Editing Gravitational lens (section)

==History==
[[File:1919 eclipse negative.jpg|right|thumb|150px|One of [[Sir Arthur Eddington|Eddington]]'s photographs of the 1919 [[solar eclipse]] experiment, presented in his 1920 paper announcing its success]]

[[Henry Cavendish]] in 1784 (in an unpublished manuscript) and [[Johann Georg von Soldner]] in 1801 (published in 1804) had pointed out that Newtonian gravity predicts that starlight will bend around a massive object<ref>{{Cite journal |author=Soldner, J. G. V. |date=1804 |title=On the deflection of a light ray from its rectilinear motion, by the attraction of a celestial body at which it nearly passes by |journal=Berliner Astronomisches Jahrbuch |pages =161–172|title-link=s:Translation:On the Deflection of a Light Ray from its Rectilinear Motion }}</ref> as had already been supposed by [[Isaac Newton]] in 1704 in his ''[[The Queries|Queries]]'' No.1 in his book ''[[Opticks]]''.<ref name=Opticks>{{cite book|last=Newton|first=Isaac|author-link=Isaac Newton |title=Opticks: or, a treatise of the reflexions, refractions, inflexions and colours of light. Also two treatises of the species and magnitude of curvilinear figures.|date=1998|publisher=Octavo|location=Palo Alto, Calif.|isbn=978-1-891788-04-8|edition=Octavo |others=Commentary by Nicholas Humez}} (''Opticks'' was originally published in 1704).</ref> The same value as Soldner's was calculated by Einstein in 1911 based on the [[equivalence principle]] alone.<ref name="Schneider, Peter; Ehlers, Jürgen; Falco, Emilio E. 1992"/>{{rp|3}} However, Einstein noted in 1915, in the process of completing general relativity, that his (and thus Soldner's) 1911-result is only half of the correct value. Einstein became the first to calculate the correct value for light bending.<ref>{{Cite journal |author=Will, C.M.|date=2006 |title=The Confrontation between General Relativity and Experiment |journal=Living Reviews in Relativity |volume =9 |issue=1 |page=39 |doi=10.12942/lrr-2006-3|doi-access=free |arxiv = gr-qc/0510072 |bibcode = 2006LRR.....9....3W |pmid=28179873 |pmc=5256066 }}</ref>

The first observation of light deflection was performed by noting the change in position of [[star]]s as they passed near the Sun on the [[celestial sphere]]. The observations were performed in 1919 by [[Sir Arthur Eddington|Arthur Eddington]], [[Frank Watson Dyson]], and their collaborators during the [[Solar eclipse of May 29, 1919|total solar eclipse on May 29]].<ref>{{cite journal| last = Dyson| first = F. W.| title = A determination of the deflection of light by the Sun's gravitational field, from observations made at the total eclipse of 29 May 1919| journal = Philosophical Transactions of the Royal Society | volume = 220A | issue = 571–581| pages = 291–333| date = 1920| author2 = Eddington, A. S. | author3 = Davidson C. | doi=10.1098/rsta.1920.0009 | bibcode=1920RSPTA.220..291D| url = https://zenodo.org/record/1432106| doi-access = free}}</ref> The [[solar eclipse]] allowed the stars near the Sun to be observed. Observations were made simultaneously in the cities of [[Sobral, Ceará]], Brazil and in [[São Tomé and Príncipe]] on the west coast of Africa.<ref>{{cite journal| last = Stanley| first = Matthew| title = 'An Expedition to Heal the Wounds of War': The 1919 Eclipse and Eddington as Quaker Adventurer| journal = Isis | volume = 94| issue = 1| pages = 57–89| date = 2003| doi = 10.1086/376099| pmid = 12725104| bibcode = 2003Isis...94...57S| s2cid = 25615643}}</ref> The observations demonstrated that the light from [[star]]s passing close to the [[Sun]] was slightly bent, so that stars appeared slightly out of position.<ref>{{cite journal|last=Dyson|first=F. W.|date=1 January 1920|title=A Determination of the Deflection of Light by the Sun's Gravitational Field, from Observations Made at the Total Eclipse of May 29, 1919|journal=Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences|volume=220|issue=571–581|pages=291–333|bibcode=1920RSPTA.220..291D|doi=10.1098/rsta.1920.0009|author2=Eddington, A. S.|author3=Davidson, C.|url=https://zenodo.org/record/1432106|doi-access=free}}</ref>

[[Image:Gravitational lens-full.jpg|thumb|left|Bending light around a massive object from a distant source. The orange arrows show the apparent position of the background source. The white arrows show the path of the light from the true position of the source.]]
[[Image:Einstein cross.jpg|thumb|In the formation known as [[Einstein's Cross]], four images of the same distant [[quasar]] appear around a foreground galaxy due to strong gravitational lensing.]]
The result was considered spectacular news and made the front page of most major newspapers. It made Einstein and his theory of general relativity world-famous. When asked by his assistant what his reaction would have been if general relativity had not been confirmed by Eddington and Dyson in 1919, Einstein said "Then I would feel sorry for the dear Lord. The theory is correct anyway."<ref>Rosenthal-Schneider, Ilse: Reality and Scientific Truth. Detroit: Wayne State University Press, 1980. p 74. (See also Calaprice, Alice: ''The New Quotable Einstein''. Princeton: Princeton University Press, 2005. p 227.)</ref> In 1912, Einstein had speculated that an observer could see multiple images of a single light source, if the light were deflected around a mass.  This effect would make the mass act as a kind of gravitational lens. However, as he only considered the effect of deflection around a single star, he seemed to conclude that the phenomenon was unlikely to be observed for the foreseeable future since the necessary alignments between stars and observer would be highly improbable. Several other physicists speculated about gravitational lensing as well, but all reached the same conclusion that it would be nearly impossible to observe.<ref name=":0">{{Cite web|url=http://www.einstein-online.info/spotlights/grav_lensing_history|title=A brief history of gravitational lensing|website=[[Einstein Online]]|publisher=[[Max Planck Institute for Gravitational Physics]]|access-date=2016-06-29|archive-url=https://web.archive.org/web/20160701154224/http://www.einstein-online.info/spotlights/grav_lensing_history|archive-date=2016-07-01|url-status=dead}}</ref>

Although Einstein made unpublished calculations on the subject,<ref name="origin of the idea"/> the first discussion of the gravitational lens in print was by Khvolson, in a short article discussing the "halo effect" of gravitation when the source, lens, and observer are in near-perfect alignment,<ref name="Early History of Gravitational Lensing" /> now referred to as the [[Einstein ring]].

In 1936, after some urging by Rudi W. Mandl, Einstein reluctantly published the short article "Lens-Like Action of a Star By the Deviation of Light In the Gravitational Field" in the journal ''Science''.<ref name=":0" />

In 1937, [[Fritz Zwicky]] first considered the case where the newly discovered [[galaxy|galaxies]] (which were called 'nebulae' at the time) could act as both source and lens, and that, because of the mass and sizes involved, the effect was much more likely to be observed.<ref>{{cite journal|title=Nebulae as Gravitational lenses|author=F. Zwicky|journal=[[Physical Review]]|year=1937|issue=4|volume=51|page=290|doi=10.1103/PhysRev.51.290|url=http://authors.library.caltech.edu/5611/1/ZWIpr37a.pdf |archive-url=https://web.archive.org/web/20131226004748/http://authors.library.caltech.edu/5611/1/ZWIpr37a.pdf |archive-date=2013-12-26 |url-status=live|bibcode = 1937PhRv...51..290Z }}</ref>

In 1963 Yu. G. Klimov, S. Liebes, and [[Sjur Refsdal]] recognized independently that quasars are an ideal light source for the gravitational lens effect.<ref name="Schneider, Peter; Kochanek, Christopher; Wambsganss, Joachim">{{cite book|title=Gravitational Lensing: Strong, Weak and Micro|pages=4|author1=Schneider Peter|author2=Kochanek, Christopher|author3=Wambsganss, Joachim|publisher=Springer Verlag Berlin Heidelberg New York Press|date=2006|isbn=978-3-540-30309-1}}</ref>

It was not until 1979 that the first gravitational lens would be discovered. It became known as the "[[Twin QSO]]" since it initially looked like two identical quasistellar objects. (It is officially named [[SBS 0957+561]].) This gravitational lens was discovered by [[Dennis Walsh]], Bob Carswell, and [[Ray Weymann]] using the [[Kitt Peak National Observatory]] 2.1 meter [[telescope]].<ref>{{cite journal|last=Walsh|first=D.|author2=Carswell, R. F. |author3=Weymann, R. J. |title=0957 + 561 A, B: twin quasistellar objects or gravitational lens?|journal=Nature|date=31 May 1979|volume=279|issue=5712|pages=381–384|doi=10.1038/279381a0|bibcode = 1979Natur.279..381W|pmid=16068158|s2cid=2142707}}</ref>

In the 1980s, astronomers realized that the combination of CCD imagers and computers would allow the brightness of millions of stars to be measured each night. In a dense field, such as the galactic center or the Magellanic clouds, many microlensing events per year could potentially be found. This led to efforts such as [[Optical Gravitational Lensing Experiment]], or OGLE, that have characterized hundreds of such events, including those of [[OGLE-2016-BLG-1190Lb]] and [[OGLE-2016-BLG-1195Lb]].