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Gravitational waves are detected using a Michelson interferometer with a laser light source. In 2020 there were three Michelson interferometer gravitational wave detectors operational, and a fourth under construction. These Michelson interferometers have arms 4 kilometers in length, set at 90 degree angles to each other, with the light passing through 1 m diameter vacuum tubes running their entire length. A passing gravitational wave will slightly stretch one arm as it shortens the other. This is precisely the motion to which these Michelson interferometers are most sensitive. As of 2020 fifteen gravitational wave events had been observed using these Michelson interferometers.