Editing Werner Heisenberg (section)

==German nuclear weapons program==
{{Main|German nuclear weapons program}}

===Pre-war work on physics===
In mid-1936, Heisenberg presented his theory of [[cosmic ray|cosmic-ray]] showers in two papers.<ref>{{harvnb|Heisenberg|1936a}}, {{harvnb|Heisenberg|1936b}}, as cited by {{harvnb|Mott|Peierls|1977|p=244}}</ref> Four more papers<ref>{{cite journal|author=Heisenberg, W. |title=Der Durchgang sehr energiereicher Korpuskeln durch den Atomkern|journal=Die Naturwissenschaften|volume =25|pages= 749–750 |doi= 10.1007/BF01789574 |year=1937 |issue=46 |bibcode=1937NW.....25..749H|s2cid=39613897}}, as cited by {{harvnb|Mott|Peierls|1977|p=244}}</ref><ref>Heisenberg, W. (1937) ''Theoretische Untersuchungen zur Ultrastrahlung'', ''Verh. Dtsch. Phys. Ges.'' Volume 18, 50, as cited by {{harvnb|Mott|Peierls|1977|p=244}}</ref><ref>{{cite journal|title=Die Absorption der durchdringenden Komponente der Höhenstrahlung|doi=10.1002/andp.19384250705|year=1938|last1=Heisenberg|first1=W. |journal=Annalen der Physik|volume=425|issue=7|pages=594–599|bibcode = 1938AnP...425..594H }}, as cited by {{harvnb|Mott|Peierls|1977|p=244}}</ref><ref>Heisenberg, W. (1938) ''Der Durchgang sehr energiereicher Korpuskeln durch den Atomkern'', ''Nuovo Cimento'' Volume 15, 31–34; ''Verh. Dtsch. Phys. Ges.'' Volume 19, 2, as cited by {{harvnb|Mott|Peierls|1977|p=244}}</ref> appeared in the next two years.<ref name=Cassidy/><ref name=MottPeierls77_231>{{harvnb|Mott|Peierls|1977|p=231}}</ref>

In December 1938, the German chemists [[Otto Hahn]] and [[Fritz Strassmann]] sent a manuscript to ''[[Die Naturwissenschaften|The Natural Sciences]]'' reporting they had detected the element [[barium]] after bombarding [[uranium]] with neutrons, leading Hahn to conclude that a ''bursting'' of the uranium nucleus had occurred;<ref>{{cite journal|author1=Hahn, O. |author2= Strassmann, F. |title=Über den Nachweis und das Verhalten der bei der Bestrahlung des Urans mittels Neutronen entstehenden Erdalkalimetalle|trans-title=On the detection and characteristics of the alkaline earth metals formed by irradiation of uranium with neutrons|journal=Naturwissenschaften|volume =27|issue= 1|pages= 11–15 |year=1939|doi= 10.1007/BF01488241 |bibcode= 1939NW.....27...11H |s2cid= 5920336 }}. The authors were identified as being at the ''Kaiser-Wilhelm-Institut für Chemie'', Berlin-Dahlem. Received 22 December 1938.</ref> simultaneously, Hahn communicated these results to his friend [[Lise Meitner]], who had in July of that year fled, first to the Netherlands, then to Sweden.<ref>{{cite journal |author=Sime, Ruth Lewin |title=Lise Meitner's Escape from Germany |journal=American Journal of Physics |volume=58 |issue=3 |pages=263–267 |date=March 1990 |doi=10.1119/1.16196 |bibcode=1990AmJPh..58..262S |author-link=Ruth Lewin Sime }}</ref> Meitner, and her nephew [[Otto Robert Frisch]], correctly interpreted Hahn's and Strassmann's results as being [[nuclear fission]].<ref>{{cite journal |author=Meitner, Lise|s2cid=4113262 |title=Disintegration of Uranium by Neutrons: a New Type of Nuclear Reaction |journal=Nature |volume=143 |issue=3615 |pages=239–240 |date=11 February 1939|doi=10.1038/143239a0 |bibcode=1939Natur.143..239M }} The paper is dated 16 January 1939. Meitner is identified as being at the Physical Institute, Academy of Sciences, Stockholm. Frisch is identified as being at the Institute of Theoretical Physics, University of Copenhagen.</ref> Frisch confirmed this experimentally on 13 January 1939.<ref>{{cite journal|author=Frisch, O.R. |s2cid=4076376 |title=Physical Evidence for the Division of Heavy Nuclei under Neutron Bombardment |journal=Nature |volume=143 |issue=3616 |page=276 |date=18 February 1939|doi=10.1038/143276a0 |bibcode=1939Natur.143..276F |doi-access=free }} The [http://dbhs.wvusd.k12.ca.us/webdocs/Chem-History/Frisch-Fission-1939.html paper] {{webarchive|url=https://web.archive.org/web/20090123165907/http://dbhs.wvusd.k12.ca.us/webdocs/Chem-History/Frisch-Fission-1939.html |date=23 January 2009 }} is dated 17 January 1939. [The experiment for this letter to the editor was conducted on 13 January 1939; see Richard Rhodes ''The Making of the Atomic Bomb'' 263 and 268 (Simon and Schuster, 1986).]</ref>

In June and July 1939, Heisenberg traveled to the United States visiting [[Samuel Abraham Goudsmit]] at the [[University of Michigan]] in [[Ann Arbor, Michigan|Ann Arbor]]. However, Heisenberg refused an invitation to emigrate to the United States. He did not see Goudsmit again until six years later, when Goudsmit was the chief scientific advisor to the American [[Operation Alsos]] at the close of World War II.<ref name=Cassidy/><ref>{{harvnb|Hentschel|Hentschel|1996|p=387}}</ref><ref>{{harvnb|Goudsmit|1986|p= picture facing p. 124}}</ref>

===Membership in the Uranverein===
The [[German nuclear weapons program]], known as ''Uranverein'', was formed on 1 September 1939, the day [[World War II]] began in Europe. The ''Heereswaffenamt'' (HWA, Army Ordnance Office) had squeezed the ''[[Reichsforschungsrat]]'' (RFR, Reich Research Council) out of the ''[[Reichserziehungsministerium]]'' (REM, Reich Ministry of Education) and started the formal German nuclear energy project under military auspices. The project had its first meeting on 16 September 1939. The meeting was organized by [[Kurt Diebner]], advisor to the HWA, and held in Berlin. The invitees included [[Walther Bothe]], [[Siegfried Flügge]], [[Hans Geiger]], [[Otto Hahn]], [[Paul Harteck]], [[Gerhard Hoffmann (physicist)|Gerhard Hoffmann]], [[Josef Mattauch]] and [[Georg Stetter]]. A second meeting was held soon thereafter and included Heisenberg, [[Klaus Clusius]], [[Robert Döpel]] and [[Carl Friedrich von Weizsäcker]]. The ''Kaiser-Wilhelm Institut für Physik'' (KWIP, Kaiser Wilhelm Institute for Physics) in [[Dahlem (Berlin)|Berlin-Dahlem]], was placed under HWA authority, with Diebner as the administrative director, and the military control of the nuclear research commenced.<ref name=Macrakis164>{{harvnb|Macrakis|1993|pp = 164–169}}</ref><ref name=Rechenberg>{{cite book |last1=Mehra |first1=Jagdish |first2=Helmut |last2=Rechenberg |title=Volume 6. The Completion of Quantum Mechanics 1926–1941. Part 2. The Conceptual Completion and Extension of Quantum Mechanics 1932–1941. Epilogue: Aspects of the Further Development of Quantum Theory 1942–1999 |series=The Historical Development of Quantum Theory |publisher=Springer |year=2001 |isbn=978-0-387-95086-0 |url-access=registration |url=https://archive.org/details/completionofquan0000mehr |pages=1010–1011}}</ref><ref>{{harvnb|Hentschel|Hentschel|1996|pp=363–364, Appendix F;}} see the entries for Diebner and Döpel. See also the entry for the KWIP in Appendix A and the entry for the HWA in Appendix B.</ref> During the period when Diebner administered the KWIP under the HWA program, considerable personal and professional animosity developed between Diebner and Heisenberg's inner circle, which included [[Karl Wirtz]] and [[Carl Friedrich von Weizsäcker]].<ref name=Cassidy/><ref name=Walker94>{{harvnb|Walker|1989|pp=19, 94–95}}</ref>

[[File:UFission.gif|upright=1.15|right|thumb|A visual representation of an induced nuclear fission event where a slow-moving neutron is absorbed by the nucleus of a uranium-235 atom, which fissions into two fast-moving lighter elements (fission products) and additional neutrons. Most of the energy released is in the form of the kinetic velocities of the fission products and the neutrons.]]
At a scientific conference on 26–28 February 1942 at the Kaiser Wilhelm Institute for Physics, called by the Army Weapons Office, Heisenberg presented a lecture to Reich officials on energy acquisition from nuclear fission.<ref>[http://www.aip.org/history/heisenberg/p14.htm American Institute for Physics, Center for History of Physics] {{Webarchive|url=https://web.archive.org/web/20080917202704/http://www.aip.org/history/heisenberg/p14.htm |date=17 September 2008 }}</ref> The lecture, entitled "Die theoretischen Grundlagen für die Energiegewinnung aus der Uranspaltung" ("The theoretical basis for energy generation from uranium fission") was, as Heisenberg wrote after the Second World War in a letter to [[Samuel Goudsmit]], "adapted to the intelligence level of a Reich Minister", as is often done when presenting complex and cutting-edge scientific concepts to laymen.<ref>{{harvnb|Macrakis|1993|p = 244}}</ref> Heisenberg lectured on the enormous energy potential of nuclear fission, stating that 250 million electron volts could be released through the fission of an atomic nucleus. Heisenberg stressed that pure U-235 had to be obtained to achieve a chain reaction. He explored various ways of obtaining isotope {{nuclide|U|235}} in its pure form, including uranium enrichment and an alternative layered method of normal uranium and a moderator in a machine. This machine, he noted, could be used in practical ways to fuel vehicles, ships and submarines. Heisenberg stressed the importance of the Army Weapons Office's financial and material support for this scientific endeavour.

A second scientific conference followed. Lectures were heard on problems of modern physics with decisive importance for the national defense and economy. The conference was attended by [[Bernhard Rust]], the Reich Minister of Science, Education and National Culture. At the conference, Reich Minister Rust decided to take the nuclear project away from the Kaiser Wilhelm Society, and give it to the Reich Research Council.<ref>{{harvnb|Macrakis|1993|p = 171}}</ref>

In April 1942 the army returned the Physics Institute to the Kaiser Wilhelm Society, naming Heisenberg as Director at the Institute.<ref name=Macrakis172/> [[Peter Debye]] was still director of the institute, but had gone on leave to the United States after he had refused to become a German citizen when the HWA took administrative control of the KWIP. Heisenberg still also had his department of physics at the University of Leipzig where work had been done for the ''Uranverein'' by [[Robert Döpel]] and his wife [[Klara Döpel]].<ref name=Cassidy/><ref name=Walker94/>

On 4 June 1942, Heisenberg was summoned to report to [[Albert Speer]], Germany's Minister of Armaments, on the prospects for converting the Uranverein's research toward developing [[nuclear weapon]]s. During the meeting, Heisenberg told Speer that a bomb could not be built before 1945, because it would require significant monetary resources and number of personnel.<ref>Albert Speer, [[Inside the Third Reich]], Macmillan, 1970, pp. 225ff.</ref><ref>[http://www.stanford.edu/~njenkins/cgi-bin/auden/individual.php?pid=I662&ged=auden-bicknell.ged Prof. Werner Carl Heisenberg (I662)] {{webarchive|url=https://web.archive.org/web/20080615112426/http://www.stanford.edu/~njenkins/cgi-bin/auden/individual.php?pid=I662&ged=auden-bicknell.ged |date=15 June 2008 }}. Stanford.edu</ref>

After the Uranverein project was placed under the leadership of the Reich Research Council, it focused on [[nuclear power]] production and thus maintained its ''kriegswichtig'' (importance for the war) status; funding therefore continued from the military. The nuclear power project was broken down into the following main areas: [[uranium]] and [[heavy water]] production, uranium [[isotope separation]] and the ''Uranmaschine'' (uranium machine, i.e., [[nuclear reactor]]). The project was then essentially split up between a number of institutes, where the directors dominated the research and set their own research agendas.<ref name=Macrakis164/><ref>{{harvnb|Hentschel|Hentschel|1996}}; see the entry for the KWIP in Appendix A and the entries for the HWA and the RFR in Appendix B. Also see p. 372 and footnote #50 on p. 372.</ref><ref>{{harvnb|Walker|1989|pp=49–53}}</ref> The point in 1942, when the army relinquished its control of the German nuclear weapons program, was the zenith of the project relative to the number of personnel. About 70 scientists worked for the program, with about 40 devoting more than half their time to nuclear fission research. After 1942, the number of scientists working on applied nuclear fission diminished dramatically. Many of the scientists not working with the main institutes stopped working on nuclear fission and devoted their efforts to more pressing war-related work.<ref>{{harvnb|Walker|1989|pp=52, Reference #40 on p. 262}}</ref>

In September 1942, Heisenberg submitted his first paper of a three-part series on the scattering matrix, or [[S-matrix]], in elementary [[particle physics]]. The first two papers were published in 1943<ref>{{cite journal |last=Heisenberg |first=W. |s2cid=120706757 |title=Die beobachtbaren Grössen in der Theorie der Elementarteilchen. I |journal=[[Z. Phys.]] |volume=120 |pages=513–538 |year=1943 |doi=10.1007/BF01329800 |issue=7–10 |bibcode=1943ZPhy..120..513H }} as cited in {{harvnb|Mott|Peierls|1977|p=245}}</ref><ref>{{cite journal |last=Heisenberg |first=W. |s2cid=124531901 |title=Die beobachtbaren Grössen in der Theorie der Elementarteilchen. II |journal=Z. Phys. |volume=120 |pages=673–702 |year=1943 |doi=10.1007/BF01336936 |issue=11–12 |bibcode=1943ZPhy..120..673H }} as cited in {{harvnb|Mott|Peierls|1977|p=245}}</ref> and the third in 1944.<ref>{{cite journal |last=Heisenberg |first=W. |s2cid=123698415 |title=Die beobachtbaren Grössen in der Theorie der Elementarteilchen. III |journal=Z. Phys. |volume=123 |issue=1–2 |pages=93–112 |year=1944 |doi=10.1007/BF01375146 |bibcode=1944ZPhy..123...93H }} as cited in {{harvnb|Mott|Peierls|1977|p=245}}</ref> The S-matrix described only the states of incident particles in a collision process, the states of those emerging from the collision, and stable [[bound state]]s; there would be no reference to the intervening states. This was the same precedent as he followed in 1925 in what turned out to be the foundation of the matrix formulation of quantum mechanics through only the use of observables.<ref name=Cassidy/><ref name=MottPeierls77_231 />

In February 1943, Heisenberg was appointed to the Chair for Theoretical Physics at the ''Friedrich-Wilhelms-Universität'' (today, the [[Humboldt University of Berlin|Humboldt-Universität zu Berlin]]). In April, his election to the ''Preußische Akademie der Wissenschaften'' ([[Prussian Academy of Sciences]]) was approved. That same month, he moved his family to their retreat in [[:de:Urfeld am Walchensee|Urfeld]] as Allied bombing increased in Berlin. In the summer, he dispatched the first of his staff at the ''Kaiser-Wilhelm Institut für Physik'' to [[Hechingen]] and its neighboring town of [[Haigerloch]], on the edge of the [[Black Forest]], for the same reasons. From 18–26 October, he travelled to [[German-occupied Netherlands]]. In December 1943, Heisenberg visited [[History of Poland (1939–1945)|German-occupied Poland]].<ref name=Cassidy/><ref>{{harvnb|Bernstein|2004|pp=300–304}}</ref>

From 24 January to 4 February 1944, Heisenberg travelled to occupied Copenhagen, after the German army confiscated [[Bohr's Institute of Theoretical Physics]]. He made a short return trip in April. In December, Heisenberg lectured in [[Switzerland during the World Wars#World War II|neutral Switzerland]].<ref name=Cassidy/> The United States [[Office of Strategic Services]] sent agent [[Moe Berg]] to attend the lecture carrying a pistol, with orders to shoot Heisenberg if his lecture indicated that Germany was close to completing an atomic bomb.<ref>{{citation|url=http://bos.sagepub.com/content/68/1/61.full|journal=[[Bulletin of the Atomic Scientists]]|title=Nuclear scientists as assassination targets|author=Tobey, William |s2cid=145583391|doi=10.1177/0096340211433019|date=January–February 2012|volume=68|issue=1|pages=63–64|bibcode=2012BuAtS..68a..61T|access-date=18 August 2014|archive-url=https://web.archive.org/web/20140723003531/http://bos.sagepub.com/content/68/1/61.full|archive-date=23 July 2014|url-status=live}}, citing [[Thomas Powers]] 1993 book "Heisenberg's War".</ref>

In January 1945, Heisenberg, with most of the rest of his staff, moved from the ''Kaiser-Wilhelm Institut für Physik'' to the facilities in the Black Forest.<ref name=Cassidy/>