Editing Positron (section)

== History ==

=== Theory ===

In 1928, [[Paul Dirac]] published a paper proposing that electrons can have both a positive and negative charge.<ref name="QuantumElectron">
{{cite journal
 |last=Dirac |first=P. A. M.
 |authorlink=Paul Dirac
 |year=1928
 |title=The quantum theory of the electron
 |journal=[[Proceedings of the Royal Society A]]
 |volume=117 |issue=778 |pages=610–624
 |bibcode=1928RSPSA.117..610D
 |doi=10.1098/rspa.1928.0023
 |doi-access=free
}}</ref> This paper introduced the [[Dirac equation]], a unification of quantum mechanics, [[special relativity]], and the then-new concept of electron [[Spin (physics)|spin]] to explain the [[Zeeman effect]]. The paper did not explicitly predict a new particle but did allow for electrons having either positive or negative energy [[Dirac spinor|as solutions]]. [[Hermann Weyl]] then published a paper discussing the mathematical implications of the negative energy solution.<ref>
{{cite journal
 |last=Weyl |first=H.
 |year=1929
 |title=Gravitation and the Electron
 |journal=PNAS
 |volume=15 |issue=4 |pages=323–334
 |bibcode=1929PNAS...15..323W
 |doi=10.1073/pnas.15.4.323
 |pmid=16587474
 |pmc=522457
 |doi-access=free
}}</ref> The positive-energy solution explained experimental results, but Dirac was puzzled by the equally valid negative-energy solution that the mathematical model allowed. Quantum mechanics did not allow the negative energy solution to simply be ignored, as classical mechanics often did in such equations; the dual solution implied the possibility of an electron spontaneously jumping between positive and negative energy states. However, no such transition had yet been observed experimentally.<ref name="QuantumElectron"/>

Dirac wrote a follow-up paper in December 1929<ref name="ElectronProton">
{{cite journal
 |last=Dirac |first=P. A. M.
 |authorlink=Paul Dirac
 |year=1930
 |title=A theory of electrons and protons
 |journal=[[Proceedings of the Royal Society A]]
 |volume=126 |issue=801 |pages=360–365
 |bibcode=1930RSPSA.126..360D
 |doi=10.1098/rspa.1930.0013
 |doi-access=free
}}</ref> that attempted to explain the unavoidable negative-energy solution for the relativistic electron. He argued that "... an electron with negative energy moves in an external [electromagnetic] field as though it carries a positive charge." He further asserted that all of space could be regarded as a [[Dirac sea|"sea" of negative energy states]] that were filled, so as to prevent electrons jumping between positive energy states (negative electric charge) and negative energy states (positive charge). The paper also explored the possibility of the [[proton]] being an island in this sea, and that it might actually be a negative-energy electron. Dirac acknowledged that the proton having a much greater mass than the electron was a problem, but expressed "hope" that a future theory would resolve the issue.<ref name="ElectronProton"/>

[[Robert Oppenheimer]] argued strongly against the proton being the negative-energy electron solution to Dirac's equation. He asserted that if it were, the hydrogen atom would rapidly self-destruct.<ref>{{Cite journal
 |last=Oppenheimer |first=J. R.
 |author-link=J. Robert Oppenheimer
 |date=March 1930
 |title=Note on the Theory of the Interaction of Field and Matter
 |journal=[[Physical Review]]
 |volume=35 |issue=5 |pages=461–477
 |doi=10.1103/PhysRev.35.461 |bibcode=1930PhRv...35..461O
 |issn=0031-899X
}}</ref> Weyl in 1931 showed that the negative-energy electron must have the same mass as that of the positive-energy electron.<ref>{{Cite journal
 |last=Weyl |first=H.
 |author-link=Hermann Weyl
 |date=November 1927
 |title=Quantenmechanik und Gruppentheorie
 |url=https://www.thphys.uni-heidelberg.de/~wolschin/qms1920_7s.pdf
 |journal=[[Zeitschrift für Physik]]
 |language=de |volume=46 |issue=1–2 |pages=1–46
 |doi=10.1007/BF02055756 |bibcode=1927ZPhy...46....1W
 |issn=1434-6001
}}</ref> Persuaded by Oppenheimer's and Weyl's argument, Dirac published a paper in 1931 that predicted the existence of an as-yet-unobserved particle that he called an "anti-electron" that would have the same mass and the opposite charge as an electron and that would mutually annihilate upon contact with an electron.<ref>
{{cite journal
 |last=Dirac |first=P. A. M.
 |authorlink=Paul Dirac
 |year=1931
 |title=Quantised Singularities in the Quantum Field
 |journal=[[Proceedings of the Royal Society A]]
 |volume=133 |issue=821 |pages=60–72
 |bibcode=1931RSPSA.133...60D
 |doi=10.1098/rspa.1931.0130
 |doi-access=free
}}</ref>

[[Ernst Stueckelberg]], and later [[Richard Feynman]], proposed an interpretation of the positron as an electron moving backward in time,<ref>{{cite journal
 |last=Feynman
 |first=R.
 |year=1949
 |title=The theory of positrons
 |journal=[[Physical Review]]
 |volume=76
 |issue=6
 |pages=749–759
 |bibcode=1949PhRv...76..749F
 |doi=10.1103/PhysRev.76.749
 |s2cid=120117564
 |url=https://authors.library.caltech.edu/3520/
 |access-date=28 December 2021
 |archive-date=9 August 2022
 |archive-url=https://web.archive.org/web/20220809030941/https://authors.library.caltech.edu/3520/
 |url-status=dead
 |url-access=subscription
 }}</ref> reinterpreting the negative-energy solutions of the Dirac equation. Electrons moving backward in time would have a positive [[electric charge]]. [[John Archibald Wheeler]] invoked this concept to explain the identical properties shared by all electrons, suggesting that [[One-electron universe|"they are all the same electron"]] with a complex, self-intersecting [[worldline]].<ref>{{cite speech |title=The Development of the Space-Time View of Quantum Electrodynamics |last=Feynman |first=R. |date=11 December 1965 |location=Nobel Lecture |url=http://nobelprize.org/nobel_prizes/physics/laureates/1965/feynman-lecture.html |access-date=2 January 2007}}</ref> [[Yoichiro Nambu]] later applied it to all production and [[annihilation]] of particle-antiparticle pairs, stating that "the eventual creation and annihilation of pairs that may occur now and then is no creation or annihilation, but only a change of direction of moving particles, from the past to the future, or from the future to the past."<ref>
{{cite journal
 |last=Nambu |first=Y.
 |year=1950
 |title=The Use of the Proper Time in Quantum Electrodynamics I
 |journal=[[Progress of Theoretical Physics]]
 |volume=5 |issue=1 |pages=82–94
 |bibcode=1950PThPh...5...82N
 |doi=10.1143/PTP/5.1.82
 |doi-access=free
}}</ref> The backwards in time point of view is nowadays accepted as completely equivalent to other pictures, but it does not have anything to do with the macroscopic terms "cause" and "effect", which do not appear in a microscopic physical description.{{citation needed|date=July 2020}}

=== Experimental clues and discovery ===
[[File:Cloud chambers played an important role of particle detectors.jpg|thumb|Wilson [[cloud chamber]]s used to be very important [[particle detector]]s in the early days of [[particle physics]]. They were used in the discovery of the positron, [[muon]], and [[kaon]].]]
{{Antimatter}}
Several sources have claimed that [[Dmitri Skobeltsyn]] first observed the positron long before 1930,<ref>
{{cite book
 |last=Wilson |first=David
 |year=1983
 |title=Rutherford, Simple Genius
 |pages=562–563
 |publisher=Hodder and Stoughton
 |isbn=0-340-23805-4
}}</ref> or even as early as 1923.<ref>
{{cite book
 |last=Close |first=F. |author-link=Frank Close
 |year=2009
 |title=Antimatter
 |pages=50–52
 |publisher=[[Oxford University Press]]
 |isbn=978-0-19-955016-6
}}</ref> They state that while using a Wilson [[cloud chamber]]<ref>
{{cite journal
 |last=Cowan |first=E.
 |date=1982
 |title=The Picture That Was Not Reversed
 |journal=[[Engineering & Science Education Journal|Engineering & Science]]
 |volume=46 |issue=2 |pages=6–28
 |url=http://calteches.library.caltech.edu/3360/
}}</ref> in order to study the [[Compton effect]], Skobeltsyn detected particles that acted like electrons but curved in the opposite direction in an applied magnetic field, and that he presented photographs with this phenomenon in a conference in the [[University of Cambridge]], on 23–27 July 1928. In his book<ref>
{{cite book
 |last=Hanson |first=Norwood Russel
 |year=1963
 |title=The Concept of the Positron
 |pages=136–139
 |publisher=[[Cambridge University Press]]
 |isbn=978-0-521-05198-9
}}</ref> on the history of the positron discovery from 1963, [[Norwood Russell Hanson]] has given a detailed account of the reasons for this assertion, and this may have been the origin of the myth. But he also presented Skobeltsyn's objection to it in an appendix.<ref>
{{cite book
 |last=Hanson |first=Norwood Russel
 |year=1963
 |title=The Concept of the Positron
 |pages=179–183
 |publisher=[[Cambridge University Press]]
 |isbn=978-0-521-05198-9
}}</ref> Later, Skobeltsyn rejected this claim even more strongly, calling it "nothing but sheer nonsense".<ref>
{{cite book
 |last1=Brown |first1=Laurie M.
 |last2=Hoddeson |first2=Lillian
 |year=1983
 |title=The Birth of Particle Physics
 |pages=118–119
 |publisher=[[Cambridge University Press]]
 |isbn=0-521-24005-0
}}</ref>

Skobeltsyn did pave the way for the eventual discovery of the positron by two important contributions: adding a magnetic field to his cloud chamber (in 1925<ref>
{{cite journal
 |last=Bazilevskaya |first=G.A.
 |year=2014
 |title=Skobeltsyn and the early years of cosmic particle physics in the Soviet Union
 |pages=61–66
 |journal=Astroparticle Physics
 |volume=53
 |doi=10.1016/j.astropartphys.2013.05.007
 |bibcode=2014APh....53...61B
}}</ref>), and by discovering charged particle [[cosmic ray]]s,<ref>
{{cite journal
 |last=Skobeltsyn |first=D.
 |year=1929
 |title=Uber eine neue Art sehr schneller beta-Strahlen
 |pages=686–702
 |journal=Z. Phys.
 |volume=54
 |issue=9–10
 |doi=10.1007/BF01341600
 |bibcode=1929ZPhy...54..686S
 |s2cid=121748135
}}</ref> for which he is credited in [[Carl David Anderson]]'s [[Nobel Prize|Nobel lecture]].<ref>
{{cite web
 |last=Anderson |first=Carl D.
 |year=1936
 |title=The Production and Properties of Positrons
 |url=https://www.nobelprize.org/prizes/physics/1936/anderson/lecture/
 |access-date=10 August 2020
}}</ref> Skobeltsyn did observe likely positron tracks on images taken in 1931,<ref>
{{cite journal
 |last=Skobeltzyn |first=D.
 |year=1934
 |title=Positive electron tracks
 |pages=23–24
 |journal=Nature
 |volume=133
 |issue=3349
 |doi=10.1038/133023a0
 |bibcode=1934Natur.133...23S
 |s2cid=4226799
}}</ref> but did not identify them as such at the time.

Likewise, in 1929 [[Chung-Yao Chao]], a Chinese graduate student at [[Caltech]], noticed some anomalous results that indicated particles behaving like electrons, but with a positive charge, though the results were inconclusive and the phenomenon was not pursued.<ref name="MehraRechenberg">
{{cite book
 |last1=Merhra |first1=J. |author-link1=Jagdish Mehra
 |last2=Rechenberg |first2=H. |author-link2=Helmut Rechenberg
 |year=2000
 |title=The Historical Development of Quantum Theory, Volume 6: The Completion of Quantum Mechanics 1926–1941
 |url=https://books.google.com/books?id=9l61Dy9FBfYC&q=Chung-Yao%20Chao%20positron&pg=PA804
 |page=804
 |publisher=Springer
 |isbn=978-0-387-95175-1
}}</ref> Fifty years later, Anderson acknowledged that his discovery was inspired by the work of his Caltech classmate [[Chung-Yao Chao]], whose research formed the foundation from which much of Anderson's work developed but was not credited at the time.<ref name="Chinese">{{Cite journal|last=Cao|first=Cong|date=2004|title=Chinese Science and the 'Nobel Prize Complex'|url=http://china-us.uoregon.edu/pdf/Minerva-2004.pdf|journal=Minerva|language=en|volume=42|issue=2|page=154|doi=10.1023/b:mine.0000030020.28625.7e|s2cid=144522961|issn=0026-4695}}</ref>

Anderson discovered the positron on 2 August 1932,<ref>
{{cite journal
 |last=Anderson |first=C. D.
 |date=1933
 |title=The Positive Electron
 |journal=[[Physical Review]]
 |volume=43 |issue=6 |pages=491–494
 |bibcode=1933PhRv...43..491A
 |doi=10.1103/PhysRev.43.491
 |doi-access=free
}}</ref> for which he won the [[Nobel Prize in Physics|Nobel Prize for Physics]] in 1936.<ref name="nobel">
{{cite web
 |title=The Nobel Prize in Physics 1936
 |url=http://nobelprize.org/nobel_prizes/physics/laureates/1936/index.html
 |access-date=21 January 2010
}}</ref> Anderson did not coin the term ''positron'', but allowed it at the suggestion of the ''[[Physical Review]]'' journal editor to whom he submitted his discovery paper in late 1932. The positron was the first evidence of [[antimatter]] and was discovered when Anderson allowed cosmic rays to pass through a cloud chamber and a lead plate. A magnet surrounded this apparatus, causing particles to bend in different directions based on their electric charge. The ion trail left by each positron appeared on the photographic plate with a curvature matching the [[mass-to-charge ratio]] of an electron, but in a direction that showed its charge was positive.<ref name="Penny_Gilmer_6-19-11">
{{cite web
 |last=Gilmer
 |first=P. J.
 |date=19 July 2011
 |title=Irène Jolit-Curie, a Nobel laureate in artificial radioactivity
 |url=http://www.chem.fsu.edu/~gilmer/PDFs/Ch%202_Irene_Curie_Penny_Gilmer_6-19-11_pg_mh.pdf
 |page=8
 |access-date=13 July 2013
 |archive-url=https://web.archive.org/web/20140519131211/http://www.chem.fsu.edu/~gilmer/PDFs/Ch%202_Irene_Curie_Penny_Gilmer_6-19-11_pg_mh.pdf
 |archive-date=19 May 2014
 |url-status=dead
}}</ref>

Anderson wrote in retrospect that the positron could have been discovered earlier based on Chung-Yao Chao's work, if only it had been followed up on.<ref name="MehraRechenberg"/> [[Frédéric Joliot-Curie|Frédéric]] and [[Irène Joliot-Curie]] in Paris had evidence of positrons in old photographs when Anderson's results came out, but they had dismissed them as protons.<ref name="Penny_Gilmer_6-19-11"/>

The positron had also been contemporaneously discovered by [[Patrick Blackett]] and [[Giuseppe Occhialini]] at the Cavendish Laboratory in 1932. Blackett and Occhialini had delayed publication to obtain more solid evidence, so Anderson was able to publish the discovery first.<ref name="AM">
{{cite web
 |date=2011–2014
 |title=Atop the Physics Wave: Rutherford Back in Cambridge, 1919–1937
 |url=http://www.aip.org/history/exhibits/rutherford/sections/atop-physics-wave.html
 |website=Rutherford's Nuclear World
 |publisher=[[American Institute of Physics]]
 |access-date=19 August 2014
 |archive-date=21 October 2014
 |archive-url=https://web.archive.org/web/20141021094704/http://www.aip.org/history/exhibits/rutherford/sections/atop-physics-wave.html
 |url-status=dead
}}</ref>