The Search for Gravity Waves
Analogous to electromagnetic waves, gravity waves were predicted by Einstein's general theory of relativity. A hypothetical particle, given the name graviton, has been suggested as the mediator of the gravitational force; it is analogous to the photon, the particle embodying the quantum properties of electromagnetic waves (see quantum theory). Tantalizing evidence for the existence of gravity waves came from astronomical observations of a binary pulsar designated 1913+16. The rate at which the two neutron stars in the binary rotate around each other changes in a manner that is consistent with the emission of gravity waves. The subsequent search for gravity waves has involved the building of large interferometers sensitive enough to detect the faint waves directly (see interference). The Laser Interferometer Gravitational Wave Observatory (LIGO), supported by the National Science Foundation, consists of two interferometers constructed in the 1990s, one in Hanford, Wash., the other in Livingston, La.; each has two 2.5-mi-long (4-km) arms at a right angle to each other. LIGO begin its work in 2002, but did not detect any gravitational waves until after an upgrade completed in 2015. Beginning in late 2015, LIGO several times detected gravitational waves that resulted from the merging of two black holes. The European Gravitational Observatory's Virgo gravitational wave detector, near Pisa, Italy, became operational in 2017. Begun by French and Italian scientific research organizations and now including personnel from institutes in other European nations, Virgo has a design similar to LIGO's, with two arms 1.86 mi (3 km) long. Later in 2017 it and LIGO detected gravitational waves from another black-hole merger and from a neutron-star merger, and detections by LIGO and Virgo increased in subsequent years. The proposed, even more ambitious Laser Interferometer Space Antenna (LISA) was originally a NASA–European Space Agency project but NASA withdrew in 2011 due to a lack of funding.
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