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Gravitational wave
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====Interferometers==== {{Main|Ground-based interferometric gravitational-wave search}} {{gravitational wave observatory principle.svg}} A more sensitive class of detector uses a laser [[Michelson interferometer]] to measure gravitational-wave induced motion between separated 'free' masses.<ref>The idea of using laser interferometry for gravitational wave detection was first mentioned by Gerstenstein and Pustovoit 1963 Sov. Phys.–JETP 16 433. Weber mentioned it in an unpublished laboratory notebook. [[Rainer Weiss]] first described in detail a practical solution with an analysis of realistic limitations to the technique in R. Weiss (1972). "Electromagetically Coupled Broadband Gravitational Antenna". Quarterly Progress Report, Research Laboratory of Electronics, MIT 105: 54.</ref> This allows the masses to be separated by large distances (increasing the signal size); a further advantage is that it is sensitive to a wide range of frequencies (not just those near a resonance as is the case for Weber bars). After years of development ground-based interferometers made the first detection of gravitational waves in 2015. Currently, the most sensitive is [[LIGO]]{{snd}} the Laser Interferometer Gravitational Wave Observatory. LIGO has three detectors: one in [[Livingston, Louisiana]], one at the [[Hanford site]] in [[Richland, Washington]] and a third (formerly installed as a second detector at Hanford) that is planned to be moved to [[INDIGO|India]]. Each observatory has two [[Fabry–Pérot interferometer|light storage arms]] that are 4 kilometers in length. These are at 90 degree angles to each other, with the light passing through 1 m diameter vacuum tubes running the entire 4 kilometers. A passing gravitational wave will slightly stretch one arm as it shortens the other. This is the motion to which an interferometer is most sensitive. Even with such long arms, the strongest gravitational waves will only change the distance between the ends of the arms by at most roughly 10<sup>−18</sup> m. LIGO should be able to detect gravitational waves as small as ''h'' ~ {{val|5|e=-22}}. Upgrades to LIGO and [[Virgo interferometer|Virgo]] should increase the sensitivity still further. Another highly sensitive interferometer, [[KAGRA]], which is located in the [[Kamioka Observatory]] in Japan, is in operation since February 2020. A key point is that a tenfold increase in sensitivity (radius of 'reach') increases the volume of space accessible to the instrument by one thousand times. This increases the rate at which detectable signals might be seen from one per tens of years of observation, to tens per year.<ref name=Abadie2010>{{cite journal |author1=LIGO Scientific Collaboration |author2=Virgo Collaboration |title=Predictions for the rates of compact binary coalescences observable by ground-based gravitational-wave detectors |journal=Classical and Quantum Gravity |volume=27 |issue=17 |page=17300 |arxiv=1003.2480 |bibcode=2010CQGra..27q3001A |doi=10.1088/0264-9381/27/17/173001 |year=2010 |s2cid=15200690 }}</ref> Interferometric detectors are limited at high frequencies by [[shot noise]], which occurs because the lasers produce photons randomly; one analogy is to rainfall{{snd}} the rate of rainfall, like the laser intensity, is measurable, but the raindrops, like photons, fall at random times, causing fluctuations around the average value. This leads to noise at the output of the detector, much like radio static. In addition, for sufficiently high laser power, the random momentum transferred to the test masses by the laser photons shakes the mirrors, masking signals of low frequencies. Thermal noise (e.g., [[Brownian motion]]) is another limit to sensitivity. In addition to these 'stationary' (constant) noise sources, all ground-based detectors are also limited at low frequencies by [[seismic]] noise and other forms of environmental vibration, and other 'non-stationary' noise sources; creaks in mechanical structures, lightning or other large electrical disturbances, etc. may also create noise masking an event or may even imitate an event. All of these must be taken into account and excluded by analysis before detection may be considered a true gravitational wave event.
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