The Laser Interferometer Gravitational-Wave Observatory consists of two detectors in Hanford, Washington, and Livingston, Louisiana, built to observe gravitational waves produced by astronomical phenomena. Like ripples created by objects traveling along water, gravitational waves are ripples produced by objects moving through spacetime. Unlike light, which can be blocked by matter, gravitational waves travel at the speed of light unimpeded, allowing scientists to use LIGO to study otherwise invisible cosmic events (e.g., black holes).
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The instruments at each site send lasers down equally long arms, which should take the same amount of time to travel between suspended mirrors. Passing gravitational waves alter the shape of space—lengthening one arm and shrinking the other—altering how long it takes the lasers to complete their path, which scientists can measure.
First detected a century after Albert Einstein predicted their existence, gravitational waves transmit information about cosmological events by altering the shape of spacetime. Like ripples in water, these waves travel outward and can be detected on Earth, even for objects that do not emit light, such as black holes.
The 1984 agreement between the National Science Foundation and both research institutions came four years after funding 1.5-meter (4.9-foot) and 40-meter (131-foot) prototypes. With an initial budget of $395M, LIGO was once one of the largest NSF-funded projects in history.
Across the 4-kilometer (2.5-mile) arms, the height of Earth's surface falls by almost a meter, preventing the lasers traveling within the arms from meeting the mirror at the end of each. Concrete slabs built beneath the arms help keep them level and provide structural support against seismic vibrations.
The devices are designed so that a laser is split into two beams that later recombine, canceling each other out and producing no signal. Changes in the lengths of the arms due to passing gravitational waves result in an interference pattern whose properties reveal features of the objects that created the waves.
Gravity is the weakest of the fundamental forces in nature, meaning the length of objects only changes by minuscule amounts as gravitational waves ripple through space from distant, albeit massive, cosmic events. LIGO's sensitivity can measure an object shifting its position by one seven-hundred-trillionth of the width of a human hair.
On February 11, 2016, scientists at the Laser Interferometer Gravitational-Wave Observatory announced the confirmed detection of ripples in spacetime, which were produced from the collision of two black holes 36 and 29 times the mass of the sun. The detection confirmed that gravitational waves travel at the speed of light, as predicted by Albert Einstein in 1915.
As ripples in spacetime, gravitational waves do not produce sound, which is the perception of specific vibrations in matter (e.g., vocal cords vibrating air molecules). The "chirp" of gravitational waves comes instead from converting the spacetime ripples into sound waves that the human ear can hear.
In "Black Hole Hunter," users try to listen for gravitational wave signals from black holes in four sample data files that include substantial background noise. With each level, learn more about black holes while being faced with ever-quieter signals.
The 1,864-mile distance meant gravitational wave signals would be detected no more than 10 milliseconds apart, ruling out many background signals. The sites needed sufficient plots of nearly flat land, proximity to infrastructure (e.g., electricity), and adequate distance from urban areas and geologically active areas to reduce environmental noise.
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