====== Gravitational Waves ======

Gravitational waves are ripples in spacetime curvature produced by accelerating masses, propagating outward from their source at the speed of light. The concept follows from [[general-relativity-overview-technical|general relativity]], which describes gravity not as a force but as the curvature of spacetime caused by mass and energy. Albert Einstein predicted their existence in 1916 as a consequence of his field equations.((Albert Einstein, "Näherungsweise Integration der Feldgleichungen der Gravitation," *Sitzungsberichte der Königlich Preußischen Akademie der Wissenschaften* (1916): 688-696.)) The interpretation of certain detection claims and the broader implications drawn from them have at times generated debate within the scientific community; see [[gravitational-waves-controversy-bicep2-controversy|Controversies]] below.

The clearest sources of gravitational waves are systems involving extreme acceleration of compact, massive objects: orbiting [[black-holes|black holes]] or neutron stars, supernova collapses, and, hypothetically, processes in the very early universe. As two massive bodies orbit each other, they lose orbital energy to gravitational radiation, causing the orbit to decay-a process that, over long timescales, brings the bodies into closer orbit and eventually merger.

===== Current State of Knowledge =====

For decades after Einstein's prediction, gravitational waves remained a theoretical construct, partly because some physicists, including Einstein himself at points, questioned whether they were physically real or merely artifacts of coordinate choice in the mathematics.((See [[gravitational-waves-history|History]] for the resolution of this debate.)) Indirect evidence accumulated starting in 1974, when Russell Hulse and Joseph Taylor discovered a binary pulsar (PSR B1913+16) whose orbital decay matched the rate predicted by gravitational-wave emission to high precision, work for which they received the 1993 Nobel Prize in Physics.((Joseph H. Taylor and Joel M. Weisberg, "A New Test of General Relativity: Gravitational Radiation and the Binary Pulsar PSR 1913+16," *The Astrophysical Journal* 253 (1982): 908-920.))

Direct detection was achieved on 14 September 2015 by the Laser Interferometer Gravitational-Wave Observatory (LIGO), which recorded a signal (GW150914) consistent with the merger of two black holes roughly 1.3 billion light-years away.((B. P. Abbott et al., "Observation of Gravitational Waves from a Binary Black Hole Merger," *Physical Review Letters* 116, no. 6 (2016): 061102.)) The detection relied on laser interferometry: measuring minute changes in the relative length of two perpendicular arms, each several kilometers long, as a passing wave alternately stretches and compresses spacetime. Since 2015, LIGO, along with the Virgo and KAGRA detectors, has recorded numerous additional events, including the first observed neutron star merger (GW170817) in 2017, which was also observed across the electromagnetic spectrum, allowing cross-checks between gravitational and conventional astronomy.((B. P. Abbott et al., "GW170817: Observation of Gravitational Waves from a Binary Neutron Star Inspiral," *Physical Review Letters* 119, no. 16 (2017): 161101.)) Pulsar timing arrays, such as NANOGrav, have separately reported evidence consistent with a background of much lower-frequency gravitational waves, potentially originating from supermassive black hole binaries.

===== Consensus Status =====

There is broad consensus among physicists and astronomers, arrived at independently across multiple detector collaborations (LIGO, Virgo, KAGRA), national funding bodies, and theoretical frameworks, that gravitational waves exist as predicted by general relativity and have been directly detected. See [[gravitational-waves-consensus-direct-detection-consensus]].

===== Viewpoints =====

  * **Mainstream general-relativistic view** - Gravitational waves are real, physical perturbations of spacetime, fully consistent with and predicted by general relativity; current detections represent confirmation of a century-old theoretical prediction. See [[gravitational-waves-mainstream-relativistic-viewpoint]].
  * **Alternative gravity skepticism** - Proponents of certain modified gravity theories (e.g., some variants of MOND or other alternatives to general relativity) have offered competing accounts of binary-merger signals or argue that gravitational-wave observations do not uniquely confirm general relativity over all rivals. See [[gravitational-waves-modified-gravity-skeptical-viewpoint]].
  * **Instrumentalist/data-skeptical view** - A minority of commentators, including some credentialed physicists, have raised methodological concerns about signal extraction, noise modeling, or statistical handling in LIGO-type analyses, without necessarily rejecting general relativity itself. See [[gravitational-waves-instrumentalist-skeptical-viewpoint]].
  * **Historical relativist-skeptic view** - Reflecting an earlier era of the field, this now largely abandoned position held that gravitational waves might be mathematical artifacts rather than physically real, observable phenomena. See [[gravitational-waves-historical-coordinate-artifact-viewpoint]].

===== Controversies =====

  * The 2014 BICEP2 announcement of purported evidence for primordial gravitational waves from cosmic inflation was later retracted after follow-up analysis attributed the signal to galactic dust, becoming a widely cited case study in the risks of premature announcement in cosmology. See [[gravitational-waves-controversy-bicep2-controversy]].

===== Related Pages =====

  * [[gravitational-waves-history|History]]
  * [[gravitational-waves-relativistic-vs-modified-gravity-debate|Debate: Mainstream Relativistic vs. Modified Gravity Skeptical]]
  * [[gravitational-waves-consensus-direct-detection-consensus|Consensus: Direct Detection]]
  * [[gravitational-waves-controversy-bicep2-controversy|Controversy: BICEP2]]
  * [[black-holes|Main Topic: Black Holes]]
  * [[general-relativity-overview-layman|General Relativity Overview Layman]]
  * [[general-relativity-overview-technical|General Relativity Overview Technical]]

===== Footnotes =====

((Albert Einstein, "Näherungsweise Integration der Feldgleichungen der Gravitation," *Sitzungsberichte der Königlich Preußischen Akademie der Wissenschaften* (1916): 688-696.))

((Joseph H. Taylor and Joel M. Weisberg, "A New Test of General Relativity: Gravitational Radiation and the Binary Pulsar PSR 1913+16," *The Astrophysical Journal* 253 (1982): 908-920.))

((B. P. Abbott et al., "Observation of Gravitational Waves from a Binary Black Hole Merger," *Physical Review Letters* 116, no. 6 (2016): 061102.))

((B. P. Abbott et al., "GW170817: Observation of Gravitational Waves from a Binary Neutron Star Inspiral," *Physical Review Letters* 119, no. 16 (2017): 161101.))