Editing Gravitational wave (section)

==History==
[[File:History of the Universe.svg|thumb|Primordial gravitational waves are hypothesized to arise from [[Inflation (cosmology)|cosmic inflation]], a phase of [[accelerated expansion]] just after the [[Big Bang]] (2014).<ref name="BICEP2-2014" /><ref name="NASA-20140317" /><ref name="NYT-20140317">{{cite news |last=Overbye |first=Dennis |author-link=Dennis Overbye |title=Detection of Waves in Space Buttresses Landmark Theory of Big Bang |url=https://www.nytimes.com/2014/03/18/science/space/detection-of-waves-in-space-buttresses-landmark-theory-of-big-bang.html |date=17 March 2014 |work=[[New York Times]] |access-date=17 March 2014}}</ref>]]

The possibility of gravitational waves and that those might travel at the speed of light was discussed in 1893 by [[Oliver Heaviside]], using the analogy between the inverse-square law of gravitation and the [[Coulomb's law|electrostatic force]].<ref>{{Cite book |last=Heaviside |first=Oliver |url=https://archive.org/details/electromagnetict01heavrich/electromagnetict01heavrich |title=Electromagnetic theory Vol 1 |date=1894 |publisher=The Electrician printing and publishing company, limited |pages=455–66 Appendix B}}</ref> In 1905, [[Henri Poincaré]] proposed gravitational waves, emanating from a body and propagating at the speed of light, as being required by the Lorentz transformations<ref>[http://www.academie-sciences.fr/pdf/dossiers/Poincare/Poincare_pdf/Poincare_CR1905.pdf (PDF)] Membres de l'Académie des sciences depuis sa création : Henri Poincare. Sur la dynamique de l' electron. Note de H. Poincaré. C.R. T.140 (1905) 1504–1508.</ref> and suggested that, in analogy to an accelerating [[electrical charge]] producing [[electromagnetic wave]]s, accelerated masses in a relativistic field theory of gravity should produce gravitational waves.<ref>{{cite web|url=http://www.academie-sciences.fr/pdf/dossiers/Poincare/Poincare_pdf/Poincare_CR1905.pdf|title=page 1507}}</ref><ref name="CGShistory">
{{cite journal
|first1=J.L. |last1=Cervantes-Cota
|first2=S. |last2=Galindo-Uribarri
|first3=G.F. |last3=Smoot
|title=A Brief History of Gravitational Waves
|journal=Universe
|volume=2
|number=3
|year=2016
|page=22
|doi=10.3390/universe2030022|bibcode=2016Univ....2...22C
|arxiv=1609.09400 |s2cid=2187981
|doi-access=free
}}</ref>

In 1915 Einstein published his [[General relativity|general theory of relativity]], a complete relativistic theory of gravitation. He conjectured, like Poincaré, that the equation would produce gravitational waves, but, as he mentions in a letter to Schwarzschild in February 1916,<ref name="CGShistory"/> these could not be similar to electromagnetic waves. Electromagnetic waves can be produced by dipole motion, requiring both a positive and a negative charge. Gravitation has no equivalent to negative charge. Einstein continued to work through the complexity of the equations of general relativity to find an alternative wave model. The result was published in June 1916,<ref name="Gravitationswellen" /> and there he came to the conclusion that the gravitational wave must propagate with the speed of light, and there must, in fact, be three types of gravitational waves dubbed longitudinal–longitudinal, transverse–longitudinal, and transverse–transverse by [[Hermann Weyl]].<ref name="CGShistory"/>

However, the nature of Einstein's approximations led many (including Einstein himself) to doubt the result. In 1922, [[Arthur Eddington]] showed that two of Einstein's types of waves were artifacts of the coordinate system he used, and could be made to propagate at any speed by choosing appropriate coordinates, leading Eddington to jest that they "propagate at the speed of thought".<ref name="Kennefick2016">{{Cite book |last=Kennefick |first=Daniel |url=https://books.google.com/books?id=T0uUCwAAQBAJ |title=Traveling at the Speed of Thought: Einstein and the Quest for Gravitational Waves |year=2016 |publisher=Princeton University Press |isbn=978-1-4008-8274-8 |language=en}}</ref>{{rp|72}} This also cast doubt on the physicality of the third (transverse–transverse) type that Eddington showed always propagate at the [[speed of light]] regardless of coordinate system. In 1936, Einstein and [[Nathan Rosen]] submitted a paper to ''[[Physical Review]]'' in which they claimed gravitational waves could not exist in the full general theory of relativity because any such solution of the field equations would have a singularity. The journal sent their manuscript to be reviewed by [[Howard P. Robertson]], who anonymously reported that the singularities in question were simply the harmless coordinate singularities of the employed cylindrical coordinates. Einstein, who was unfamiliar with the concept of peer review, angrily withdrew the manuscript, never to publish in ''Physical Review'' again. Nonetheless, his assistant [[Leopold Infeld]], who had been in contact with Robertson, convinced Einstein that the criticism was correct, and the paper was rewritten with the opposite conclusion and published elsewhere.<ref name="CGShistory"/><ref name="Kennefick2016" />{{rp|79ff}} In 1956, [[Felix Pirani]] remedied the confusion caused by the use of various coordinate systems by rephrasing the gravitational waves in terms of the manifestly observable [[Riemann curvature tensor]].<ref>{{Cite journal |last=F.A.E. |first=Pirani |year=1956 |title=On the physical significance of the Riemann tensor |journal=Acta Physica Polonica |volume=15 |pages=389–405 |bibcode=1956AcPP...15..389P}}</ref>

At the time, Pirani's work was overshadowed by the community's focus on a different question: whether gravitational waves could transmit [[energy]]. This matter was settled by a thought experiment proposed by [[Richard Feynman]] during the first "GR" conference at [[University of North Carolina at Chapel Hill|Chapel Hill]] in 1957. In short, his argument known as the "[[sticky bead argument]]" notes that if one takes a rod with beads then the effect of a passing gravitational wave would be to move the beads along the rod; friction would then produce heat, implying that the passing wave had done [[work (physics)|work]]. Shortly after, [[Hermann Bondi]] published a detailed version of the "sticky bead argument".<ref name="CGShistory"/> This later led to a series of articles (1959 to 1989) by Bondi and Pirani that established the existence of plane wave solutions for gravitational waves.<ref>{{Cite journal |last=Robinson |first=D.C. |date=2019 |title=Gravitation and general relativity at King's College London |journal=The European Physical Journal H |language=en |volume=44 |issue=3 |pages=181–270 |arxiv=1811.07303 |doi=10.1140/epjh/e2019-100020-1 |bibcode=2019EPJH...44..181R |issn=2102-6459}}</ref>

[[Paul Dirac]] further postulated the existence of gravitational waves, declaring them to have "physical significance" in his 1959 lecture at the [[Lindau Nobel Laureate Meetings|Lindau Meetings]].<ref>{{Cite web |last=Skuse |first=Ben |date=2022-09-01 |title=Black Holes – Topic |url=https://mediatheque.lindau-nobel.org/topics/black-holes |access-date=2023-11-02 |website=Lindau Nobel Mediatheque |language=en}}</ref> Further, it was Dirac who predicted gravitational waves with a well-defined energy density in 1964.<ref>{{Cite journal |last=Debnath |first=Lokenath |author-link=Lokenath Debnath |date=2013 |title=A short biography of Paul A.M. Dirac and historical development of Dirac delta function |url=http://www.tandfonline.com/doi/abs/10.1080/0020739X.2013.770091 |journal=International Journal of Mathematical Education in Science and Technology |language=en |volume=44 |issue=8 |pages=1201–23 |doi=10.1080/0020739X.2013.770091 |bibcode=2013IJMES..44.1201D |s2cid=121423215 |issn=0020-739X}}</ref>

After the Chapel Hill conference, [[Joseph Weber]] started designing and building the first gravitational wave detectors now known as [[Weber bar]]s. In 1969, Weber claimed to have detected the first gravitational waves, and by 1970 he was "detecting" signals regularly from the [[Galactic Center]]; however, the frequency of detection soon raised doubts on the validity of his observations as the implied rate of energy loss of the [[Milky Way]] would drain our galaxy of energy on a timescale much shorter than its inferred age. These doubts were strengthened when, by the mid-1970s, repeated experiments from other groups building their own Weber bars across the globe failed to find any signals, and by the late 1970s consensus was that Weber's results were spurious.<ref name="CGShistory"/>

In the same period, the first indirect evidence of gravitational waves was discovered. In 1974, [[Russell Alan Hulse]] and [[Joseph Hooton Taylor, Jr.]] discovered the [[Hulse–Taylor binary|first binary pulsar]], which earned them the 1993 [[Nobel Prize in Physics]].<ref>Nobel Prize Award (1993) [https://www.nobelprize.org/prizes/physics/1993/press-release/ Press Release] The Royal Swedish Academy of Sciences.</ref> Pulsar timing observations over the next decade showed a gradual decay of the orbital period of the Hulse–Taylor pulsar that matched the loss of energy and angular momentum in gravitational radiation predicted by general relativity.<ref name="auto">{{Cite journal |last1=Taylor |first1=J. H. |last2=Weisberg |first2=J.M. |last3=McCulloch, P.M. |date=1982 |title=A new test of general relativity – Gravitational radiation and the binary pulsar PSR 1913+16 |journal=[[The Astrophysical Journal]] |language=en |volume=253 |page=908 |bibcode=1982ApJ...253..908T |doi=10.1086/159690 |issn=0004-637X}}</ref><ref>{{Cite journal |last1=Taylor |first1=J. H. |last2=Fowler |first2=L.A. |last3=McCulloch |first3=P.M. |date=1979 |title=Measurements of general relativistic effects in the binary pulsar PSR1913 + 16 |journal=[[Nature (journal)|Nature]] |language=en |volume=277 |issue=5696 |pages=437–440 |bibcode=1979Natur.277..437T |doi=10.1038/277437a0 |issn=0028-0836 |s2cid=22984747}}</ref><ref name="CGShistory"/>

This indirect detection of gravitational waves motivated further searches, despite Weber's discredited result. Some groups continued to improve Weber's original concept, while others pursued the detection of gravitational waves using laser interferometers. The idea of using a laser interferometer for this seems to have been floated independently by various people, including M.E. Gertsenshtein and V. I. Pustovoit in 1962,<ref>{{cite journal |last1=Gertsenshtein |first1=M.E. |last2=Pustovoit |first2=V.I. |year=1962 |title=On the detection of low frequency gravitational waves |journal=JETP |volume=43 |pages=605–07 }}</ref> and Vladimir B. Braginskiĭ in 1966. The first prototypes were developed in the 1970s by [[Robert L. Forward]] and Rainer Weiss.<ref>{{Cite web |last=Cho |first=Adrian |date=3 October 2017 |title=Ripples in space: U.S. trio wins physics Nobel for discovery of gravitational waves |url=https://www.science.org/content/article/ripples-space-us-trio-wins-physics-nobel-discovery-gravitational-waves |access-date=20 May 2019 |website=[[Space.com]]}}</ref><ref>{{Cite journal |last1=Cervantes-Cota |first1=Jorge |last2=Galindo-Uribarri |first2=Salvador |last3=Smoot |first3=George |date=2016-09-13 |title=A Brief History of Gravitational Waves |journal=Universe |language=en |volume=2 |issue=3 |page=22 |arxiv=1609.09400 |doi=10.3390/universe2030022 |doi-access=free |bibcode=2016Univ....2...22C |issn=2218-1997}}</ref> In the decades that followed, ever more sensitive instruments were constructed, culminating in the construction of [[GEO600]], [[LIGO]], and [[Virgo interferometer|Virgo]].<ref name="CGShistory"/>

After years of producing null results, improved detectors became operational in 2015. On 11 February 2016, the [[LIGO Scientific Collaboration|LIGO-Virgo]] collaborations announced the [[first observation of gravitational waves]],<ref name="BBC_11Feb16">{{cite news|date=11 February 2016|title=Gravitational waves from black holes detected|work=BBC News|url=https://www.bbc.co.uk/news/science-environment-35524440}}</ref><ref name="Abbot">{{Cite journal |last=Abbott |first=B.P. |display-authors=etal |year=2016 |title=Observation of Gravitational Waves from a Binary Black Hole Merger |journal=[[Physical Review Letters]] |language=en |volume=116 |issue=6 |page=061102 |arxiv=1602.03837 |bibcode=2016PhRvL.116f1102A |doi=10.1103/PhysRevLett.116.061102 |issn=0031-9007 |pmid=26918975 |s2cid=124959784 |collaboration=LIGO Scientific Collaboration and Virgo Collaboration}}</ref><ref name="NSF">{{Cite web |date=2016-02-11 |title=Gravitational waves detected 100 years after Einstein's prediction |url=https://www.nsf.gov/news/news_summ.jsp?cntn_id=137628 |access-date=2016-02-11 |website=[[US National Science Foundation]]}}</ref><ref name="Discovery 2016">{{cite journal|last1=Castelvecchi|first1=Davide|last2=Witze|first2=Witze|date=11 February 2016|title=Einstein's gravitational waves found at last|url=http://www.nature.com/news/einstein-s-gravitational-waves-found-at-last-1.19361|journal=Nature News|doi=10.1038/nature.2016.19361|access-date=2016-02-11|s2cid=182916902}}</ref> from a signal (dubbed [[GW150914]]) detected at 09:50:45 GMT on 14 September 2015 of two black holes with masses of 29 and 36 [[solar mass]]es merging about 1.3&nbsp;billion light-years away. During the final fraction of a second of the merger, it released more than 50 times the [[Power (physics)|power]] of all the stars in the observable universe combined.<ref>{{cite web|title=This collision was 50 times more powerful than all the stars in the universe combined|url=http://www.techinsider.io/black-hole-collision-energy-50-times-universe-2016-2}}</ref> The signal increased in frequency from 35 to 250&nbsp;Hz over 10 cycles (5 orbits) as it rose in strength for a period of 0.2 second.<ref name="Abbot" /> The mass of the new merged black hole was 62 solar masses. Energy equivalent to three solar masses was emitted as gravitational waves.<ref name="Wired">{{cite magazine|last1=Scoles|first1=Sarah|date=2016-02-11|title=LIGO's First-Ever Detection of Gravitational Waves Opens a New Window on the Universe|url=https://www.wired.com/2016/02/scientists-spot-the-gravity-waves-that-flex-the-universe/|magazine=Wired}}</ref> The signal was seen by both LIGO detectors in Livingston and Hanford, with a time difference of 7 milliseconds due to the angle between the two detectors and the source. The signal came from the [[Southern Celestial Hemisphere]], in the rough direction of (but much farther away than) the [[Magellanic Clouds]].<ref name="Discovery 2016" /> The confidence level of this being an observation of gravitational waves was 99.99994%.<ref name="Wired" />

A year earlier, the BICEP2 collaboration claimed that they had detected the imprint of gravitational waves in the [[cosmic microwave background]]. However, they were later forced to retract this result.<ref name="BICEP2-2014">{{cite web |title=BICEP2 2014 Results Release |url=http://bicepkeck.org |date=17 March 2014 |website=[[National Science Foundation]] |access-date=18 March 2014}}</ref><ref name="NASA-20140317">{{cite web |last=Clavin |first=Whitney |title=NASA Technology Views Birth of the Universe |url=http://www.jpl.nasa.gov/news/news.php?release=2014-082 |date=17 March 2014 |website=[[NASA]] |access-date=17 March 2014}}</ref><ref>{{Cite web |last=Moskowitz |first=Clara |author-link=Clara Moskowitz |date=17 March 2014 |title=Gravity Waves from Big Bang Detected |url=https://www.scientificamerican.com/article/gravity-waves-cmb-b-mode-polarization/ |access-date=21 March 2016 |website=[[Scientific American]]}}</ref><ref>{{Cite news |last=Sample |first=Ian |date=2014-06-04 |title=Gravitational waves turn to dust after claims of flawed analysis |url=https://www.theguardian.com/science/2014/jun/04/gravitational-wave-discovery-dust-big-bang-inflation |work=[[The Guardian]]}}</ref>

In 2017, the [[Nobel Prize in Physics]] was awarded to [[Rainer Weiss]], [[Kip Thorne]] and [[Barry Barish]] for their role in the detection of gravitational waves.<ref name="BBC-20171003">{{cite news |last1=Rincon |first1=Paul |last2=Amos |first2=Jonathan |url=https://www.bbc.co.uk/news/science-environment-41476648|title=Einstein's waves win Nobel Prize |work=[[BBC News]] |date=3 October 2017 |access-date=3 October 2017}}</ref><ref name="NYT-20171003">{{cite news |last=Overbye |first=Dennis |author-link=Dennis Overbye |title=2017 Nobel Prize in Physics Awarded to LIGO Black Hole Researchers |url=https://www.nytimes.com/2017/10/03/science/nobel-prize-physics.html |date=3 October 2017 |work=[[The New York Times]] |access-date=3 October 2017 }}</ref><ref name="NYT-20171003dk">{{cite news |last=Kaiser |first=David |author-link=David Kaiser (physicist) |title=Learning from Gravitational Waves |url=https://www.nytimes.com/2017/10/03/opinion/gravitational-waves-ligo-funding.html |date=3 October 2017 |work=[[The New York Times]] |access-date=3 October 2017 }}</ref>

In 2023, NANOGrav, EPTA, PPTA, and IPTA announced that they found evidence of a universal [[gravitational wave background]].<ref name="SA-20230804">{{cite news |last=O'Callaghan |first=Jonathan |title=A Background 'Hum' Pervades the Universe. Scientists Are Racing to Find Its Source – Astronomers are now seeking to pinpoint the origins of an exciting new form of gravitational waves that was announced earlier this year |url=https://www.scientificamerican.com/article/a-background-hum-pervades-the-universe-scientists-are-racing-to-find-its-source/ |date=4 August 2023 |work=[[Scientific American]] |url-status=live |archive-url=https://archive.today/20230804144053/https://www.scientificamerican.com/article/a-background-hum-pervades-the-universe-scientists-are-racing-to-find-its-source/ |archive-date=4 August 2023 |access-date=4 August 2023 }}</ref> [[North American Nanohertz Observatory for Gravitational Waves]] states, that they were created over cosmological time scales by supermassive black holes, identifying the distinctive [[Hellings-Downs curve]] in 15 years of radio observations of 25 pulsars.<ref>{{cite web |url=https://www.jpl.nasa.gov/news/15-years-of-radio-data-reveals-evidence-of-spacetime-murmur |title=15 Years of Radio Data Reveals Evidence of Spacetime Murmur |publisher=NASA Jet Propulsion Laboratory |access-date=2023-06-30}}</ref>
Similar results are published by European Pulsar Timing Array, who claimed a [[Three-sigma rule|<math>3\sigma</math>-significance]]. They expect that a <math>5\sigma</math>-significance will be achieved by 2025 by combining the measurements of several collaborations.<ref>[https://cloud.mpifr-bonn.mpg.de/index.php/s/5BS4QnZaKWnn3Ti The second data release from the European Pulsar Timing Array III. Search for gravitational wave signals]</ref><ref>{{cite web |url=https://www.mpifr-bonn.mpg.de/7919388/news_publication_20524892_transferred |title=Ein neuer Zugang zum Universum }}</ref>