GW150914 - The first direct detection of gravitational waves

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.


LIGO: The First Observation of Gravitational Waves (3:35)

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. (Credit: Caltech)


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Signal observed by each LIGO detector

Signal observed by each LIGO detector. These plots show the signals of gravitational waves detected by the twin LIGO observatories at Livingston, Louisiana, and Hanford, Washington. The signals came from two merging black holes, each about 30 times the mass of our sun, lying 1.3 billion light-years away.

The top two plots show data received at Livingston and Hanford, along with the predicted shapes for the waveform. These predicted waveforms show what two merging black holes should look like according to the equations of Albert Einstein's general theory of relativity, along with the instrument's ever-present noise. Time is plotted on the X-axis and strain on the Y-axis. Strain represents the fractional amount by which distances are distorted. As the plots reveal, the LIGO data very closely match Einstein's predictions.

The final plot compares data from both detectors. The Hanford data have been inverted for comparison, due to the differences in orientation of the detectors at the two sites. The data were also shifted to correct for the travel time of the gravitational-wave signals between Livingston and Hanford (the signal first reached Livingston, and then, traveling at the speed of light, reached Hanford seven thousandths of a second later). As the plot demonstrates, both detectors witnessed the same event, confirming the detection. (Image Credit: Caltech/MIT/LIGO Lab.) [Download additional file sizes here.]

The approximate location of GW150914 on a sky map of the southern hemisphere.

The approximate location of GW150914 on a sky map of the southern hemisphere. The colored lines represent different probabilities for where the signal originated: the purple line defines the region where the signal is predicted to have come from with a 90 percent confidence level; the inner yellow line defines the target region at a 10 percent confidence level. The gravitational waves were produced by a pair of merging black holes located 1.3 billion light-years away.

A small galaxy near our own, called the Large Magellanic Cloud, can be seen as a fuzzy blob underneath the marked area, while an even smaller galaxy, called the Small Magellanic Cloud, is below it.

Researchers were able to home in on the location of the gravitational-wave source using data from the LIGO observatories in Livingston, Louisiana, and Hanford, Washington. The gravitational waves arrived at Livingston 7 milliseconds before arriving at Hanford. This time delay revealed a particular slice of sky, or ring, from which the signal must have arisen. Further analysis of the varying signal strength at both detectors ruled out portions of the ring, leaving the remaining patch shown on this map. (Image credit: LIGO/Axel Mellinger.) [Download additional file sizes here.]

Computer simulation image showing the merger of two black holes.

Computer simulation image showing the merger of two black holes. LIGO detected gravitational waves, or ripples in space and time generated as the black holes spiraled in toward each other, collided, and merged. This simulation shows how the merger would appear to our eyes if we could somehow travel in a spaceship for a closer look. It was created by solving equations from Albert Einstein's general theory of relativity using the LIGO data.

The stars appear warped due to the incredibly strong gravity of the black holes. The black holes warp space and time, and this causes light from the stars to curve around the black holes in a process called gravitational lensing. (Image Credit: SXS, the Simulating eXtreme Spacetimes (SXS) project.) [Download additional file sizes here.]

Network of gravitational-wave observatories during LIGO's first observing run.

Network of gravitational-wave observatories during LIGO's first observing run. During the detection of GW150914, facilities in the global network include the twin LIGO detectors—in Hanford, Washington, and Livingston, Louisiana—and GEO600 in Germany. The Virgo detector in Italy and the Kamioka Gravitational Wave Detector (KAGRA) in Japan are undergoing upgrades and are expected to begin operations in 2016 and 2018, respectively. A sixth observatory is being planned in India. Having more gravitational-wave observatories around the globe helps scientists pin down the locations and sources of gravitational waves coming from space. (Image Credit: Caltech/MIT/LIGO Lab.) [Download additional file sizes here.]


Computer simulation showing the warping of space and time around two colliding black holes observed by LIGO on September 14, 2015. LIGO detected gravitational waves generated by this black hole merger—humanity's first contact with gravitational waves and black-hole collisions. Gravitational waves are ripples in the shape of space and flow of time.

The colored surface is the space of our universe, as viewed from a hypothetical, flat, higher-dimensional universe, in which our own universe is embedded. Our universe looks like a warped two-dimensional sheet because one of its three space dimensions has been removed. Around each black hole, space bends downward in a funnel shape, a warping produced by the black hole's huge mass. Near the black holes, the colors depict the rate at which time flows. In the green regions outside the holes, time flows at its normal rate. In the yellow regions, it is slowed by 20 or 30 percent. In the red regions, time is hugely slowed. Far from the holes, the blue and purple bands depict outgoing gravitational waves, produced by the black holes' orbital movement and collision.

Our universe's space, as seen from the hypothetical higher-dimensional universe, is dragged into motion by the orbital movement of the black holes, and by their gravity and by their spins. This motion of space is depicted by silver arrows, and it causes the plane of the orbit to precess gradually, as seen in the video.

The upper left numbers show time, as measured by a hypothetical person near the black holes (but not so near that time is warped). The bottom portion of the movie shows the waveform, or wave shape, of the emitted gravitational waves.

The gravitational waves carry away energy, causing the black holes to spiral inward and collide. The movie switches to slow motion as the collision nears, and is paused at the moment the black holes' surfaces (their "horizons") touch. At the pause, space is enormously distorted. After the pause, again seen in slow motion, the shapes of space and time oscillate briefly but wildly, and then settle down into the quiescent state of a merged black hole. Returning to fast motion, we see the gravitational waves from the collision, propagating out into the universe.

The collision and wild oscillations constitute a "storm" in the fabric of space and time—an enormously powerful but brief storm. During the storm, the power output in gravitational waves is far greater than the luminosity of all the stars in our observable universe put together. In other words, this collision of two black holes, each the size of a large city on Earth, is the most powerful explosion that astronomers have ever seen, aside from our universe's birth in the Big Bang. (Credit: SXS, the Simulating eXtreme Spacetimes (SXS) project.)


Animation illustrating how the twin LIGO observatories work. One observatory is in Hanford, Washington, the other in Livingston, Louisiana. Each houses a large-scale interferometer, a device that uses the interference of two beams of laser light to make the most precise distance measurements in the world.

The animation begins with a simplified depiction of the LIGO instrument. A laser beam of light is generated and directed toward a beam splitter, which splits it into two separate and equal beams. The light beams then travel perpendicularly to a distant mirror, with each arm of the device being 4 kilometers in length. The mirrors reflect the light back to the beam splitter, repeating this process 200 times.

When gravitational waves pass through this device, they cause the length of the two arms to alternately stretch and squeeze by infinitesimal amounts, tremendously exaggerated here for visibility. This movement causes the light beam that hits the detector to flicker.

The second half of the animation explains the flickering, and this is where light interference comes into play. After the two beams reflect off the mirrors, they meet at the beam splitter, where the light is recombined in a process called interference. Normally, when no gravitational waves are present, the distance between the beam splitter and the mirror is precisely controlled so that the light waves are kept out of phase with each other and cancel each other out. The result is that no light hits the detectors.

But when gravitational waves pass through the system, the distance between the end mirrors and the beam splitter lengthen in one arm and at the same time shorten in the other arm in such a way that the light waves from the two arms go in and out of phase with each other. When the light waves are in phase with each other, they add together constructively and produce a bright beam that illuminates the detectors. When they are out of phase, they cancel each other out and there is no signal. Thus, the gravitational waves from a major cosmic event, like the merger of two black holes, will cause the signal to flicker, as seen here.

By digitizing and recording the specific patterns of signals that hit the LIGO detectors, researchers can then analyze what they see and compare the data to computer models of predicted gravitational wave signals. The effects of the gravitational waves on the LIGO instrument have been vastly exaggerated in this video to demonstrate how it works. In reality, the changes in the lengths of the instrument's arms is only 1/1000th the size of a proton. Other characteristics of LIGO, such as the exquisite stability of its mirrors, also contribute to its ability to precisely measure distances. In fact, LIGO can be thought of as the most precise "ruler" in the world. (Animation created by T. Pyle, Caltech/MIT/LIGO Lab.)


Computer simulation showing the collision of two black holes and lensing of the background star light. LIGO detected gravitational waves, or ripples in space and time generated as the black holes spiraled in toward each other, collided, and merged. This simulation shows how the merger would appear to our eyes if we could somehow travel in a spaceship for a closer look. It was created by solving equations from Albert Einstein's general theory of relativity using the LIGO data.

The two merging black holes are each roughly 30 times the mass of the sun, with one slightly larger than the other. Time has been slowed down by a factor of about 100. The event took place 1.3 billion years ago. The stars appear warped due to the incredibly strong gravity of the black holes. The black holes warp space and time, and this causes light from the stars to curve around the black holes in a process called gravitational lensing. The ring around the black holes, known as an Einstein ring, arises from the light of all the stars in a small region behind the holes, where gravitational lensing has smeared their images into a ring.

The gravitational waves themselves would not be seen by a human near the black holes and so do not show in this video, with one important exception. The gravitational waves that are traveling outward toward the small region behind the black holes disturb that region’s stellar images in the Einstein ring, causing them to slosh around, even long after the collision. The gravitational waves traveling in other directions cause weaker, and shorter-lived sloshing, everywhere outside the ring.(Credit: SXS, the Simulating eXtreme Spacetimes (SXS) project.)


The Sound of Two Black Holes Colliding. Gravitational waves sent out from a pair of colliding black holes have been converted to sound waves, as heard in this animation. On September 14, 2015, LIGO observed gravitational waves from the merger of two black holes, each about 30 times the mass of our sun. The incredibly powerful event, which released 50 times more energy than all the stars in the observable universe, lasted only fractions of a second.

In the first two runs of the animation, the sound-wave frequencies exactly match the frequencies of the gravitational waves. The second two runs of the animation play the sounds again at higher frequencies that better fit the human hearing range. The animation ends by playing the original frequencies again twice.

As the black holes spiral closer and closer together, the frequency of the gravitational waves increases. Scientists call these sounds "chirps," because some events that generate gravitation waves would sound like a bird's chirp. (Audio Credit: Caltech/MIT/LIGO Lab.)


Journey of a Gravitational Wave. LIGO scientist David Reitze takes us on a 1.3 billion year journey that begins with the violent merger of two black holes in the distant universe. The event produced gravitational waves, tiny ripples in the fabric of space and time, which LIGO detected as they passed Earth on September 14, 2015. (Credit: LIGO/SXS/R. Hurt and T. Pyle.)


LIGO: Opening a New Window Onto the Universe. This video narrative tells the story of the history and legacy of LIGO from the genesis of the idea to the detection in September 2015. (Credit: Caltech Strategic Communications and Caltech AMT.)


Black Hole Waves Simulation. This simulation depicts the birth, 1.3 billion years ago, of the gravitational waves discovered by LIGO on September 14, 2015. The waves are generated by two black holes that spiral around each other, then collide and merge. In the bright green regions, the waves stretch space; in the dark green regions, they squeeze space. As the black holes approach each other, the waves get stronger and the separation between them gets shorter, giving rise to what scientists refer to as a "chirp." The two black holes collide and merge into a new black hole, bringing the waves to a crescendo. The newborn black hole vibrates briefly, then becomes quiet and stops generating waves. The waves all depart from the black hole’s vicinity, traveling out into the universe, carrying news of the black holes’ collision. (Credit: SXS, the Simulating eXtreme Spacetimes (SXS) project.)


Zooming into an Atom. On September 14, 2015, LIGO became the first instrument to detect gravitational waves on Earth. When two black holes—each about 30 times more massive than our sun—merged, they generated gravitational waves—ripples in space and time. More than a billion years later, those waves reached LIGO's detectors, causing the distance between its mirrors—separated by 4 kilometers—to change by roughly 1/1000th the diameter of a proton. This annotated animation zooms in on the proton of a hydrogen atom. The movement of the proton shows the tiny changes measured by LIGO. (Simulation created by T. Pyle, Caltech/MIT/LIGO Lab.)

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