Editing Positron (section)

=== 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}}