Gravitational wave

Lore & Background

The possibility of gravitational waves traveling at the speed of light was discussed as early as 1893 by Oliver Heaviside, using an analogy between the inverse-square law of gravitation and the electrostatic force. In 1905, Henri Poincaré proposed gravitational waves emanating from a body and propagating at the speed of light as required by Lorentz transformations, suggesting that accelerated masses should produce gravitational waves analogous to accelerating electrical charges producing electromagnetic waves. Einstein published his general theory of relativity in 1915 and conjectured that the equation would produce gravitational waves, though he noted they could not be similar to electromagnetic waves because gravitation has no equivalent to negative charge. His 1916 publication concluded that gravitational waves must propagate at the speed of light and identified three types, later termed longitudinal–longitudinal, transverse–longitudinal, and transverse–transverse by Hermann Weyl. However, in 1922 Arthur Eddington showed that two of these types were artifacts of the coordinate system and could be made to propagate at any speed, casting doubt on the physicality of the third type. In 1936, Einstein and Nathan Rosen submitted a paper claiming gravitational waves could not exist in the full general theory of relativity because any such solution would have a singularity.

Reader's Guide

Gravitational waves have transformed astronomy by enabling observation of events that cannot be studied with electromagnetic radiation, as they do not strongly interact with intervening matter. Sources include binary star systems of white dwarfs, neutron stars, and black holes; supernovae; and the early universe shortly after the Big Bang. Because the universe was opaque to electromagnetic radiation before recombination, gravitational waves offer a possible way to observe the very early universe. The first indirect evidence came in 1974 from the orbital decay of the Hulse–Taylor binary pulsar, matching general relativity's prediction for energy lost to gravitational radiation, leading to the 1993 Nobel Prize in Physics for Russell Alan Hulse and Joseph Hooton Taylor Jr. The first direct observation occurred in September 2015, when a signal from two merging black holes was received by LIGO detectors in Livingston, Louisiana, and Hanford, Washington; the 2017 Nobel Prize in Physics was awarded to Rainer Weiss, Kip Thorne, and Barry Barish for their role in this detection. Gravitational waves can exist at any frequency; very low frequency waves are detected using pulsar timing arrays, which monitor timing changes from approximately 100 pulsars to locate sources such as merging supermassive black holes. The speed of gravitational waves equals the speed of light in vacuum, as confirmed by the 2017 event GW170817, which constrained the difference to less than one part in 10^15.

Did You Know?

• The first indirect evidence for gravitational waves came in 1974 from the observed orbital decay of the Hulse–Taylor binary pulsar.
• The first direct observation of gravitational waves was made in September 2015 by the LIGO detectors.
• Gravitational waves are not affected by intervening matter, unlike electromagnetic radiation.