Editing Werner Heisenberg (section)

===Göttingen, Copenhagen and Leipzig===
From 1924 to 1927, Heisenberg was a [[Privatdozent]] at [[Göttingen]], meaning he was qualified to teach and examine independently, without having a chair. From 17 September 1924 to 1 May 1925, under an International Education Board [[Rockefeller Foundation]] fellowship, Heisenberg went to do research with [[Niels Bohr]], director of the Institute of Theoretical Physics at the [[University of Copenhagen]]. On June 7, after weeks of failing to alleviate a severe bout of [[hay fever]] with aspirin and cocaine,<ref>{{cite book |last1=Rechenberg |first1=Helmut |title=Werner Heisenberg – Die Sprache der Atome. Leben und Wirken |date=2010 |publisher=Springer |isbn=978-3-540-69221-8 |page=322}}</ref> Heisenberg retreated to the pollen-free [[North Sea]] island of [[Heligoland|Helgoland]] to focus on quantum mechanics.<ref>{{Cite web|last=Prescod-Weinstein|first=Chanda |author-link=Chanda Prescod-Weinstein|date=2021-07-07|title=No man is an island – the early days of the quantum revolution|url=https://physicsworld.com/no-man-is-an-island/ |access-date=2022-02-03|website=[[Physics World]]|language=en-GB}}</ref><ref name=crease>{{cite web |last1=Crease |first1=Robert P. |author-link=Robert P. Crease |title=Return to Helgoland: celebrating 100 years of quantum mechanics |url=https://physicsworld.com/a/return-to-helgoland-celebrating-100-years-of-quantum-mechanics/ |website=[[Physics World]] |date=1 December 2024 |archive-url=https://web.archive.org/web/20241201152618/https://physicsworld.com/a/return-to-helgoland-celebrating-100-years-of-quantum-mechanics/ |archive-date=2024-12-01 |url-status=dead }}</ref><!-- [[Helgoland (book)]] --> His seminal paper, "[[Über quantentheoretische Umdeutung kinematischer und mechanischer Beziehungen]]" ("Quantum theoretical re-interpretation of kinematic and mechanical relations") also called the ''Umdeutung'' (reinterpretation) paper, was published in September 1925.<ref>Kragh, H. (2004) "[[Dirac, Paul Adrien Maurice]] (1902–1984)", ''[[Oxford Dictionary of National Biography]]'', Oxford University Press. {{doi|10.1093/ref:odnb/31032}}</ref> He returned to Göttingen and, with [[Max Born]] and [[Pascual Jordan]] over a period of about six months, developed the [[matrix mechanics]] formulation of [[quantum mechanics]]. On 1 May 1926, Heisenberg began his appointment as a university lecturer and assistant to Bohr in Copenhagen. It was in Copenhagen, in 1927, that Heisenberg developed his [[uncertainty principle]], while working on the mathematical foundations of quantum mechanics. On 23 February, Heisenberg wrote a letter to fellow physicist [[Wolfgang Pauli]], in which he first described his new principle.<ref>{{Cite journal | url=http://www.aps.org/publications/apsnews/200802/physicshistory.cfm | title=February 1927: Heisenberg's Uncertainty Principle | publisher=American Physics Society | journal=APS News | date=February 2008 | volume=17 | issue=2 | access-date=23 February 2011 | archive-url=https://web.archive.org/web/20110130195156/http://aps.org/publications/apsnews/200802/physicshistory.cfm | archive-date=30 January 2011 | url-status=live }}</ref> In his paper on the principle,<ref>{{harvnb|Heisenberg|1927}}, cited in {{harvnb|Mott|Peierls|1977|p=243}}</ref> Heisenberg used the word "''Ungenauigkeit''" (imprecision), not uncertainty, to describe it.<ref name=Biography/><ref name=Cassidy>{{harvnb|Cassidy|1992|loc=Appendix A}}</ref><ref>{{harvnb|Mott|Peierls|1977|p=224}}</ref>

In 1927, Heisenberg was appointed ''ordentlicher Professor'' (professor ordinarius) of theoretical physics and head of the department of physics at the [[Leipzig University|University of Leipzig]]; he gave his inaugural lecture there on 1 February 1928. In his first paper published from Leipzig,<ref>{{harvnb|Heisenberg|1928}}, as cited in {{harvnb|Mott|Peierls|1977|p=243}}</ref> Heisenberg used the [[Pauli exclusion principle]] to solve the mystery of [[ferromagnetism]].<ref name=Biography/><ref name=Hentschel/><ref name=Cassidy/><ref>{{harvnb|Mott|Peierls|1977|pp=226–227}}</ref>

At 25 years old, Heisenberg gained the title of the youngest full-time professor in Germany and professorial chair<ref name="Valiunas-2019">{{Cite journal |last=Valiunas |first=Algis |date=2019 |title=The Most Dangerous Possible German |url=https://www.jstor.org/stable/26609101 |journal=The New Atlantis |issue=57 |pages=36–74 |jstor=26609101 |issn=1543-1215}}</ref> of the Institute for Theoretical Physics at the University of Leipzig. He gave lectures that were attended by physicists like [[Edward Teller]] and [[Robert Oppenheimer]],<ref name="Valiunas-2019" /> who would later work on the [[Manhattan Project]]<ref name="Groves-1962" /> for the United States. 

During Heisenberg's tenure at Leipzig, the high quality of the doctoral students and [[Postgraduate education|post-graduate]] and research associates who studied and worked with him is clear from the acclaim that many later earned. They included [[Erich Bagge]], [[Felix Bloch]], [[Ugo Fano]], [[Siegfried Flügge]], [[William Vermillion Houston]], [[Friedrich Hund]], [[Robert S. Mulliken]], [[Rudolf Peierls]], [[George Placzek]], [[Isidor Isaac Rabi]], [[Fritz Sauter]], [[John C. Slater]], [[Edward Teller]], [[John Hasbrouck van Vleck]], [[Victor Frederick Weisskopf]], [[Carl Friedrich von Weizsäcker]], [[Gregor Wentzel]], and [[Clarence Zener]].<ref name=MottPeierls77_227>{{harvnb|Mott|Peierls|1977|p=227}}</ref>

In early 1929, Heisenberg and Pauli submitted the first of two papers laying the foundation for relativistic [[quantum field theory]].<ref>{{harvnb|Heisenberg|Pauli|1929}}, {{harvnb|Heisenberg|Pauli|1930}}, as cited in {{harvnb|Mott|Peierls|1977|p=243}}</ref> Also in 1929, Heisenberg went on a lecture tour of China, Japan, India, and the United States.<ref name=Cassidy/><ref name=MottPeierls77_227 /> In the spring of 1929, he was a visiting lecturer at the [[University of Chicago]], where he lectured on quantum mechanics.<ref>{{cite book|last1=Kursunoglu|first1=Behram N.|last2=Wigner|first2=Eugene P.|title=Paul Adrien Maurice Dirac: Reminiscences about a Great Physicist|date=26 April 1990|publisher=Cambridge University Press|isbn=978-0-521-38688-3|page=132}}</ref>

In 1928, the British [[Mathematical Physics|mathematical physicist]] [[Paul Dirac]] had derived his [[Dirac equation|relativistic wave equation]] of quantum mechanics, which implied the existence of positive electrons, later to be named [[positron]]s. In 1932, from a [[cloud chamber]] photograph of [[cosmic ray]]s, the American physicist [[Carl David Anderson]] identified a track as having been made by a [[positron]]. In mid-1933, Heisenberg presented his theory of the positron. His thinking on Dirac's theory and further development of the theory were set forth in two papers. The first, "Bemerkungen zur Diracschen Theorie des Positrons" ("Remarks on Dirac's theory of the positron") was published in 1934,<ref>{{harvnb|Heisenberg|1934}}</ref> and the second, "Folgerungen aus der Diracschen Theorie des Positrons" ("Consequences of Dirac's Theory of the Positron"), was published in 1936.<ref name=Cassidy/><ref>{{harvnb|Heisenberg|Euler|1936}}</ref><ref>{{cite book |first=Emilio G. |last=Segrè |author-link=Emilio G. Segrè |title=From X-rays to Quarks: Modern Physicists and Their Discoveries |publisher=W.H. Freeman |year=1980 |isbn=978-0-7167-1146-9 |url=https://archive.org/details/fromxraystoquark0000segr }}</ref> In these papers Heisenberg was the first to reinterpret the [[Dirac equation]] as a "classical" [[field equation]] for any point particle of [[Spin (physics)|spin]] ħ/2, itself subject to quantization conditions involving anti-[[commutator]]s. Thus reinterpreting it as a (quantum{{clarify|date=January 2021}}) field equation accurately describing electrons, Heisenberg put matter on the same footing as [[electromagnetism]]: as being described by relativistic quantum field equations which allowed the possibility of particle creation and destruction. ([[Hermann Weyl]] had already described this in a 1929 letter to [[Albert Einstein]].)