# What is the detection threshold of gravitational waves for LIGO?

Since now two neutron stars have been detected merging via gravitational waves, I was wondering what is the current detection threshold that the LIGO detectors can achieve.

Considering that the first observed objects were two black holes with a combined mass of more than 60 solar masses and they now detected two neutron stars with a combined mass of only about 3 solar masses I was wondering what was the threshold that these detectors can actually detect.

Obviously there are much larger stars out there which orbit each other, but their size and distance from each other make gravitational waves too difficult to detect. So what masses and at what distances can we expect to be detected in the future?

I'm afraid this is not straightforward

The amplitude of the gravitational wave strain signal from a merging compact binary (neutrons star or black hole) is $$h sim 10^{-22} left(frac{M}{2.8M_{odot}} ight)^{5/3}left(frac{0.01{ m s}}{P} ight)^{2/3}left(frac{100 { m Mpc}}{d} ight),$$ where $M$ is the total mass of the system in solar masses, $P$ is the instantaneous orbital period in seconds and $d$ is the distance in 100s of Mpc. $h sim 10^{-22}$ is a reasonable number for the sensitivity of LIGO to gravitational wave strain where it is most sensitive (at frequencies of 30-300 Hz).

So you can see that to increase the observability you can increase the mass, decrease the period or decrease the distance.

But here are the complications. LIGO is only sensitive between about 30-300 Hz and the GW frequencies are twice the orbital frequency. Thus you cannot shorten the period to something very small because it would fall outside the LIGO frequency range and you also cannot increase the mass to something too much bigger than the black holes that have been already seen because they merge before they can attain high enough orbital frequencies to be seen. (The frequency at merger is $propto M^{-1}$).

A further complication is that the evolution of the signals is more rapid at lower masses. That is - the rate of change of frequency and amplitude increase rapidly with total mass. That is why the recent neutron star merger was detectable for 100s by LIGO, whereas the more massive black hole mergers could only be seen for about 1 second. But what this means is that you have fewer cycles of the black hole signal that can be "added up" to improve the signal to noise, which means that higher mass sources are less detectable than a simple application of the formula I gave above would suggest. A further complication is that there is a geometric factor depending on how the source and detectors are orientated with respect to each other.

OK, these are complications, but the formula can still be used as an approximation. So if we take the GW170817 signal, the total mass was about $2.8M_{odot}$, the source was at 40 Mpc, so at frequencies of 200 Hz (corresponding to a period of 0.01 s) you might have expected a strain signal of about $3 imes 10^{-22}$. This gave a very readily detectable signal. The discovery paper (Abbot et al. 2017) says the "horizon" for detection was approximately 218 Mpc for LIGO-Livingston and 107 Mic for LIGO-Hanford. As the source was much closer than these numbers then it is unsurprising that the detection was strong.

Taking the formula above and a fixed orbital period of 0.01 s, we can see that the horizon distance will scale as $sim M^{5/3}$. So a $10 M_{odot} + 10 M_{odot}$ black hole binary might be seen out to $218 imes (20/2.8)^{5/3} = 5.7$ Gpc (this will be an overestimate by a factor of a few because of the issue of the rapidity of the evolution towards merger that I discussed above).

A more through and technical discussion can be read here, although this is a couple of years out of date and LIGO's reach has been extended by about a factor of five since these calculations were done.

Figure 1 of this paper shows the horizon distance (distance to which a circularly polarised overhead signal would be detected at SNR 8) for larger mass systems up to total mass of 1000 solar masses, assuming a search with compact binary coalescence templates. For higher masses the signal amplitude is generally larger, but they merge at lower frequencies so the signals are generally shorter-lived in the sensitive band of the detectors. As they're shorter they also, unfortunately, look a lot more like classes of instrumental glitches, so if they're not that strong (just above a threshold of roughly SNR 8) the background level can be large and lead to lower significance of any candidates.

## What is the detection threshold of gravitational waves for LIGO? - Astronomy

The Laser Interferometer Gravitational-Wave Observatory (LIGO) is a large-scale physics experiment and observatory designed to detect cosmic gravitational waves and to develop gravitational-wave observations as an astronomical tool. [1] Two large observatories were built in the United States with the aim of detecting gravitational waves by laser interferometry. These observatories use mirrors spaced four kilometers apart which are capable of detecting a change of less than one ten-thousandth the charge diameter of a proton. [2]

The initial LIGO observatories were funded by the National Science Foundation (NSF) and were conceived, built and are operated by Caltech and MIT. [3] [4] They collected data from 2002 to 2010 but no gravitational waves were detected.

The Advanced LIGO Project to enhance the original LIGO detectors began in 2008 and continues to be supported by the NSF, with important contributions from the United Kingdom's Science and Technology Facilities Council, the Max Planck Society of Germany, and the Australian Research Council. [5] [6] The improved detectors began operation in 2015. The detection of gravitational waves was reported in 2016 by the LIGO Scientific Collaboration (LSC) and the Virgo Collaboration with the international participation of scientists from several universities and research institutions. Scientists involved in the project and the analysis of the data for gravitational-wave astronomy are organized by the LSC, which includes more than 1000 scientists worldwide, [7] [8] [9] as well as 440,000 active [email protected] users as of December 2016 [update] . [10]

LIGO is the largest and most ambitious project ever funded by the NSF. [11] [12] In 2017, the Nobel Prize in Physics was awarded to Rainer Weiss, Kip Thorne and Barry C. Barish "for decisive contributions to the LIGO detector and the observation of gravitational waves". [13]

Observations are made in "runs". As of December 2019 [update] , LIGO has made 3 runs, and made 50 detections of gravitational waves. Maintenance and upgrades of the detectors are made between runs. The first run, O1, which ran from 12 September 2015 to 19 January 2016, made the first 3 detections, all black hole mergers. The second run, O2, which ran from 30 November 2016 to 25 August 2017, made 8 detections, 7 black hole mergers, and the first neutron star merger. [14] The third run, O3, began on 1 April 2019 it is divided (so far) into O3a, from 1 April to 30 September 2019, and O3b, from 1 November 2019 [15] until it was suspended in March 2020 due to COVID-19. [16]

## What's so cool about gravitational waves?

The first significant thing about LIGO's direct detection of gravitational waves is that it happened at all.

But first, let's back up a bit and talk about Albert Einstein. He was a smart guy &mdash he figured out a lot of really subtle stuff about the universe, including that space is not a fixed, rigid backdrop, like a stage on which cosmic events play out. Instead, Einstein showed that space is flexible and influenced by the objects and events within it. Very massive objects create curves in space, kind of like the way a bowling ball curves a mattress when placed on top of it.

(Einstein also showed that space and time are intimately linked &mdash both are threads in the universal fabric that he called space-time. We'll gloss over this relationship for the sake of brevity.)

So what does this have to do with gravitational waves? If a massive object can curve space-time, then moving a massive object can create ripples in space-time. Think of a canoe moving across a lake, sending ripples across the surface of the water or a mallet striking a drum, creating vibrations on the surface.

The Laser Interferometer Gravitational-Wave Observatory, better known as LIGO, was the first experiment ever to directly detect these ripples in space-time, so it's the first direct physical evidence that they actually exist. Its first detection came in September 2015, 100 years after Einstein first predicted their existence. It's also been 40 years since people started working on the early incantations of the technology that LIGO uses to detect gravitational waves.

So these ripples in space-time confirm Einstein's theory (although it had already been shown to be fairly airtight). Gravitational waves are an extreme illustration of general relativity in the past, those extreme examples existed only on paper, in the theoretical world. Data can always help scientists learn more about the universe, and if Einstein's theory needs to be adjusted (to make it compatible with quantum mechanics, for example), it's possible LIGO could find where. (LIGO's executive director said he's doubtful that LIGO will find these kinds of cracks or lose ends in Einstein's theory, but it is a possibility.)

## A Bright Future For Gravitational-Wave Astronomy

As surreal as it seems, the detection of gravitational waves has now become commonplace, only five years after the first detection in September 2015. With now 50 gravitational-wave detections we are better able to explore the population of black holes and neutron stars throughout the universe (see this summary). Additional gravitational-wave detections also increase our understanding of the General Theory of Relativity (see this summary).

The future of gravitational-wave astronomy is increasingly promising after the addition of 39 events from the first six months of the O3 observing period. Analysis of the second portion of O3 (called O3b) is currently in progress and will further expand our growing gravitational-wave transient catalog. Following O3, detectors will undergo additional engineering improvements to further increase astrophysical reach in time for the fourth observing run. While we await instrumental improvements and the construction of new detectors, the gravitational-wave community will continue to explore the nature of black holes and neutron stars throughout the universe.

## Observations

Signal traces of the first confirmed gravitational wave detection. T-shirts were made celebrating the find, and this author is lucky enough to have one.

The first direct gravitational wave detection was made on the 14th of September 2015, detected by both the LIGO observatories at the time. Since then, many more successful observations have been made, with the most recent observation run netting 55 detections before being curtailed by COVID-19. With the existence of gravitational waves confirmed by direct observation, work is ongoing to better understand the phenomena. Each detection comes from a different stellar event, and sheds more light on the underlying physical processes at play. The biggest detection yet was from May 21, 2019, likely due to the merger of two black holes. Data is compared between the two LIGO observatories, as well as a similar facility in Italy by the name of VIRGO, to help pinpoint the source of any observed waves.

Being able to detect gravitational waves enables the investigation of phenomena that were difficult to access with traditional optical or electromagnetic astronomy.

Before LIGO, we didn’t really know how many binary black holes there were in the universe… you can’t really see them. What LIGO is sensitive to is that final moment where they merge … We’re doing some cool stuff with the distribution of binary black holes… they could be isotropically distributed through the universe, or there could be more black holes in certain systems.

As more measurements roll in, and physical theories evolve, the data collected from the project may shed further insights on the very structure of the universe itself.

The LIGO experiment is a great example of the level of sophistication required to investigate the phenomena at the cutting edge of physics. Often, there’s a huge lag between theories being proposed and successful experimental confirmations in this case, a full century went by before gravitational waves could be directly detected. It’s hard work untangling the secrets of the universe, but as always, scientists stand ready to rise to the challenge.

## Astronomy

Along with gravitational waves, it is expected that a powerful burst of gamma rays is emitted when two binary neutron stars merge. Detection of such a gamma ray burst coincident with the detection of gravitational waves from an inspiral would confirm this hypothesis.

Inspiraling compact binaries consisting of black holes and/or neutron stars are one of the most promising sources of gravitational radiation for the first generation of gravitational wave detectors, such as LIGO. On time scales of 10 7 years, a compact binary system loses energy by emitting gravitational waves causing its components to spiral together. As the orbit shrinks, it circularizes and the period decreases. With LIGO, we search for the gravitational waves that would be emitted during the final tens of seconds of this inspiral. The stars orbit hundreds of times per second at separations of tens of km before plunging together. The first generation of detectors can observe binary neutron star systems with a reasonable signal-to-noise ratio to about 20 Mpc, with an estimated rate which could be as high as one every 1.5 years, although the true rate is unknown and could be lower.

The coalescence of neutron star–black hole (NS-BH) binaries is believed to be the most promising progenitor of short-hard gamma ray bursts. The direct detection of gravitational waves associated with a GRB would provide compelling evidence for this hypothesis, solving the long-standing mystery of the short-hard GRB origin. The gravitational waves from such systems are likely to be complex, however. Coupling of the orbital angular momentum of a NS-BH binary to the spin of the black hole causes the binary to precess. The resulting modulation of the waveform presents significant challenges for detection, increasing the dimension of the waveform parameter space by an order of magnitude.

The LSC/Virgo Compact Binary Coalescence Group is responsible for searching for the gravitational waves produced by inspiral sources using matched-filter techniques. Members of the Syracuse group collaborate with other members of the Compact Binary Coalescence Group to develop, implement and use algorithms sift through gravitational-wave detector noise for inspiral signals, and to study the relativity and astrophysics that can be obtained from a detection. We are particularly interested in developing search techniques for the spinning binaries described above, as well contributing to the binary neutron star and binary black hole searches.

## Scientists Claim Sensational Detection of Gravitational Waves and Herald New Era in Astronomy

The Laser Interferometer Gravitational-wave Observatory in Hanford, Washington. Credit: Caltech/MIT/LIGO Lab

On May 6, 1981, the physicists Kip Thorne and Jeremiah Ostriker made a bet * . If extraterrestrial gravitational waves were detected before January 1, 2000, by at least two experimental groups, Thorne would win the bet and a case of red wine. If not, then Ostriker would have it. As it happens, Thorne would’ve smelled victory if only they’d decided to wait 16 years more.

At a press conference in Washington, DC on February 11, a global collaboration of scientists and engineers officially declared that they’d detected gravitational waves, ripples of energy flying through the fabric of space-time.

“The first direct detection of gravitational waves by the LIGO science collaboration is a breathtaking discovery because it opens a brand new window on the universe,” said Abhay Ashtekar, the director of the Institute for Gravitational Physics and Geometry at Pennsylvania State University. “It will reveal secrets from the farthest regions of the universe that we cannot access by conventional astronomy.”

Apart from better understanding how neutron stars and black-holes evolve and merge, the finding will allow astronomers to compare them against predictions made by Albert Einstein’s theory of general relativity. In short, such studies will help determine if the theory presents a perfect picture of gravity. It was first published 100 years ago.

In the month leading up to the declaration, rumours of the detection’s details had been making the rounds and were frequently hailed as “huge” Martin Rees, the United Kingdom’s Astronomer Royal, wrote it would be the “scientific highlight of the decade”. If they are verified by other experiments in the future – even though the collaboration claims a very reliable result – February 11, 2016, could be the first day of the era of gravitational-wave astronomy.

And the time at which the observation, designated GW150914, was made was September 14, 2015, 3.21 pm IST, by the twin Advanced Laser Interferometer Gravitational-wave Observatories (aLIGO) in Hanford, Washington, and Livingston, Louisiana. The observatories are funded by the National Science Foundation of the US. Their working principles are based on ideas formulated by Thorne, Rainer Weiss and Ronald Drever in the 1980s.

The gravity of massive objects in the universe deforms the space-time around them. The motion of other objects in the vicinity is influenced by this deformation and they feel it as the force of gravity. The work of Einstein as well as a group of other mathematicians and physicists in the early 20th century helped elucidate this picture as the way gravity works.

However, it was Einstein’s theory of general relativity that predicted that when massive objects accelerate, they set off disturbances in space-time that propagate outward, and throughout the universe. These disturbances are the manifestations of the objects losing gravitational energy, and the energy being carried off in the form of gravitational waves. As the waves move through the continuum, they temporarily distort distances in the regions they pass through.

The LIGO project was set up in 1992 for detecting these passing distortions, and upgraded to a more sensitive ‘Advanced’ avatar, aLIGO, by 2014. Each of its observatories has a common design: two long tunnels connected at a vertex, shaped like an ‘L’. A source at the vertex fires a laser pulse down each tunnel and waits for them to be reflected back by a mirror at the end. When the pulses reconvene, they form an interference pattern registered by a detector. In the absence of a gravitational wave, the interference is fully destructive and the detector draws a blank.

When a gravitational wave passes through aLIGO, it temporarily (and alternately) contracts and expands the length of the arms by a tiny amount. As a result, one of the laser pulses ends up travelling a longer distance than the other. When they reconvene, one pulse is slightly out of step relative to the other and their interference isn’t destructive. The detector lodges an interference pattern. According to the February 11 announcement, that’s what happened on September 14, 2015.

A simulation of GW150914 by the Numerical Relativity group at the Georgia Institute of Technology.

According to the data released, the waves likely originated from a pair of black-holes 1.2-1.3 billion lightyears away. They were orbiting each other, reaching speeds of about 180 million metres per second, and eventually merged to form a larger black-hole. While they initially weighed 29 and 36 solar masses, the resulting monster weighed 62 solar masses. The remaining 3 solar masses (equivalent to 178.7 billion trillion trillion trillion joules of energy) were released as gravitational waves during the merger and subsequent ringdown, when the resultant settles down to form a stable shape. The entire event spanned a few seconds, which means – as Thorne figured during the press conference – the power output was 50-times as much as the output of all the stars in the universe put together.

“The coolest thing for me is that the signal was emitted some 1.3 billion years ago. Back then, there was no major life-form on Earth. The signal travelled for 1.3 billion years and passed through Earth in less than half a second,” said Karan Jani, a PhD candidate at the Georgia Institute of Technology and an analyst with the LIGO collaboration.

Clifford Burgess, a theoretical physicist at the McMaster University in Hamilton, Canada, had leaked in an email to his students – eventually circulated on the Internet – ahead of the announcement that the signals registered at aLIGO were made at a statistical significance of more than 5 sigma. This means that the odds that the detection was a false signal were at most 1 in 3.5 million, sufficient among physicists to claim a discovery.

As a summary of results accompanying the announcement noted, “We expect an event as strong as GW150914 to appear by chance only once in about 200,000 years of such data.”

The detection took as long as it did to be made because, of the four fundamental forces in nature, gravity is the weakest by far. As a result, the effect of a gravitational wave is also extremely small and requires super-sensitive instruments to pick up on it. At the same time, any gravitational wave detector needs to be at least as large as the source of the wave it’s detecting.

Because two black-holes orbiting each other can be separated by only a few kilometres before smashing, the aLIGO’s arms are 4 km long, and are maintained with a perfect vacuum. The lasers and the detectors are configured to pick up on changes in the length of space of the order of 10 -20 metres – that’s about 10,000 times smaller than the nucleus of a hydrogen atom. Such sensitivity means the detectors pick up on a lot of noise as well – from vehicles moving on the surface in the vicinity, minor seismic disturbances underground, disturbances left behind by ancient cosmic events, and other activity that for pretty much any other purposes wouldn’t be bothersome.

So even when a bona fide detection is made, scientists will have to apply advanced data-filtering techniques to spot it in the sea of noise logged by the detectors. Satya Mohapatra, a staff technician at the LIGO Lab at the Massachusetts Institute of Technology, Boston, explained that different groups within the collaboration studied “thousands of channels in the LIGO instruments to characterise different noise sources that could affect a potential gravitational wave signal”. Additional groups also studied how gravitational waves originating from sources other than black-hole-mergers would look like so they could be filtered out better.

Source: LIGO/Georgia Institute of Technology

Mohapatra continued, “The exact shape of the gravitational wave that comes from the collision of two black-holes remained elusive until 2005 as general relativity is a very non-linear theory.” In that year, “the first complete simulation of the merger of two black-holes was demonstrated by Frans Pretorius.” Pretorius is now a professor of physics at Princeton University, New Jersey.

But that wasn’t the end of that road. “Black-holes and neutron stars also have spins. So the shapes of waveforms for different combinations of masses and spins have not all been simulated,” Mohapatra said. So the February 11 announcement was effectively the result of great advancements in numerical astrophysics.

The existence of gravitational waves was assured since the 1970s, when two astronomers from the University of Massachusetts-Amherst discovered a pair of neutron stars orbiting each other whose orbits were shrinking at a rate predicted by Einstein’s equations for general relativity. The astronomers would go on to win the 1993 Nobel Prize for physics for making the connection that the neutron stars were losing gravitational energy – probably by emitting gravitational waves.

So a great part of the excitement now isn’t because the waves have finally been directly detected but because we now have an instrument that can probe deeper into the mysterious sources of the waves themselves.

For example, though Einstein was satisfied by how his theory of general relativity seemed able to explain the behaviour of gravity in the universe, he wasn’t comfortable with one of its direct consequences: black-holes. The ability of these freaks of nature to distort space-time to the point of bending electromagnetic radiation into themselves has made it very difficult to study them using telescopes that ‘see’ using electromagnetic radiation.

Possible areas in the southern sky wherefrom the gravitational waves of GW150914 could’ve originated. The colours represent confidence intervals, with purple representing a 90% chance that the merger happened in the volume of space within it and yellow, 50%. Source: LIGO/Georgia Institute of Technology

Gravitational-wave observatories, on the other hand, ‘hear’ using the nature of gravity, which “couples to everything and cannot be masked”, according to Ghanashyam Date, a professor at the Institute for Mathematical Sciences, Chennai. And configuring detectors like aLIGO to better detect and investigate the waves opens up a new way to investigate the cosmos. As David Reitze, the executive director of the LIGO Laboratory, California Institute of Technology, said at the press conference, “This is the first time the universe has spoken to us – through gravitational waves.”

For one, general relativity predicts that gravitational waves should set off at the speed of light, which means the hypothesised particles carrying gravitational energy – gravitons – should have no mass. The waves in GW150914 arrived at the Louisiana and Washington detectors some seven-thousandths of a second apart, consistent with the time light would take to travel the same distance.

However, if the waves are detected to be passing through slower in the future, then theoretical physicists will have to return to the proverbial drawing board for new ideas of particulate gravity.

In another case, given how sensitive to gravitational waves the current generation of LIGO is, astronomers can also measure how many black-holes there are of different masses and how often they’re involved in intense events like mergers. “Black-holes in astrophysics were thought to belong in two extreme classes – stellar black-holes weighing less than 20 solar masses and those at the centres of galaxies weighing millions to billions of solar masses,” said Jani. There’s circumstantial evidence for these black-holes as well from conventional telescopes – which is what made the current detection more unlikely.

“We just didn’t have strong astrophysical bounds on whether black-holes of such masses can exist in the universe,” explained Jani. They weigh an intermediary 50-10,000 solar masses and haven’t been studied much with telescopes. But at LIGO, they generate the ‘loudest’ signals. “With [this finding] of black-holes just lighter than the intermediate mass, we now have a smooth range of possible masses for black-holes in the universe,” said Jani.

Currently, there are five gravitational-wave observatories: two in the US and one each in Italy (Virgo), Germany (GEO600) and Japan (KAGRA). The Japanese observatory has a different detection technique. Meanwhile, the American and German observatories form a network of observatories that’s blind to about a few hundred degrees of the sky. That is, the network won’t be able to pinpoint the source of gravitational waves from this patch of the sky.

Locations of existing gravitational-wave detectors, and how far out a LIGO in India would be. Source: LIGO

As Jani explained in the context of GW150914, which was logged by the two American observatories: the gravitational waves “that we observed came from 1.2 billion lightyears away. Based on the mass of the two black holes, each almost 30-times the mass of the Sun, they must’ve been formed through the evolution of very-heavy stars. This implies the black-holes must be residing in some host galaxy, but it’s difficult to locate it.”

Many upgrades have been proposed for the aLIGO network to become better in this sense. One is the Evolved Laser Interferometer Space Antenna (ELISA), comprising three spacecraft orbiting the Sun in an equilateral triangle. Because of the distances between them, ELISA will be able to look for gravitational waves from very large sources. Ahead of its 2034 launch, a test mission called LISA Pathfinder was launched on December 3, 2015.

The other is an aLIGO in India. According to Tarun Souradeep, of the Inter-University Centre for Astronomy and Astrophysics, Pune, its location would reduce the aLIGO network’s blindspot by an order of magnitude. The project, worth about Rs.1,500 crore, is being funded by the Department of Atomic Energy and received clearance from the erstwhile Planning Commission in its 12th Five Year Plan. At the moment, it’s waiting for clearance from the Union Cabinet.

In the meantime, pending future tests confirming the detection, the aLIGO announcement is unquestionably a Nobel moment. What’s questionable is whom the eventual Nobel Prize will end up overlooking. The LIGO Scientific Collaboration involves over 1,000 scientists from 19 countries, with over 250 research institutes involved in developing technology and analysing results. The observatories are operated by the Massachusetts Institute of Technology, Boston, and the California Institute of Technology.

## Scientists make first direct detection of gravitational waves

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Almost 100 years ago today, Albert Einstein predicted the existence of gravitational waves — ripples in the fabric of space-time that are set off by extremely violent, cosmic cataclysms in the early universe. With his knowledge of the universe and the technology available in 1916, Einstein assumed that such ripples would be “vanishingly small” and nearly impossible to detect. The astronomical discoveries and technological advances over the past century have changed those prospects.

Now for the first time, scientists in the LIGO Scientific Collaboration — with a prominent role played by researchers at MIT and Caltech — have directly observed the ripples of gravitational waves in an instrument on Earth. In so doing, they have again dramatically confirmed Einstein’s theory of general relativity and opened up a new way in which to view the universe.

But there’s more: The scientists have also decoded the gravitational wave signal and determined its source. According to their calculations, the gravitational wave is the product of a collision between two massive black holes, 1.3 billion light years away — a remarkably extreme event that has not been observed until now.

The researchers detected the signal with the Laser Interferometer Gravitational-wave Observatory (LIGO) — twin detectors carefully constructed to detect incredibly tiny vibrations from passing gravitational waves. Once the researchers obtained a gravitational signal, they converted it into audio waves and listened to the sound of two black holes spiraling together, then merging into a larger single black hole.

“We’re actually hearing them go thump in the night,” says Matthew Evans, an assistant professor of physics at MIT. “We’re getting a signal which arrives at Earth, and we can put it on a speaker, and we can hear these black holes go, ‘Whoop.’ There’s a very visceral connection to this observation. You’re really listening to these things which before were somehow fantastic.”

By further analyzing the gravitational signal, the team was able to trace the final milliseconds before the black holes collided. They determined that the black holes, 30 times as massive as our sun, circled each other at close to the speed of light before fusing in a collision and giving off an enormous amount of energy equivalent to about three solar masses — according to Einstein’s equation E=mc 2 — in the form of gravitational waves.

“Most of that energy is released in just a few tenths of a second,” says Peter Fritschel, LIGO’s chief detector scientist and a senior research scientist at MIT’s Kavli Institute for Astrophysics and Space Research. “For a very short amount of time, the actual power in gravitational waves was higher than all the light in the visible universe.”

These waves then rippled through the universe, effectively warping the fabric of space-time, before passing through Earth more than a billion years later as faint traces of their former, violent origins.

“It’s a spectacular signal,” says Rainer Weiss, a professor emeritus of physics at MIT. “It’s a signal many of us have wanted to observe since the time LIGO was proposed. It shows the dynamics of objects in the strongest gravitational fields imaginable, a domain where Newton’s gravity doesn’t work at all, and one needs the fully non-linear Einstein field equations to explain the phenomena. The triumph is that the waveform we measure is very well-represented by solutions of these equations. Einstein is right in a regime where his theory has never been tested before.”

The new results are published today in the journal Physical Review Letters.

“Magnificently in alignment”

The first evidence for gravitational waves came in 1974, when physicists Russell Hulse and Joseph Taylor discovered a pair of neutron stars, 21,000 light years from Earth, that seemed to behave in a curious pattern. They deduced that the stars were orbiting each other in such a way that they must be losing energy in the form of gravitational waves — a detection that earned the researchers the Nobel Prize in physics in 1993.

Now LIGO has made the first direct observation of gravitational waves with an instrument on Earth. The researchers detected the gravitational waves on September 14, 2015, at 5:51 a.m. EDT, using the twin LIGO interferometers, located in Livingston, Louisiana and Hanford, Washington.

Each L-shaped interferometer spans 4 kilometers in length and uses laser light split into two beams that travel back and forth through each arm, bouncing between precisely configured mirrors. Each beam monitors the distance between these mirrors, which, according to Einstein’s theory, will change infinitesimally when a gravitational wave passes by the instrument.

“You can almost visualize it as if you dropped a rock on the surface of a pond, and the ripple goes out,” says Nergis Malvalvala, the Curtis and Kathleen Marble Professor of Astrophysics at MIT. “[It’s] something that distorts the space time around it, and that distortion propagates outward and reaches us on Earth, hundreds of millions of years later.”

Last March, researchers completed major upgrades to the interferometers, known as Advanced LIGO, increasing the instruments’ sensitivity and enabling them to detect a change in the length of each arm, smaller than one-ten-thousandth the diameter of a proton. By September, they were ready to start observing with them.

“The effect we’re measuring on Earth is equivalent to measuring the distance to the closest star, Alpha Centauri, to within a few microns,” Evans says. “It’s a very tough measurement to make. Einstein expected this to never have been pulled off.”

Nevertheless, a signal came through. Using Einstein’s equations, the team analyzed the signal and determined that it originated from a collision between two massive black holes.

“We thought it was going to be a huge challenge to prove to ourselves and others that the first few signals that we saw were not just flukes and random noise,” says David Shoemaker, director of the MIT LIGO Laboratory. “But nature was just unbelievably kind in delivering to us a signal that’s very large, extremely easy to understand, and absolutely, magnificently in alignment with Einstein’s theory.”

For LIGO’s hundreds of scientists, this new detection of gravitational waves marks not only a culmination of a decades-long search, but also the beginning of a new way to look at the universe.

“This really opens up a whole new area for astrophysics,” Evans says. “We always look to the sky with telescopes and look for electromagnetic radiation like light, radio waves, or X-rays. Now gravitational waves are a completely new way in which we can get to know the universe around us.”

Tiny detection, massive payoff

LIGO research is carried out by the LIGO Scientific Collaboration (LSC), a group of some 950 scientists at universities around the United States, including MIT, and in 15 other countries. The LIGO Observatories are operated by MIT and Caltech. The instruments were first explored as a means to detect gravitational waves in the 1970s by Weiss, who along with Kip Thorne and Ronald Drever from Caltech proposed LIGO in the 1980s.

“This has been 20 years of work, and for some of us, even more,” Evans says. “It’s been a long time working on these detectors, without seeing anything. So it’s a real sea change and an interesting psychological change for the whole collaboration.”

“The project represents a triumph for federally funded research,” says Maria Zuber, vice president for research and E. A. Griswold Professor of Geophysics at MIT. “LIGO is an example of a high-risk, high-return investment in discovery-driven science. In this case the investment was major and sustained over many years, with a successful outcome far from assured. But the scientific payoff is shaping up to be extraordinary. While the discoveries reported here are already magnificent, they represent the tip of the iceberg of what will be learned about fundamental physics and the nature of the universe.”

The LIGO Observatories are due for more upgrades in the near future. Currently, the instruments are performing at one-third of their projected sensitivity. Once they are fully optimized, Shoemaker predicts that scientists will be able to detect gravitational waves emanating “from the edge of the universe.”

“In a few years, when this is fully commissioned, we should be seeing events from a whole variety of objects: black holes, neutron stars, supernova, as well as things we haven’t imagined yet, on the frequency of once a day or once a week, depending on how many surprises are out there.” Shoemaker says. “That’s our dream, and so far we don’t have any reason to know that that’s not true.”

As for this new gravitational signal, Weiss, who first came up with the rudimentary design for LIGO in the 1970s as part of an experimental exercise for one of his MIT courses, sees the tiny detection as a massive payoff.

“This is the first real evidence that we’ve seen now of high-gravitational field strengths: monstrous things like stars, moving at the velocity of light, smashing into each other and making the geometry of space-time turn into some sort of washing machine,” Weiss says. “And this horrendously strong thing made a very tiny effect in our apparatus, a relative motion of 10 to the minus 18 meters between the mirrors in the interferometer arms. It’s sort of unbelievable to think about.”