must operate harmoniously and with unprecedented precision. It all begins with the main laser.

LASER: We all encounter lasers daily in laser pointers, in cat toys, or in the barcode scanners at the grocery store. Because of their omnipresence most of us tend to take them for granted without really knowing how they work. If you're one of those people, you are not alone! If you want to know how they work, Cambridge University’s “Naked Science Scrapbook” video, “How do lasers work?” provides a fun, easy-to-understand explanation. Once you grasp the basic principles, understanding LIGO's laser is a snap!

The first thing to understand is that the word, "laser" is actually an acronym for “Light Amplification by the Stimulated Emission of Radiation”. This means that "laser" refers to a process more than a thing, but most of us now use the term ubiquitously to refer to the device that generates the laser beam, or the beam itself. This might be splitting hairs, but when it comes to understanding LIGO's laser, it's an important distinction.

The heart of LIGO is its 200 W laser beam. But you might be surprised to know that the beam doesn't start out at 200 W. It actually takes four steps to amplify its power and refine its wavelength to a level of precision never before seen in a laser of this kind.

The very first glimmer of light that ultimately becomes LIGO's powerful laser emerges from a laser diode, which uses electricity to generate an 808 nanometer (nm) near-infrared beam of about 4 W. This is the same kind of device that a typical laser pointer uses. While 4 W doesn't seem like a lot, the laser in the average laser pointer shines at less than 5 mW (milliWatt). So LIGO's 4 W beam is 800 times more powerful than that laser pointer you use to entertain your pet!

The second step in boosting LIGO's laser up to 200 W occurs when the 4 W beam enters a device called a Non-Planar Ring Oscillator (NPRO). The NPRO consists of a remarkably small boat-shaped crystal about the size of a pinky fingernail! The 4 W beam bounces around inside this crystal and stimulates the emission (the S and E in LASER) of a 2 W beam with a wavelength of 1064 nm (in the invisible infrared part of the spectrum).

Step three in LIGO's laser amplification occurs when the now 2 W beam enters another amplifying device that boosts the 1064 nm beam from 2 to 35 W. Getting from 35 W to 200 W requires a different kind of device, however. So the 35 W beam is sent through a device called a High Powered Oscillator (HPO), which performs further amplification and refinement, and generates the 200 W beam of pristine "lased" light. This is the beam that ultimately enters LIGO's interferometer.

This multi-stage amplified laser is required for LIGO because of its need to continually produce a pristine single wavelength of light. In fact, LIGO's laser is the most stable ever made to produce light at this wavelength. This stability is one of several factors critical for LIGO's ability to detect gravitational waves.

Mirrors: LIGO’s mirrors are the highest quality available, both in material and shape. Made of very pure fused silica glass, they absorb just one in 3-million photons that hit them. This is important because it means that most of the laser light is reflected. Reflecting most of the light means that the mirrors are not prone to heating. Too much heat from the laser could alter the mirror shapes enough that they degrade the quality of the laser light. Any degradation would hamper LIGO's ability to distinguish a gravitational wave from environmental noise. Lastly, the highly reflective surface preserves laser power — again, the more power, the better LIGO's resolution.

The mirrors also refocus the laser, keeping the beam traveling coherently, meaning that it doesn't spread out as it travels throughout its multiple reflections before encountering the photodetector.

Finally, the mirrors were polished so precisely that the difference between the theoretical design (the perfect mirror shape as designed on a computer) and the actual polished mirror surface is measured in atoms! This is critical because, with all the reflections it goes through, each laser in each arm travels about 1120 km before being merged with its partner and reflected one last time to the photodetector. Maintaining the stability and purity of the laser light is one of LIGO's biggest challenges.

Computation and Data Collection: Computers are required both to run the LIGO instruments and to process the data that it collects.

When it is in 'observing' mode, LIGO generates terabytes (1000's of gigabytes) of data every day. All of this information must be transferred to a network of supercomputers for storage and archiving. Such supercomputers are located at each of the observatories, at Caltech, at MIT, and at various other institutions. Once the data is secured, scientists can use customized computer programs to scour the data for gravitational waves.

The amount of data LIGO collects is as incomprehensively large as gravitational wave signals are small. LIGO's archive already holds the equivalent over over 1-million DVDs of data and will add the equivalent of about 178-thousand DVDs each year to its archive. In actual numerical terms, the data archive at Caltech holds over 4.5 Petabytes (PB, or 4.5 x 1015bytes) of data, and will grow at a rate of about 0.8 PB (800 terabytes) per year. What's a petabyte? If you wanted to count up to a petabyte by counting one byte per second, it would take you 35.7 million years to reach one petabyte!

Storing information is one thing; processing it is another. Processing and analyzing all of LIGO's data requires a vast computing infrastructure. For LIGO's first observing run in 2015, the LIGO Lab will provide 35 MSU (million service units) worth of computing cycles/time. This is equivalent to running a modern 4-core laptop computer for 1,000 years! The amount of computing time is expected to grow by a factor of 10 to around 400 MSU by the time LIGO has completed its third observing run.

LIGO Facility

LIGO's original instrument, a largely 'proof of concept' model dubbed "Initial LIGO", engaged in "science observations" from 2002 to 2010. No detections were made in that time, but enormous strides in detector engineering were achieved as a result of what was learned during that initial run. 2010 marked the end of the Initial LIGO project, and as planned, between 2010 and 2014, both interferometers were completely overhauled to incorporate much more sophisticated engineering.

This "Advanced LIGO" project successfully improved the capabilities of the detectors, and within days of turning on the new and improved instruments, LIGO made its first detection of gravitational waves, generated by a pair of colliding black holes some 1.3 billion light years away. Since that historic day, LIGO's engineers have continued to improve the detectors' sensitivities. The success of these improvements is evidenced by the many more gravitational wave detections that have since been made. Ultimately, with continued refinement and upgrading, Advanced LIGO's detectors will achieve a sensitivity 10 times greater than Initial LIGO, bringing 1000 times more galaxies into LIGO's observational range.


Figure 3: 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. 4) 5)

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.

The existence of gravitational waves was first demonstrated in the 1970s and 80s by Joseph Taylor, Jr., and colleagues. Taylor and Russell Hulse discovered in 1974 a binary system composed of a pulsar in orbit around a neutron star. Taylor and Joel M. Weisberg in 1982 found that the orbit of the pulsar was slowly shrinking over time because of the release of energy in the form of gravitational waves. For discovering the pulsar and showing that it would make possible this particular gravitational wave measurement, Hulse and Taylor were awarded the Nobel Prize in Physics in 1993.

• October 3, 2017: The LIGO Laboratory, comprising LIGO Hanford, LIGO Livingston, Caltech, and MIT are excited to announce that LIGO’s three longest-standing and greatest champions have been awarded the 2017 Nobel Prize in Physics: Barry Barish and Kip Thorne of Caltech and Rainer Weiss of MIT. The LIGO Scientific Collaboration is absolutely delighted to congratulate Rainer Weiss, Barry Barish, and Kip Thorne on winning the 2017 Nobel Prize in Physics. Weiss and Thorne are two of the founders of the LIGO project. Barish was the Principal Investigator of LIGO from 1994 to 2005, during the period of its construction and initial operation. 6)


Figure 4: Image credit: LIGO/Caltech/MIT

Decades in the Making: While from the outside, it may seem surprising that this Nobel Prize was awarded a scant 2 years after the discovery of gravitational waves (often, Nobel Prizes are awarded many years after discoveries), for the three laureates, it actually comes at the culmination of decades of effort. LIGO may have only recently detected gravitational waves, but its journey to doing so began nearly 45 years ago.

The very idea for LIGO came to Rainer Weiss in the early 1970’s when, as associate professor of physics at MIT, he had to find a way to explain gravitational waves (a prediction of general relativity) to his students. In an interview with MIT news writer, Jennifer Chu, Weiss recalled his revelation:

“That was my quandary at the time, and that’s when the invention was made. I said, ‘What’s the simplest thing I can think of to show these students that you could detect the influence of a gravitational wave? … The obvious thing to me was, let’s take freely floating masses in space and measure the time it takes light to travel between them. The presence of a gravitational wave would change that time. [Later] knowing what you could do with lasers, I worked it out: Could you actually detect gravitational waves this way? And I came to the conclusion that yes, you could detect gravitational waves ....”

Sometime later, in 1972, Weiss carefully thought through and wrote down his idea, subsequently publishing it as a paper titled, "Electromagnetically Coupled Broadband Gravitational Antenna" . In this paper, Weiss described in great detail, the design and promise of using laser interferometry to detect gravitational waves. Within its 22 pages, the paper laid out the blueprint for the Laser Interferometer Gravitational-Wave Observatory (at the time, Weiss called it an antenna.) — And with that, LIGO was born (at least on paper).

Today, no one disputes the fact that LIGO owes its very existence to Rainer Weiss.

Transforming LIGO from concept to reality, however, would take another 20 years. There is no way that one man, even the enigmatic Rai Weiss, could get LIGO built by himself. So early on in those first two decades of navigating a circuitous path, trying to keep the idea of a radical scientific experiment alive, Rai found an ideal partner. In 1975, a fateful meeting between Weiss and Kip Thorne of Caltech would set into motion the development of one of the most complicated and risky scientific experiments ever conceived. Thorne, already a highly respected, accomplished, and influential theoretical physicist (with expertise in gravitational waves since the late 1960s), commanded a level of respect among peers, colleagues, research groups, and funding agencies that was unequalled. Kip’s contributions in setting the astrophysics goals were central to the design of the Observatories and the first instruments, and his presence lent a high level of validity and credence to the idea.

As much as Rai is responsible for conceiving of LIGO, Kip Thorne is equally responsible for convincing countless others of LIGO’s potential for success. As a result of his interactions with Weiss, Thorne convinced Caltech to create a gravitational-wave research group. The group would be led by Ron Drever, from Glasgow University (Drever, considered a co-founder of LIGO, would go on to co-invent the Pound–Drever–Hall technique for laser stabilization, which was critical to LIGO’s ability to detect gravitational waves). Sadly, Drever passed away earlier this year, but not before he learned of LIGO’s success.

Thus was formed the LIGO triumvirate. With Rai’s vision and original idea, Thorne’s brilliant theoretical physics mind, Drever’s brilliant engineering, and all of their remarkable capacities to champion the effort, nothing would stand in the way of LIGO’s evolution from concept to reality.

In 1989, Weiss and Thorne, along with Ron Drever, Fred Raab, and Robbie Vogt, submitted a proposal for LIGO to the U.S. National Science Foundation (NSF) which has enthusiastically supported this effort from the start. The proposal included Rai’s original design for the instrument, updated to include engineering and technology innovations that had occurred in the intervening 17 years. To their credit, and at great risk, the NSF approved the proposal. In 1994 construction began on the twin LIGO detectors in Hanford, Washington and Livingston, Louisiana.

That’s when Barry Barish joined the trio. Barish became LIGO Principal Investigator in 1994, helping the growing cadre move from a small intensely focused group cooking up basic ideas to a large and broad team that could actually deliver the Observatories and hardware. Barry came with knowledge of how to build a project that would succeed, but more importantly, he came with vision and an incredible ability to strategize and make the science happen. His insights led not only to construction of the Observatories and the initial detectors, but also to the creation of the LIGO Scientific Collaboration and to the successful proposal to build Advanced LIGO—the instrument that would finally make the first detection. Barry was the perfect complement to Kip and Rai to bring LIGO to success.

The rest is history: Another 21 years would pass before the efforts of Rai Weiss, Kip Thorne, and Barry Barish finally paid off. On September 14, 2015, LIGO detected the gossamer flutters of spacetime created by the merging of two massive black holes some 1.3 billion light years away. Nearly half-a-century after its conception, LIGO had fulfilled its destiny.

So, while this Nobel Prize is being awarded barely two years after LIGO’s historic detection, it acknowledges 45 years of effort: from conception, through design, planning, testing and prototyping, through decades of research and engineering, invention and innovation, advances in computing, lasers, and optics, and especially the championing and advocating of three remarkable men: Barry Barish, Kip Thorne, and Rai Weiss. There can be no doubt; their recognition by the Nobel Committee is well earned. — Congratulations to you all!

• By the early 2000s, a set of initial detectors was completed, including TAMA 300 in Japan, GEO600 in Hannover, 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.

Status of the LIGO gravity-wave detection

• September 9, 2019: The final chapter of the historic detection of the powerful merger of two neutron stars in 2017 officially has been written. After the extremely bright burst finally faded to black, an international team led by Northwestern University painstakingly constructed its afterglow — the last bit of the famed event’s life cycle. 7)


Figure 5: An artistic rendering of two neutron stars merging (image credit: NSF/LIGO/Sonoma State/A. Simonnet)

- Not only is the resulting image the deepest picture of the neutron star collision’s afterglow to date, it also reveals secrets about the origins of the merger, the jet it created and the nature of shorter gamma ray bursts.

- “This is the deepest exposure we have ever taken of this event in visible light,” said Northwestern’s Wen-fai Fong, who led the research. “The deeper the image, the more information we can obtain.”

- The study will be published this month in The Astrophysical Journal Letters. Fong is an assistant professor of physics and astronomy in Northwestern’s Weinberg College of Arts and Sciences and a member of CIERA (Center for Interdisciplinary Exploration and Research in Astrophysics), an endowed research center at Northwestern focused on advancing studies with an emphasis on interdisciplinary connections.

- Many scientists consider the 2017 neutron-star merger, dubbed GW170817, as LIGO’s (Laser Interferometer Gravitational-Wave Observatory) most important discovery to date. It was the first time that astrophysicists captured two neutron stars colliding. Detected in both gravitational waves and electromagnetic light, it also was the first-ever multi-messenger observation between these two forms of radiation.

- The light from GW170817 was detected, partly, because it was nearby, making it very bright and relatively easy to find. When the neutron stars collided, they emitted a kilo nova — light 1,000 times brighter than a classical nova, resulting from the formation of heavy elements after the merger. But it was exactly this brightness that made its afterglow — formed from a jet travelling near light-speed, pummeling the surrounding environment — so difficult to measure.

- “For us to see the afterglow, the kilonova had to move out of the way,” Fong said. “Surely enough, about 100 days after the merger, the kilonova had faded into oblivion, and the afterglow took over. The afterglow was so faint, however, leaving it to the most sensitive telescopes to capture it.”

Hubble to the rescue

- Starting in December 2017, NASA’s Hubble Space Telescope detected the visible light afterglow from the merger and revisited the merger’s location 10 more times over the course of a year and a half.


Figure 6: The box indicates where the now-faded afterglow was located (image credit: Northwestern University)

- At the end of March 2019, Fong’s team used the Hubble to obtain the final image and the deepest observation to date. Over the course of seven-and-a-half hours, the telescope recorded an image of the sky from where the neutron-star collision occurred. The resulting image showed — 584 days after the neutron-star merger — that the visible light emanating from the merger was finally gone.

- Next, Fong’s team needed to remove the brightness of the surrounding galaxy, in order to isolate the event’s extremely faint afterglow.

- “To accurately measure the light from the afterglow, you have to take all the other light away,” said Peter Blanchard, a postdoctoral fellow in CIERA and the study’s second author. “The biggest culprit is light contamination from the galaxy, which is extremely complicated in structure.”

- Fong, Blanchard and their collaborators approached the challenge by using all 10 images, in which the kilonova was gone and the afterglow remained as well as the final, deep Hubble image without traces of the collision. The team overlaid their deep Hubble image on each of the 10 afterglow images. Then, using an algorithm, they meticulously subtracted — pixel by pixel — all light from the Hubble image from the earlier afterglow images.

- The result: a final time-series of images, showing the faint afterglow without light contamination from the background galaxy. Completely aligned with model predictions, it is the most accurate imaging time-series of GW170817’s visible-light afterglow produced to date.

- “The brightness evolution perfectly matches our theoretical models of jets,” Fong said. “It also agrees perfectly with what the radio and X-rays are telling us.”

Illuminating information

- With the Hubble’s deep space image, Fong and her collaborators gleaned new insights about GW170817’s home galaxy. Perhaps most striking, they noticed that the area around the merger was not densely populated with star clusters.

- “Previous studies have suggested that neutron star pairs can form and merge within the dense environment of a globular cluster,” Fong said. “Our observations show that’s definitely not the case for this neutron star merger.”

- According to the new image, Fong also believes that distant, cosmic explosions known as short gamma ray bursts are actually neutron star mergers — just viewed from a different angle. Both produce relativistic jets, which are like a fire hose of material that travels near the speed of light. Astrophysicists typically see jets from gamma ray bursts when they are aimed directly, like staring directly into the fire hose. But GW170817 was viewed from a 30-degree angle, which had never before been done in the optical wavelength.

- “GW170817 is the first time we have been able to see the jet ‘off-axis,’” Fong said. “The new time-series indicates that the main difference between GW170817 and distant short gamma-ray bursts is the viewing angle.”

- The study was primarily supported by the National Science Foundation (award numbers AST-1814782 and AST-1909358) and NASA (award numbers HST-GO-15606.001-A and SAO-G09-20058A). 8)

• On April 25, 2019, NFS's (National Science Foundation's) LIGO (Laser Interferometer Gravitational-Wave Observatory) and the European-based Virgo detector registered gravitational waves from what appears likely to be a crash between two neutron stars—the dense remnants of massive stars that previously exploded. One day later, on April 26, the LIGO-Virgo network spotted another candidate source with a potentially interesting twist: it may in fact have resulted from the collision of a neutron star and black hole, an event never before witnessed. 9) 10)

- "The universe is keeping us on our toes," says Patrick Brady, spokesperson for the LIGO Scientific Collaboration and a professor of physics at the University of Wisconsin-Milwaukee. "We're especially curious about the April 26 candidate. Unfortunately, the signal is rather weak. It's like listening to somebody whisper a word in a busy café; it can be difficult to make out the word or even to be sure that the person whispered at all. It will take some time to reach a conclusion about this candidate."

- "NSF's LIGO, in collaboration with Virgo, has opened up the universe to future generations of scientists," says NSF Director France Cordova. "Once again, we have witnessed the remarkable phenomenon of a neutron star merger, followed up closely by another possible merger of collapsed stars. With these new discoveries, we see the LIGO-Virgo collaborations realizing their potential of regularly producing discoveries that were once impossible. The data from these discoveries, and others sure to follow, will help the scientific community revolutionize our understanding of the invisible universe."

- The discoveries come just weeks after LIGO and Virgo turned back on. The twin detectors of LIGO—one in Washington and one in Louisiana—along with Virgo, located at the European Gravitational Observatory (EGO) in Italy, resumed operations April 1, after undergoing a series of upgrades to increase their sensitivities to gravitational waves—ripples in space and time. Each detector now surveys larger volumes of the universe than before, searching for extreme events such as smash-ups between black holes and neutron stars.

- "Joining human forces and instruments across the LIGO and Virgo collaborations has been once again the recipe of an incomparable scientific month, and the current observing run will comprise 11 more months," says Giovanni Prodi, the Virgo Data Analysis Coordinator, at the University of Trento and the Istituto Nazionale di Fisica Nucleare (INFN) in Italy. "The Virgo detector works with the highest stability, covering the sky 90 percent of the time with useful data. This is helping in pointing to the sources, both when the network is in full operation and at times when only one of the LIGO detectors is operating. We have a lot of groundbreaking research work ahead."

- In addition to the two new candidates involving neutron stars, the LIGO-Virgo network has, in this latest run, spotted three likely black hole mergers. In total, since making history with the first-ever direct detection of gravitational waves in 2015, the network has spotted evidence for two neutron star mergers; 13 black hole mergers; and one possible black hole-neutron star merger.

- When two black holes collide, they warp the fabric of space and time, producing gravitational waves. When two neutron stars collide, they not only send out gravitational waves but also light. That means telescopes sensitive to light waves across the electromagnetic spectrum can witness these fiery impacts together with LIGO and Virgo. One such event occurred in August 2017: LIGO and Virgo initially spotted a neutron star merger in gravitational waves and then, in the days and months that followed, about 70 telescopes on the ground and in space witnessed the explosive aftermath in light waves, including everything from gamma rays to optical light to radio waves.

- In the case of the two recent neutron star candidates, telescopes around the world once again raced to track the sources and pick up the light expected to arise from these mergers. Hundreds of astronomers eagerly pointed telescopes at patches of sky suspected to house the signal sources. However, at this time, neither of the sources has been pinpointed.

- "The search for explosive counterparts of the gravitational-wave signal is challenging due to the amount of sky that must be covered and the rapid changes in brightness that are expected," says Brady. "The rate of neutron star merger candidates being found with LIGO and Virgo will give more opportunities to search for the explosions over the next year."


Figure 7: How to catch a gravitational wave. The world’s first captured gravitational waves were created in a violent collision between two black holes, 1.3 billion light-years away. When these waves passed the Earth, 1.3 billion years later, they had weakened considerably: the disturbance in spacetime that LIGO measured was thousands of times smaller than an atomic nucleus (image credit: LIGO)

- The April 25 neutron star smash-up, dubbed S190425z, is estimated to have occurred about 500 million light-years away from Earth. Only one of the twin LIGO facilities picked up its signal along with Virgo (LIGO Livingston witnessed the event but LIGO Hanford was offline). Because only two of the three detectors registered the signal, estimates of the location in the sky from which it originated were not precise, leaving astronomers to survey nearly one-quarter of the sky for the source.

- The possible April 26 neutron star-black hole collision (referred to as S190426c) is estimated to have taken place roughly 1.2 billion light-years away. It was seen by all three LIGO-Virgo facilities, which helped better narrow its location to regions covering about 1,100 square degrees, or about 3 percent of the total sky.

- "The latest LIGO-Virgo observing run is proving to be the most exciting one so far," says David H. Reitze of Caltech, Executive Director of LIGO. "We're already seeing hints of the first observation of a black hole swallowing a neutron star. If it holds up, this would be a trifecta for LIGO and Virgo—in three years, we'll have observed every type of black hole and neutron star collision. But we've learned that claims of detections require a tremendous amount of painstaking work—checking and rechecking—so we'll have to see where the data takes us."

- LIGO is funded by NSF and operated by Caltech and MIT, which conceived of LIGO and led the Initial and Advanced LIGO projects. Financial support for the Advanced LIGO project was led by the NSF with Germany (Max Planck Society), the U.K. (Science and Technology Facilities Council) and Australia (Australian Research Council-OzGrav) making significant commitments and contributions to the project. Approximately 1,300 scientists from around the world participate in the effort through the LIGO Scientific Collaboration, which includes the GEO Collaboration. A list of additional partners is available at

- The Virgo Collaboration is currently composed of approximately 350 scientists, engineers, and technicians from about 70 institutes from Belgium, France, Germany, Hungary, Italy, the Netherlands, Poland, and Spain. The European Gravitational Observatory (EGO) hosts the Virgo detector near Pisa in Italy, and is funded by Centre National de la Recherche Scientifique (CNRS) in France, the Istituto Nazionale di Fisica Nucleare (INFN) in Italy, and Nikhef in the Netherlands. A list of the Virgo Collaboration members can be found at More information is available on the Virgo website at

• April 26, 2019: In the few weeks since LIGO kicked off its third observing run, it’s also already detected three potential black hole collisions and a neutron star merger, bringing its total lifetime gravitational wave haul to 14. 11)

- It took astronomers a century to make the first-ever gravitational wave detection, confirming a core prediction of Albert Einstein’s theory of general relativity. But this month, the floodgates have opened.

- On Friday (26 April), scientists with the LIGO announced they’ve likely detected a second gravitational wave event in as many days. Detectors at three locations around the world caught the arrival of a probable ripple in space-time around 11:20 a.m. E.T. (~15:20 GMT). It followed right on the heels of a gravitational wave detection Thursday that sent astronomers racing to observe the event with their telescopes.

- In all, it’s the fifth gravitational wave detection this month. And the influx has astronomers excited about kickstarting the era of multi-messenger astronomy, where scientists can combine gravitational wave data with observations from conventional telescopes to gain new insights into extreme cosmic events like colliding black holes and neutron stars.

- Scientists suspect Thursday’s event marked the second-ever gravitational wave detection of two colliding neutron stars, the collapsed cores left behind when giant stars go supernova. The merger would have likely spawned a new black hole. Astronomers spent Thursday searching for any signs of the collision on the sky. They’re less certain about the celestial event that led to today’s detection: There’s about a one in seven chance that it was a false alarm caused by earthly vibrations. Its signal is right at the threshold of what LIGO can pick out.

- If this latest signal does turn out to be real cosmic collision, though, scientists say that there’s a chance it may be the hallmark of a never-before-seen event: the collision of a neutron star and a black hole. But odds still favor it as a third neutron star merger.

• April 26, 2019: For just the second time, physicists working on the LIGO (Laser Interferometer Gravitational-Wave Observatory) and at Virgo have caught the gravitational waves of two neutron stars colliding to likely form a black hole. 12) 13)

- The ripples in space-time traveled some 500 million light-years and reached the detectors at LIGO, as well as its Italian sister observatory, Virgo, at around 4 a.m. E.T. Thursday (~8:00 GMT), 25 April. Team members say there’s a more than 99 percent chance that the gravitational waves were created from a binary neutron star merger.

Shot at a Kilonova

- In the moments after the event, a notice went out alerting astronomers around the world to turn their telescopes to the heavens in hopes of catching light from the explosion, called a kilonova. Kilonovae are 1,000 times brighter than normal novae, and they create huge amounts of heavy elements, like gold and platinum. That brightness makes it easy for astronomers to find these events in the night sky — provided they’ve been given a heads-up and location from LIGO first.

- LIGO’s twin L-shaped observatories — one in Washington state and one in Louisiana — work by shooting a laser beam down the long legs of their “L.” Their experimental setup is precise enough that even the minimal disturbance caused by a passing gravitational wave is enough to trigger a slight change in the laser’s appearance. It made the first ever detection of gravitational waves in 2016. Then it followed up by detecting merging neutron stars in 2017.

- Scientists use any slight delays between when signals reach the detectors to help them better triangulate where the waves originated in the sky. But one of LIGO’s twin detectors was offline Thursday when the gravitational wave reached Earth, making it hard for astronomers to triangulate exactly where the signal was coming from. That sent astronomers racing to image as many galaxies as they could across a region covering one-quarter of the sky.

- And instead of finding one potential binary neutron star merger, astronomers turned up at least two different candidates. Now the question is which, if any, are related to the gravitational wave that LIGO saw. Sorting that out will require more observations, which were already happening around the world as darkness fell.

- “I would assume that every observatory in the world is observing this now,” says astronomer Josh Simon of the Carnegie Observatories. “These two candidates (they’ve) found are relatively close to the equator, so they can be seen from both the Northern and Southern Hemisphere.”

- Simon also says that, as of Thursday afternoon in the United States, telescopes in Europe and elsewhere should be gathering spectra on these objects. His fellow astronomers at the Carnegie Observatories turned their telescopes at Chile’s Las Campanas Observatory to the event Thursday night.


Figure 8: An artist’s illustration of two colliding neutron stars (image credit: NASA/Swift/Dana Berry)

• March 26, 2019: The National Science Foundation's LIGO (Laser Interferometer Gravitational-Wave Observatory) is set to resume its hunt for gravitational waves—ripples in space and time—on April 1, after receiving a series of upgrades to its lasers, mirrors, and other components. LIGO—which consists of twin detectors located in Washington and Louisiana—now has a combined increase in sensitivity of about 40 percent over its last run, which means that it can survey an even larger volume of space than before for powerful, wave-making events, such as the collisions of black holes. 14)

- Joining the search will be Virgo, the European-based gravitational-wave detector, located at the European Gravitational Observatory (EGO) in Italy, which has almost doubled its sensitivity since its last run and is also starting up April 1.

- "For this third observational run, we achieved significantly greater improvements to the detectors' sensitivity than we did for the last run," says Peter Fritschel, LIGO's chief detector scientist at MIT. "And with LIGO and Virgo observing together for the next year, we will surely detect many more gravitational waves from the types of sources we've seen so far. We're eager to see new events too, such as a merger of a black hole and a neutron star."

- In 2015, after LIGO began observing for the first time in an upgraded program called Advanced LIGO, it soon made history by making the first direct detection of gravitational waves. The ripples traveled to Earth from a pair of colliding black holes located 1.3 billion light-years away. For this discovery, three of LIGO's key players—Caltech's Barry C. Barish, the Ronald and Maxine Linde Professor of Physics, Emeritus, and Kip S. Thorne, the Richard P. Feynman Professor of Theoretical Physics, Emeritus, along with MIT's Rainer Weiss, professor of physics, emeritus—were awarded the 2017 Nobel Prize in Physics.

- Since then, the LIGO-Virgo detector network has uncovered nine additional black hole mergers and one explosive smashup of two neutron stars. That event, dubbed GW170817, generated not just gravitational waves but light, which was observed by dozens of telescopes in space and on the ground.

- "With our three detectors now operational at a significantly improved sensitivity, the global LIGO-Virgo detector network will allow more precise triangulation of the sources of gravitational waves," says Jo van den Brand of Nikhef (the Dutch National Institute for Subatomic Physics) and VU University Amsterdam, who is the spokesperson for the Virgo collaboration. "This will be an important step toward our quest for multi-messenger astronomy."


Figure 9: Current operating facilities in the global network include the twin LIGO detectors—in Hanford, Washington, and Livingston, Louisiana— Virgo in Italy and GEO600 in Germany (image credit: LIGO-Virgo-GEO600 collaboration)

- Now, with the start of the next joint LIGO-Virgo run, the observatories are poised to detect an even greater number of black hole mergers and other extreme events, such as additional neutron-neutron star mergers or a yet-to-be-seen black hole-neutron star merger. One of the metrics the team uses for measuring increases in sensitivity is to calculate how far out they can detect neutron-neutron star mergers. In the next run, LIGO will be able to see those events out to an average of 550 million light-years away, or more than 190 million light-years farther out than before.

- A key to achieving this sensitivity involves lasers. Each LIGO installation consists of two long arms that form an L shaped interferometer. Laser beams are shot from the corner of the "L" and bounced off mirrors before traveling back down the arms and recombining. When gravitational waves pass by, they stretch and squeeze space itself, making imperceptibly tiny changes to the distance the laser beams travel and thereby affecting how they recombine. For this next run, the laser power has been doubled to more precisely measure these distance changes, thereby increasing the detectors’ sensitivity to gravitational waves.

- Other upgrades were made to LIGO's mirrors at both locations, with a total of five of eight mirrors being swapped out for better-performing versions. "We had to break the fibers holding the mirrors and very carefully take out the optics and replace them," says Calum Torrie, LIGO's mechanical-optical engineering head at Caltech. "It was an enormous engineering undertaking."

- This next run also includes upgrades designed to reduce levels of quantum noise. Quantum noise occurs due to random fluctuations of photons, which can lead to uncertainty in the measurements and can mask faint gravitational-wave signals. By employing a technique called "squeezing," initially developed for gravitational-wave detectors at the Australian National University, and matured and routinely used since 2010 at the GEO600 detector, researchers can shift the uncertainty in the photons around, making their amplitudes less certain and their phases, or timing, more certain. The timing of photons is what is crucial for LIGO's ability to detect gravitational waves.

- Torrie says that the LIGO team has spent months commissioning all of these new systems, making sure everything is aligned and working correctly. "One of the things that is satisfying to us engineers is knowing that all of our upgrades mean that LIGO can now see farther into space to find the most extreme events in our universe."

• February 27, 2019: LIGO and Virgo are pleased to announce that the strain data from the O2 observing run have been released. These data are now available through the Gravitational Wave Open Science Center ( 15)

- The O2 observing run began on November 30, 2016 and ended on August 25, 2017. The release includes over 150 days of data from each of the two LIGO observatories, as well as 20 days of data from Virgo, making this the largest data set of "advanced" gravitational wave detectors to date. Observations in O2 include seven binary black hole mergers, as well as the first binary neutron star merger observed in gravitational waves, all recently published with the GWTC-1 catalog. The LIGO Scientific Collaboration and Virgo Collaboration have published a number of papers based on these data; please see the LIGO Scientific Collaboration web pages for a list of these papers, and several more will be appearing soon. Along with the strain data, the release contains detailed documentation and links to open source software tools.

- O2 is the second observing run of Advanced LIGO, and the first observing run of Advanced Virgo, which joined O2 on August 1st, 2017. Data from Advanced LIGO's first observing run (O1) are already available online, and have been used in a number of scientific publications, text books, artistic projects, and classroom activities. As with previous data releases, the O2 data set should be useful for both scientific investigations and educational activities.


Figure 10: Time-frequency maps and reconstructed signal waveforms for the ten BBH (Binary Body Hole) events (image credit: LIGO Scientific Collaboration and the Virgo Collaboration)

• 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. 16)

- The results are published in this week’s issue of „Science“. 17)

- 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 11: 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 12: 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, marking major progress since the first gravitational wave detection in 2015 (SN: 3/5/16, p. 6). 18)

- 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). 19)

- 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. 20)

- 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 13: 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)

• August 17, 2018: Today LIGO commemorates the one-year anniversary of its most important discovery to-date: The detection of a merging pair of neutron stars, aka a BNS (Binary Neutron Star) merger. 21)

After a 130 million year journey, the gravitational waves generated by these exotic stars arrived at LIGO’s Hanford and Livingston detectors in the United States, and the Virgo detector in Italy on 17 August 2017. Dubbed a ‘kilonova’ (a term coined in 2010 in a paper wherein it was theorized that a pair of merging neutron stars would emit light about 1000 times brighter than a classical nova), the detection also led to a massive explosion of multimessenger astronomy results gathered by astronomers from all around the globe. LIGO announced the discovery to the world with papers published on 16 October 2017.

How important was this detection? Well, on the day of the announcement, 84 scientific papers were published about it.

Today, an internet search for “GW170817” will yield over 110,000 results, all related to this one event that captured the world scientific community’s attention (incidentally, a search for “GW150914”, LIGO’s first detection, yields a mere 80,000 hits).

Why all the Excitement? Up until that day in August 2017, LIGO’s detections had all been gravitational waves caused by merging black holes. While there’s no doubt that those discoveries have been monumental, the scope and magnitude of this discovery would prove unprecedented. The LIGO and Virgo detection has become probably the most widely studied astronomical event in human history. Within days, this object was being examined by nearly one-third of the world’s electromagnetic (EM) astronomers. The fact that EM astronomers were able to observe the phenomenon alongside GW astronomers is what truly elevated this event to history-making levels. LIGO’s previous detections of merging black holes did not result in such widespread study because, by their very nature, black holes are believed not to emit electromagnetic waves (i.e., light of any wavelength). No amount of searching by astronomers using telescopes designed to observe EM radiation has revealed anything. Only gravitational wave observatories like LIGO and Virgo can ‘observe’ black holes colliding.

Neutron stars are different, however. Unlike black holes, neutron stars are made up of actual matter, including copious amounts of neutrons (hence their moniker), and when you accelerate or slam matter together you get electromagnetic radiation (again, not something one expects to detect from colliding black holes).


Figure 14: Basic anatomy of a neutron star. By Robert Schulze [CC BY-SA 3.0 (]

But like black holes, neutron stars are massive and compact enough to generate gravitational waves when they collide. This combination of properties (material composition and density) means that colliding neutron stars can emit both gravitational waves AND electromagnetic radiation: “light” in all its forms from gamma rays to radio waves. In fact, it was a burst of gamma rays, arriving 1.7 seconds after the gravitational waves, which alerted the broader astronomical community to something truly extraordinary and led to an unprecedented global effort to study the phenomenon.

• February 22, 2018: The National Science Foundation funded Advanced LIGO Documentary Project released a new video looking at information coming from first multi-messenger detection of colliding neutron stars. On August 8, 2017, LIGO joined forces with Virgo and over 70 astronomical observatories to look at a neutron star merger through gravitational waves and electromagnetic waves (light). This video explores the significance of that event. 22)

Figure 15: LIGO: A Discovery that Shook the World. This is the third video in Advanced LIGO Documentary Project's eight-part series on LIGO's historic discovery of gravitational waves and the birth of the new age of gravitational wave astronomy. In August 2017, LIGO and its Italian partner, VIRGO, made a discovery as important as its historic first detection of gravitational waves in 2015. They detected gravitational waves from two colliding neutron stars, which ejected a spectacular gamma ray burst that was seen by seven space-based telescopes and dozens of astronomical observatories on earth. It was the long dreamed-of marriage of gravitational wave astronomy with conventional astronomy, and the results were spectacular (video credit: Advanced LIGO Documentary Project)

• December 11, 2017: Physics World announced that the first multi-messenger detection of a neutron star merger was 2017's breakthrough of the year. On August 17, 2017 LIGO detected a gravitational wave that was expected to come from a neutron star merger. Around 2 seconds later a Gamma Ray Burst occurred and was detected by the Fermi Gamma-ray Space Telescope. Together with LIGO, Virgo and Fermi's information astronomers were able to piece together approximately where in the sky the neutron star merger occurred. Telescopes around the world pointed there scopes at the spot and soon identified precisely where the neutron stars had collided. In the coming months more than 70 telescopes observed multiple frequencies of electromagnetic radiation coming from the neutron star merger, yielding a treasure trove of information about the kilonova that occurred when the two neutron stars collided. Over 50 collaborations, including LIGO participated in this venture. For more on this detection check out our news article, or the press release. Last year Physics World awarded LIGO with the 2016 breakthrough of the year, due to the detection it's detection of gravitational waves. 23)

• 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. 24)

- 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 16: 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 17). 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 17: 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 18: 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 19: "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 20: 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.

Deeper Meaning

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’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.

• October 16, 2017: LIGO’s latest 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. 25)

- 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.

When It Rains, It Pours

- 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 3x10-12, 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 a 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!”

September 27, 2017: The Virgo Collaboration and the LIGO Scientific Collaboration have jointly observed the merger of two black holes. This is the fourth confirmed detection of a binary black hole merger, and the first detection made using a network of three interferometers. 26) 27)

The detected waves—observed on August 14th, 2017 at 10:30:43 UTC (6:30AM EDT) —were produced by a pair of black holes with 31 and 25 solar masses. They merged to produce a spinning black hole of 53 solar masses. Combining the signal from Virgo with the signal observed in the two LIGO observatories improved the sky localization of the source by over a factor of 10. 28)

The Virgo and LIGO Scientific Collaborations have been observing since November 30, 2016 in the second Advanced Detector Observing Run ‘O2’ , searching for gravitational-wave signals, first with the two LIGO detectors, then with both LIGO and Virgo instruments operating together since 1 August 2017. Some promising gravitational-wave candidates have been identified in data from both LIGO and Virgo during our preliminary analysis, and we have shared what we currently know with astronomical observing partners. We are working hard to assure that the candidates are valid gravitational-wave events, and it will require time to establish the level of confidence needed to bring any results to the scientific community and the greater public. We will let you know as soon we have information ready to share. 29) 30)

The detected gravitational waves—ripples in space and time—were emitted during the final moments of the merger of two black holes with masses about 31 and 25 times the mass of the sun and located about 1.8 billion light-years away. The newly produced spinning black hole has about 53 times the mass of our sun, which means that about 3 solar masses were converted into gravitational-wave energy during the coalescence.


Figure 21: The GWevent GW170814 observed by LIGO Hanford, LIGO Livingston, and Virgo. Times are shown from August 14, 2017, 10:30:43 UTC. Top row: SNR time series produced in low latency and used by the low-latency localization pipeline on August 14, 2017. The time series were produced by time shifting the best-match template from the online analysis and computing the integrated SNR at each point in time. The single-detector SNRs in Hanford, Livingston, and Virgo are 7.3, 13.7, and 4.4, respectively. Second row: Time-frequency representation of the strain data around the time of GW170814. Bottom row: Time-domain detector data (in color), and 90% confidence intervals for waveforms reconstructed from a morphology-independent wavelet analysis (light gray) and BBH (Binary Black Hole) models (image credit: LIGO and Virgo Collaboration)

The era of gravitational-wave (GW) astronomy began with the detection of binary black hole (BBH) mergers, by the Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO) detectors, during the first of the Advanced Detector Observation Runs. 31) Three detections, GW150914, GW151226, and GW170104, and a lower significance candidate, LVT151012, have been announced so far. The Advanced Virgo detector joined the second observation run on August 1, 2017.

• June 15, 2016: 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. 32) 33)

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. 34) 35)

The team, including engineers and scientists from Northwestern University in Illinois, published their results in the journal Physical Review Letters. 36)

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.”

• In February 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. 37)

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 spaceborne gravitational wave detector, eLISA (evolved Laser Interferometer Space Antenna), set to launch sometime in the 2030s.

On February 11, 2016, the LIGO Scientific Collaboration and Virgo Collaboration announced the first confirmed observation of gravitational waves from colliding black holes. The gravitational wave signals were observed by the LIGO's twin observatories on September 14, 2015. This confirms a key prediction of Einstein's theory of general relativity and provides the first direct evidence that black holes merge. 38)

• Already on 14 September 2015 at 09:50:45 UTC, the LIGO Hanford, WA, and Livingston, LA, observatories detected the coincident signal GW150914 shown in Figure 23. The initial detection was made by low-latency searches for generic gravitational-wave transients and was reported within three minutes of data acquisition. 39)

Figure 22: The First Observation of Gravitational Waves. On September 14, 2015, LIGO observed ripples in the fabric of spacetime. This video narrative tells the story of the science behind that important detection. (video credit: Caltech)


Figure 23: 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).

• February 11, 2016: 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. 40)

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 5:51 a.m. Eastern Daylight Time (09:51 UTC) by both of the twin Laser Interferometer Gravitational-wave Observatory (LIGO) detectors, located in Livingston, Louisiana, and Hanford, Washington, USA. The LIGO Observatories are funded by the National Science Foundation (NSF), 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.

Status of the LIGO / Virgo Facilities

• April 1, 2019: A bit more than three years after the first, landmark detection of gravitational waves (GWs), the LIGO and Virgo laser interferometer GW observatories today kick off their third observation run. 41)

- Known in the GW community simply as “O3,” the year-long observation run will likely yield a bumper crop of new astronomical observations—the result of a 40 percent improvement in the already jaw-dropping sensitivity of the two LIGO facilities in the United States and a near-doubling of the sensitivity of the Virgo facility in Italy. The O3 period also could see the long-awaited on-streaming of the KAGRA GW observatory in Japan. And, in a new twist, the LIGO/Virgo Scientific Collaboration (LSC) will be making data about possible GW detections publicly available in near realtime.

Boosting laser power

- The O3 run will add to the impressive string of milestones achieved in the first two GW observation runs. These include the detection of gravitational waves from ten binary black-hole mergers, as well as from the collision of a pair of ultra-dense neutron stars. The latter detection—coordinated with observations from more traditional optical, X-ray and gamma-ray telescopes in a path-breaking example of “multi-messenger astronomy”—resulted in a breathtaking harvest of new scientific information.

- LSC scientists are confident that the LIGO and Virgo observatories will log observations at an even faster clip in O3, as a result of technical improvements implemented since the end of the last observation run, O2, in August 2017.

- The improvements include a doubling of the power of the facilities’ lasers, which in these observatories are amplified in two Fabry–Pérot cavities 3 to 4 km long that form the arms of a gigantic, L-shaped Michelson-interferometer. Also installed in the upgrade round were scattered-light suppressors, or “baffles,” designed to control stray light within the huge interferometers.

Hammering down noise

- In addition to laser power, other recent upgrades have centered on efforts to boost sensitivity by ferreting out and eliminating noise sources in a range of subsystems.

- At LIGO, this has included the huge engineering challenge of swapping out a number of the 40-kg mirrors, or test masses, exquisitely suspended at either end of the laser interferometer arms. As a passing gravitational wave ripples through spacetime, tiny movements in these hefty mirrors result in infinitesimal changes of the interferometer arms’ lengths, which are read as picowatt-scale power fluctuations at the dark port of the interferometer. The new, better-performing versions of the mirrors include improved coatings to diminish thermal noise.


Figure 24: Engineers Hugh Radkins and Betsy Weaver at work inside the vacuum system of the detector at LIGO Hanford Observatory, Washington, USA, during recent hardware upgrades. With the upgrades complete, the two U.S. LIGO facilities, in partnership with the Virgo facility in Italy, are now poised to begin third observation run (image credit: LIGO/Caltech/MIT/Jeff Kissel)

- At Virgo, meanwhile, the steel wires suspending the main mirrors have been replaced with fused-silica versions that quiet down vibrational noise and extend the facility’s ability to pick up low- and medium-frequency GWs. And during O3, both LIGO and Virgo will use a trick of quantum mechanics, the injection of a “squeezed” state of light at the photodetector, to narrow down the uncertainties in photon arrival times attributable to fluctuations in the quantum vacuum.

- These and other technical improvements were partly developed and matured at yet another facility, GEO 600, a smaller GW observatory in Europe that has served as a vital testbed for technologies to sharpen the observing power of the larger sites. GEO 600 will also participate in the O3 run.

Sampling more of the cosmos

- The recent sensitivity upgrades will enable the global GW network to sample a much-expanded slice of the cosmos for evidence of high-energy astronomical events. In the O3 run, for example, LIGO’s sensitivity in the wake of the recent upgrades should enable it to sniff out binary neutron-star mergers to a distance of 550 million light-years—more than 190 million light-years further than in O2.

- That, coupled with an eightfold expansion of the volume of space now visible to Virgo, could increase the rate of detection of binary black-hole collisions to anywhere from a few events per month to a few per week, and binary neutron-star mergers to between one per year and one per month. There’s also the possibility of picking up more exotic, previously inaccessible events, such as the merger of a black hole and a neutron star.

Instant access to data

- In another change, the public will have near-immediate access to this harvest of discoveries, through new software developed by LSC scientists. The software will be “able to send open public alerts within five minutes” after a GW detection, according to Sarah Antier, a postdoctoral research associate at the Université Paris Diderot, France.

- That will allow rapid public access to parameters such as type of signal, sky position and estimated distance for a given GW event. Those parameters, in turn, will let both professional and amateur astronomers looking at various slices of the electromagnetic spectrum quickly train their instruments on the right patch of sky to follow up on the GW observation.

• 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. 42) 43)

- 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 25: Aerial view of LIGO Hanford Observatory (image credit: LIGO)

• November 20, 2018: The LIGO Laboratory congratulates Derek Davis of Syracuse University and T.J. Massinger of Caltech for winning the first LIGO Laboratory Award for Excellence in Detector Characterization and Calibration. 44)

- The LIGO gravitational wave detectors have registered gravitational wave signals from multiple black hole mergers and the spectacular collision of two neutron stars since Advanced LIGO first began observing in 2015. Davis and Massinger’s work to reduce the noise present in LIGO detector data was key to making these discoveries possible by allowing searches to more easily distinguish the signatures of true astrophysical gravitational wave events in noisy detector data.

- By improving how deep in space the LIGO-Hanford detector could sense by up to 50%, at least three gravitational wave signals were confidently detected during Advanced LIGO's second observing run (O2) that would not have been otherwise. Their efforts are an outstanding example of the detector characterization work needed to lay the groundwork for future discoveries in gravitational wave astrophysics and multi-messenger astronomy.

- Davis and Massinger will share a $1000 prize and are invited to present colloquia at one of the the LIGO Laboratory sites (LIGO-Hanford, LIGO-Livingston, Caltech, or MIT) to share their achievements with LIGO Laboratory members. They will each receive an award certificate at the LIGO-Virgo Collaboration meeting in March 2019.

- Derek Davis is currently a Ph.D. student at Syracuse University. As a part of the LIGO Scientific Collaboration, they serve as the Event Validation Lead for the LIGO Detector Characterization group, leading follow-up investigations of candidate gravitational-wave detections.

- T.J. Massinger is currently a postdoctoral scholar at Caltech. He earned his PhD in 2016 from Syracuse University. Within the LIGO Detector Characterization group, he serves as instrument science lead and as a liaison to the compact binary coalescence data analysis working group.

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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 (

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