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Your use of the machine generated PDF is subject to all use restrictions contained in The Cengage LearningSubscription and License Agreement and/or the Gale In Context: Science Terms and Conditions and by using the machinegenerated PDF functionality you agree to forgo any and all claims against Cengage Learning or its licensors for your use of themachine generated PDF functionality and any output derived therefrom.Gravitational wavesAuthor: K. Lee LernerDate: 2021 From: Gale Science Online CollectionPublisher: Gale, a Cengage CompanyDocument Type: Topic overview Length: 975 wordsContent Level: (Level 5)Lexile Measure: 1520LFull Text: Gravitational waves are normally described as ripples in the fabric of space-time. Although discussed abstractly as early as 1893 byBritish electrical engineer Oliver Heaviside (1850–1925), and in 1905 by French mathematician and theoretical physicist HenriPoincaré (1854'n1912) as "ondes gravifiques," German-born American Physicist Albert Einstein (1879—1955) formally predicted theexistence of gravitational waves in 1916 as part of his General Theory of Relativity.In part, the imagery of ripples across fabric or waves across the surface of a calm lake are devices needed to describe phenomenathat take place in space-time (the fusion of three dimensional space with a fourth dimension of time). Space-time, also championedby Einstein, is often visualized as a fabric stretched taught upon which masses such as stars and plants sit. Their masses individuallysink into a slight depression depending on the mass and so distort the fabric around them. Gravity is represented by the attractioncreated by depressions or wells surrounding each mass, in which objects are accelerating toward the mass.When masses are accelerated (a change in velocity created by a change in either magnitude of speed and/or direction or travel) theydisrupt the fabric of space-time much as a boat generates waves when accelerating on a calm sea. Gravitational waves then travel atthe speed of light through space-time, distorting space as they go somewhat like waves distort the surface of calm water.Gravitational waves are produced by violent and energetic processes. Just as one might not be able to see a stone plunging intowater, gravitational waves traveling from events distant from us in space and time provide evidence that something did disturb thespace-time surface or fabric. The nature of the waves, especially their energy level, then tell us about what type of process createdthe initial disturbance.The most intense, and therefor most easily detected waves result from the colliding black holes, the collapse and explosion of stars,condensing neutron stars, etc. Einstein even predicted residual gravitational waves from creation of the Cosmos in accord with BigBang theory.General relativity predicts a number of phenomena—such as the bending of light by massive objects like stars and the precession ofthe planet Mercury—both which have been well verified by direct observations. While Einstein predicted gravitational waves existedbut he did not think it would be possible to measure the subatomic distortions produced by waves reaching Earth from distantastronomical events.The first proof of the existence of detectable gravitational waves came in 1974, when Princeton University astronomers JosephTaylor and Russell Hulse, both working at the Arecibo Radio Observatory in Puerto Rico (decommissioned in 2020 after damage tocables supporting the main dish), discovered and observed a binary pulsar created by two extremely massive neutron stars in orbitabout each other. One of the neutron stars was a pulsar sending out measurable radio waves. Careful observations of the rotationshowed deviations exactly as predicted by Einstein's general relativity theory, and subsequent observations since that time are inexact accord with the emanation of gravitational waves produced by predicted changes (accelerations) in the respective orbits.Observations of slight distortions in the timing of pulsar radio emissions since recorded further strengthened evidence for gravitationalwaves.The first direct evidence of gravitational waves came in September 2015 via the Laser Interferometer Gravitational Observatory(LIGO), a consortium of observing stations including a pair of vary large and sensitive large laser interferometers based in the UnitedStates. LIGO was capable of physically sensed the sub-protonic distortions in space-time caused by gravitational waves generatedfrom the collision of two black holes nearly 1.3 billion light years away (and hence 1.3 billion years ago).Each LIGO apparatus consists of an L-shaped tunnel with two arms. At the end of each arm is a mirror, and where the two arms meetthere is a laser beam splitter. Each arm has the same length. When a laser beam is shot into the tunnel, it goes through the beamsplitter, so both beams have the same phase initially. The beams are reflected from the mirrors and recombine at the original entrypoint, where the intensity of the beam is measured by a photodetector. As long as both arms have the same length, the beamsinterfere constructively and the maximum intensity will be measured. If a gravitational wave passes through the apparatus, the lengthof the tunnel arms will change; one arm will become slightly shorter, and the other will become slightly longer. The laser beams willthen be out of phase when they recombine (because they traveled different distances) and there will be some measurable destructiveinterference.The challenge in making these measurements is to minimize vibrations caused by seismic activity or human activity that could disruptthe experiment and lead to false signals. To minimize these effects, sophisticated automatic mirror control systems were