Gravitational-Wave Observatories Across the Globe

Current operating 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.

Most Precise Ruler Ever Constructed

This animation illustrates how the twin observatories of LIGO 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 measures distances. In fact, LIGO can be thought of as the most precise "ruler" in the world.

Two Black Holes Merge into One

The collision of two black holes holes—a tremendously powerful event detected for the first time ever by the Laser Interferometer Gravitational-Wave Observatory, or LIGO—is seen in this still from a computer simulation. 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. 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.

Warped Space and Time Around Colliding Black Holes

This computer simulation shows 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.

Gravitational Waves, As Einstein Predicted

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.

Where the Gravitational Waves Came From

The approximate location of the source of gravitational waves detected on September 14, 2015, by the twin LIGO facilities is shown on this 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.

In the future, when additional gravitational-wave detectors are up and running, scientists will be able to pinpoint more precisely the locations and sources of signals.

Massive Bodies Warp Space-Time OTW

How our sun and Earth warp space and time, or spacetime, is represented here with a green grid. As Albert Einstein demonstrated in his theory of general relativity, the gravity of massive bodies warps the fabric of space and time—and those bodies move along paths determined by this geometry. His theory also predicted the existence of gravitational waves, which are ripples in space and time. These waves, which move at the speed of light, are created when massive bodies accelerate through space and time.

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 in 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.

Exaggerated Effects of Gravitational Waves on Earth

Gravitational waves stretch and squeeze the fabric of space and time on very minuscule scales; visually exaggerating these effects reveals how Earth is squeezed and stretched. Gravitational waves are generated when massive objects, such as pairs of black holes, accelerate through space and time. On September 14, 2015, LIGO became the first instrument on Earth to detect these waves, in this instance originating from the collision of two black holes more than a billion light-years away. LIGO detectors were able to measure the stretching and squeezing of space -- caused by the passage of these gravitational waves -- with a precision equivalent to 1/1000th the diameter of a proton. That's like measuring the distance to the nearest star down to a precision level of just a fraction of the thickness of a human hair.