Editing Max Planck (section)

== Career ==
[[File:Max Planck 1878.GIF|left|thumb|Planck in 1878]]
With the completion of his habilitation thesis, Planck became an unpaid [[Privatdozent]] (German academic rank comparable to lecturer/assistant professor) in Munich, waiting until he was offered an academic position. Although he was initially ignored by the academic community, he furthered his work on the field of [[theory of heat|heat theory]] and discovered one after another the same [[Thermodynamics|thermodynamic]]al formalism as [[Josiah Willard Gibbs|Gibbs]] without realizing it. Clausius's ideas on [[entropy]] occupied a central role in his work.

In April 1885, the [[University of Kiel]] appointed Planck as associate professor of [[theoretical physics]]. Further work on entropy and its treatment, especially as applied in [[physical chemistry]], followed. He published his ''Treatise on Thermodynamics'' in 1897.<ref>{{cite book|last=Planck|first=Max|title=Vorlesungen über Thermodynamik|year=1897|publisher=Verlag Von Veit & Company|location=Leipzig|url=http://gutenberg.org/ebooks/31564|access-date=27 June 2012|archive-url=https://web.archive.org/web/20120624020935/http://www.gutenberg.org/ebooks/31564|archive-date=24 June 2012|url-status=live}} English translation: {{cite book|last=Planck|first=Max|title=Treatise on Thermodynamics|year=1903|publisher=Longmans, Green, and Company|location=London|url=https://archive.org/details/treatiseonthermo00planrich|quote=Treatise on Thermodynamics.|access-date=27 June 2012|archive-url=https://web.archive.org/web/20120220105403/http://www.archive.org/details/treatiseonthermo00planrich|archive-date=20 February 2012|url-status=live}}</ref> He proposed a thermodynamic basis for [[Svante Arrhenius]]'s theory of [[electrolyte|electrolytic]] [[dissociation (chemistry)|dissociation]].

In 1889, he was named the successor to Kirchhoff's position at the [[Humboldt University of Berlin|Friedrich-Wilhelms-Universität in Berlin]]<ref>{{cite web |url=https://www.nobelprize.org/nobel_prizes/physics/laureates/1918/planck-bio.html |title=Max Planck – Biographical |website=Nobelprize.org |publisher=Nobel Prize Organisation |access-date=26 February 2017 |archive-url=https://web.archive.org/web/20170226040835/http://www.nobelprize.org/nobel_prizes/physics/laureates/1918/planck-bio.html |archive-date=26 February 2017 |url-status=live }}</ref> – presumably thanks to Helmholtz's intercession – and by 1892 became a full professor. In 1907 Planck was offered [[Ludwig Boltzmann]]'s position in [[Vienna]], but turned it down to stay in Berlin. During 1909, as a University of Berlin professor, he was invited to become the [[Ernest Kempton Adams Lectures|Ernest Kempton Adams Lecturer in Theoretical Physics]] at [[Columbia University]] in [[New York City]]. A series of his lectures were translated and co-published by Columbia University professor [[Albert Potter Wills|A. P. Wills]].<ref>{{cite book|author=Jacques Hadamard|title=Four lectures on mathematics: delivered at Columbia University in 1911|url=https://archive.org/details/cu31924060184375|access-date=5 July 2011|year=1915|publisher=Columbia University Press|pages=[https://archive.org/details/cu31924060184375/page/n24 7]–}}</ref> He was elected to the [[American Academy of Arts and Sciences]] in 1914.<ref>{{Cite web |date=2023-02-09 |title=Max Karl Ernst Ludwig Planck |url=https://www.amacad.org/person/max-karl-ernst-ludwig-planck |access-date=2023-06-22 |website=American Academy of Arts & Sciences |language=en}}</ref> He retired from Berlin on 10 January 1926,<ref>{{cite web|title=Max Planck – Humboldt-Universität zu Berlin|url=https://www.hu-berlin.de/en/about/history/nobel-laureates/planck/|website=www.hu-berlin.de|access-date=15 May 2016|language=en|archive-url=https://web.archive.org/web/20160531054648/https://www.hu-berlin.de/en/about/history/nobel-laureates/planck/|archive-date=31 May 2016|url-status=live}}</ref> and was succeeded by [[Erwin Schrödinger]].<ref>{{cite web|title=Erwin Schrödinger – Humboldt-Universität zu Berlin|url=https://www.hu-berlin.de/en/about/history/nobel-laureates/schroedinger|website=www.hu-berlin.de|access-date=15 May 2016|language=en|archive-url=https://web.archive.org/web/20160531080108/https://www.hu-berlin.de/en/about/history/nobel-laureates/schroedinger|archive-date=31 May 2016|url-status=live}}</ref> He was elected to the United States [[National Academy of Sciences]] in 1926 and the [[American Philosophical Society]] in 1933.<ref>{{Cite web |title=Max Planck |url=http://www.nasonline.org/member-directory/deceased-members/20001902.html |access-date=2023-06-22 |website=www.nasonline.org}}</ref><ref>{{Cite web |title=APS Member History |url=https://search.amphilsoc.org/memhist/search?creator=+Max+Planck&title=&subject=&subdiv=&mem=&year=&year-max=&dead=&keyword=&smode=advanced |access-date=2023-06-22 |website=search.amphilsoc.org}}</ref>

=== Professor at Berlin University ===
As a professor at the [[Humboldt University of Berlin|Friedrich-Wilhelms-Universität in Berlin]], Planck joined the local Physical Society. He later wrote about this time: "In those days I was essentially the only theoretical physicist there, whence things were not so easy for me, because I started mentioning entropy, but this was not quite fashionable, since it was regarded as a mathematical spook".<ref>
{{cite journal
 |last1     = Verband Deutscher Elektrotechniker
 |last2     = Elektrotechnischer Verein (Berlin, Germany)
 |year      = 1948
 |journal   = ETZ: Elektrotechnische Zeitschrift
 |title     = ETZ: Elektrotechnische Zeitschrift: Ausg. A.
 |volume    = 69
 |issue     = A
 |publisher = VDE-Verlag
 |language  = de
 |url       = https://books.google.com/books?id=ZFE7AAAAMAAJ
}}, [https://www.google.com/search?&tbs=bks%3A1&q=planck++%22damals+der+einzige+theoretische%22 Snipped extract] {{Webarchive|url=https://web.archive.org/web/20170405025654/http://www.google.com/search?&tbs=bks%3A1&q=planck++%22damals+der+einzige+theoretische%22 |date=5 April 2017 }}</ref> Thanks to his initiative, the various local Physical Societies of Germany merged in 1898 to form the German Physical Society ([[Deutsche Physikalische Gesellschaft]], DPG); from 1905 to 1909 Planck was the president.

[[File:Max Planck Wirkungsquantums 20050815.jpg|thumb|right|Plaque at the [[Humboldt University of Berlin]]: "Max Planck, discoverer of the elementary quantum of action ''h'', taught in this building from 1889 to 1928."]]

Planck started a six-semester course of lectures on theoretical physics, "dry, somewhat impersonal"{{Citation needed|date=March 2025}} according to [[Lise Meitner]], "using no notes, never making mistakes, never faltering; the best lecturer I ever heard"{{Citation needed|date=March 2025}} according to an English participant, [[J. R. Partington|James R. Partington]], who continues: "There were always many standing around the room. As the lecture-room was well heated and rather close, some of the listeners would from time to time drop to the floor, but this did not disturb the lecture."{{Citation needed|date=March 2025}} Planck did not establish an actual "school"; the number of his graduate students was only about 20, among them:{{Citation needed|date=March 2025}}
* 1897 [[Max Abraham]] (1875–1922)
* 1903 [[Max von Laue]] (1879–1960)
* 1904 [[Moritz Schlick]] (1882–1936)
* 1906 [[Walther Meissner]] (1882–1974)
* 1907 [[Fritz Reiche]] (1883–1960)
* 1912 [[Walter Schottky]] (1886–1976)
* 1914 [[Walther Bothe]] (1891–1957)<ref>{{cite web|url=https://www.genealogy.math.ndsu.nodak.edu/id.php?id=20750|title=Max Planck – The Mathematics Genealogy Project|website=www.genealogy.math.ndsu.nodak.edu|access-date=5 June 2017|archive-url=https://web.archive.org/web/20170608023459/https://www.genealogy.math.ndsu.nodak.edu/id.php?id=20750|archive-date=8 June 2017|url-status=live}}</ref>

=== Entropy ===
[[Thermodynamics]], also known as the "mechanical theory of heat" at the end of the 19th century, had emerged at the beginning of this century from an attempt to understand the functioning of steam engines and to improve their efficiency. In the 1840s, several researchers independently discovered and formulated the law of conservation of energy, which is now also known as the [[first law of thermodynamics]]. In 1850, [[Rudolf Clausius]] formulated the so-called [[second law of thermodynamics]], which states that a voluntary (or spontaneous) transfer of energy is only possible from a warmer to a colder body, but not vice versa. In England at this time [[William Thomson, 1st Baron Kelvin|William Thomson]] came to the same conclusion.

Clausius generalized his formulation further and further and came up with a new formulation in 1865. To this end, he introduced the concept of [[entropy]], which he defined as a measure of the reversible supply of heat in relation to the absolute temperature.

The new formulation of the second law, which is still valid today, was: "Entropy can be created, but never destroyed". Clausius, whose work Planck read as a young student during his stay in Berlin, successfully applied this new law of nature to mechanical, thermoelectric and chemical processes.

In his dissertation in 1879, Planck summarized Clausius' writings, pointing out contradictions and inaccuracies in their formulation and then clarifying them. In addition, he generalized the validity of the second law to all processes in nature; Clausius had limited its application to reversible processes and thermal processes. Furthermore, Planck dealt intensively with the new concept of entropy and emphasized that entropy is not only a property of a physical system, but at the same time a measure of the irreversibility of a process: If entropy is generated in a process, it is irreversible, since entropy cannot be destroyed according to the second law. In reversible processes, the entropy remains constant. He presented this fact in detail in 1887 in a series of treatises entitled "On the Principle of the Increase of Entropy".<ref>{{cite journal |last1=Vlasak |first1=Weldon |title=Planck's theory and thermodynamics |journal=Chemical Innovation |date=February 2001 |volume=31 |issue=2 |pages=56–59 |url=https://pubsapp.acs.org/subscribe/archive/ci/31/i02/html/02learning.html |access-date=7 August 2024}}</ref>

In his study of the concept of entropy, Planck did not follow the molecular, probabilistic interpretation that prevailed at the time, as these do not provide absolute proof of universality. Instead, he pursued a phenomenological approach and was also skeptical of atomism. Even though he later abandoned this attitude in the course of his work on the law of radiation, his early work impressively shows the possibilities of thermodynamics in solving concrete physicochemical problems.<ref name="Hoffmann29f">Hoffmann: ''Max Planck.'' Munich 2008, p.&nbsp;29.</ref><ref name="Hartmann156f">Hartmann: ''Max Planck als Mensch und Denker.'' 3. revised edition, Basel 1953, p.&nbsp;156.</ref>

Planck's understanding of entropy included the realization that the maximum of entropy corresponds to the equilibrium state. The accompanying conclusion that knowledge of the Entropy allows all laws of thermodynamic equilibrium states to be derived corresponds to the modern understanding of such states. Planck therefore chose equilibrium processes as his research focus and, based on his habilitation thesis, researched the coexistence of aggregate states and the equilibrium of gas reactions, for example. This work on the frontier of chemical thermodynamics also received great attention due to the rapidly expanding chemical work at that time.

Independently of Planck, [[Josiah Willard Gibbs]] had also discovered almost all the knowledge Planck gained about the properties of physicochemical equilibria and published them from 1876 onwards. Planck was unaware of these essays, and they did not appear in German until 1892. However, both scientists approached the topic in different ways, while Planck dealt with irreversible processes, Gibbs looked at equilibria. This approach was finally able to prevail because of its simplicity, but Planck's approach is attributed the greater universality.<ref>Hoffmann: ''Max Planck.'' Munich 2008, p.&nbsp;31&nbsp;f.</ref>

=== Black-body radiation ===
[[File:Max Planck 1901.GIF|thumb|Planck in 1901]]
In 1894, Planck turned his attention to the problem of [[black-body radiation]]. The problem had been stated by Kirchhoff in 1859: "how does the intensity of the electromagnetic radiation emitted by a [[black body]] (a perfect absorber, also known as a cavity radiator) depend on the [[frequency]] of the radiation (i.e., the color of the light) and the temperature of the body?". The question had been explored experimentally, but no theoretical treatment had agreed with the experimentally observed evidence. [[Wilhelm Wien]] proposed [[Wien approximation|Wien's law]], which correctly predicted the behaviour at high frequencies, but failed at low frequencies. The [[Rayleigh–Jeans law]], another approach to the problem, agreed with experimental results at low frequencies, but created what was later known as the "[[ultraviolet catastrophe]]" at high frequencies, as predicted by [[classical physics]]. However, contrary to many textbooks, this was not a motivation for Planck.<ref name="Kragh">For a solid approach to the complexity of Planck's intellectual motivations for the quantum, for his reluctant acceptance of its implications, see Helge Kragh, 
[https://physicsworld.com/a/max-planck-the-reluctant-revolutionary/ Max Planck: the reluctant revolutionary] {{Webarchive|url=https://web.archive.org/web/20181105061830/https://physicsworld.com/a/max-planck-the-reluctant-revolutionary/ |date=5 November 2018 }}, ''Physics World''. December 2000.</ref>

Planck's first proposed solution to the problem in 1899 followed from what he called the "principle of elementary disorder", which allowed him to derive Wien's law from a number of assumptions about the [[entropy]] of an ideal oscillator, creating what was referred to as the Wien–Planck law. Soon, however, it was found that experimental evidence did not confirm the new law at all, to Planck's frustration. He revised his approach and now derived the first version of the famous [[Planck's law of black-body radiation|Planck black-body radiation law]], which described clearly the experimentally observed black-body spectrum. It was first proposed in a meeting of the DPG on 19 October 1900 and published in 1901. (This first derivation did not include energy quantisation, and did not use [[statistical mechanics]], to which he held an aversion.) In November 1900 Planck revised this first version, now relying on [[Ludwig Boltzmann|Boltzmann]]'s statistical interpretation of the [[second law of thermodynamics]] as a way of gaining a more fundamental understanding of the principles behind his radiation law. Planck was deeply suspicious of the philosophical and physical implications of such an interpretation of Boltzmann's approach; thus his recourse to them was, as he later put it, "an act of despair ... I was ready to sacrifice any of my previous convictions about physics".<ref name="Kragh"/>

The central assumption behind his new derivation, presented to the DPG on 14 December 1900, was the supposition, now known as the [[Planck postulate]], that electromagnetic energy could be emitted only in [[Quantization (physics)|quantized]] form, in other words, the energy could only be a multiple of an elementary unit:
: <math>E = h\nu</math>
where {{math|''h''}} is the [[Planck constant]], also known as Planck's action quantum (introduced already in 1899), and {{math|''ν''}} is the frequency of the radiation. Note that the elementary units of energy discussed here are represented by {{math|''hν''}} and not simply by {{math|''ν''}}. Physicists now call these quanta photons, and a photon of frequency {{math|''ν''}} will have its own specific and unique energy. The total energy at that frequency is then equal to {{math|''hν''}} multiplied by the number of photons at that frequency.

[[File:Max Planck Nobel 1918.jpg|thumb|left|Planck in 1918, the year he was awarded the [[Nobel Prize in Physics]] for his work on [[Quantum mechanics|quantum theory]]]]
At first Planck considered that quantisation was only "a purely formal assumption ... actually I did not think much about it ..."; nowadays this assumption, incompatible with [[classical physics]], is regarded as the birth of [[quantum physics]] and the greatest intellectual accomplishment of Planck's career. ([[Ludwig Boltzmann|Boltzmann]] had been discussing in a theoretical paper in 1877 the possibility that the energy states of a physical system could be discrete). The discovery of the Planck constant enabled him to define a new universal set of [[Planck units|physical units]] (such as the Planck length and the Planck mass), all based on fundamental physical constants, upon which much of quantum theory is based. In a discussion with his son in December 1918 Planck described his discovery as 'a discovery of the first rank, comparable perhaps only to the discoveries of Newton'.<ref>Egginton, William, ''[https://us5.campaign-archive.com/?e=01376ceb1d&u=6557fc90400ccd10e100a13f4&id=dc364d6f8b The Rigor of Angels: Max Planck unleashed a revolution in physics]'', pp. 52–54, Pantheon, Delancy Place, 2023</ref> In recognition of Planck's fundamental contribution to a new branch of physics, he was awarded the Nobel Prize in Physics for 1918; (he received the award in 1919).<ref>Kragh, Helge (1 December 2000), Max Planck: the reluctant revolutionary, PhysicsWorld.com</ref><ref>{{cite web|url=https://www.nobelprize.org/nobel_prizes/physics/laureates/1918/|title=The Nobel Prize in Physics 1918|website=www.nobelprize.org|access-date=11 June 2017|archive-url=https://web.archive.org/web/20170609233007/http://www.nobelprize.org/nobel_prizes/physics/laureates/1918/|archive-date=9 June 2017|url-status=live}}</ref>

Subsequently, Planck tried to grasp the meaning of energy quanta, but to no avail. "My unavailing attempts to somehow reintegrate the action quantum into classical theory extended over several years and caused me much trouble." Even several years later, other physicists such as [[John William Strutt, 3rd Baron Rayleigh|Rayleigh]], [[James Jeans|Jeans]], and [[Hendrik Lorentz|Lorentz]] set the Planck constant to zero in order to align with classical physics, but Planck knew well that this constant had a precise nonzero value. "I am unable to understand Jeans' stubbornness – he is an example of a theoretician as should never be existing, the same as [[Hegel]] was for philosophy. So much the worse for the facts if they don't fit."<ref>Heilbron, 2000, [https://books.google.com/books?id=d5zKH2Bx2AwC&pg=PA8 p. 8] {{Webarchive|url=https://web.archive.org/web/20180417074508/https://books.google.com/books?id=d5zKH2Bx2AwC&pg=PA8 |date= 2018 }}</ref>

[[Max Born]] wrote about Planck: "He was, by nature, a conservative mind; he had nothing of the revolutionary and was thoroughly skeptical about speculations. Yet his belief in the compelling force of logical reasoning from facts was so strong that he did not flinch from announcing the most revolutionary idea which ever has shaken physics."<ref name="frs"/>

=== Einstein and the theory of relativity ===
<!-- Commented out: [[File:Max-Planck-und-Albert-Einstein.jpg|thumb|Max Planck presents [[Albert Einstein]] with the Max-Planck medal, Berlin 28 June 1929]] -->
{{Special relativity sidebar}}

In 1905, three papers by [[Albert Einstein]] were published in the journal ''[[Annalen der Physik]]''. Planck was among the few who immediately recognized the significance of the [[special theory of relativity]]. Thanks to his influence, this theory was soon widely accepted in Germany. Planck also contributed considerably to extend the special theory of relativity. For example, he recast the theory in terms of classical [[Action (physics)|action]].<ref>''Einstein and the Quantum'', A.Douglas Stone, Princeton University Press, Princeton and Oxford, chapter 9, ''Tripping the light heuristic'', 2013.</ref>

Einstein's hypothesis of light ''quanta'' ([[photons]]), based on [[Heinrich Hertz#Electromagnetic research|Heinrich Hertz's]] 1887 discovery (and further investigation by [[Philipp Lenard#Photoelectric investigations|Philipp Lenard]]) of the [[photoelectric effect]], was initially rejected by Planck. He was unwilling to discard completely [[James Clerk Maxwell|Maxwell]]'s theory of [[electrodynamics]]. "The theory of light would be thrown back not by decades, but by centuries, into the age when [[Christiaan Huygens]] dared to fight against the mighty emission theory of [[Isaac Newton]] ..."<ref>{{Cite book|title=Atoms and Photons and Quanta, Oh My!: Ask the physicist about atomic, nuclear, and quantum physics|last=Baker|first=F. Todd|publisher=Morgan & Claypool Publishers|year=2015|isbn=978-1-62705-940-4}}</ref>

In 1910, Einstein pointed out the anomalous behavior of [[specific heat]] at low temperatures as another example of a phenomenon which defies explanation by classical physics. Planck and [[Walther Nernst]], seeking to clarify the increasing number of contradictions, organized the First [[Solvay Conference]] (Brussels 1911). At this meeting Einstein was able to convince Planck.

Meanwhile, Planck had been appointed dean of Berlin University, whereby it was possible for him to call Einstein to Berlin and establish a new professorship for him (1914). Soon the two scientists became close friends and met frequently to play music together.

=== First World War ===
At the onset of the [[First World War]] Planck endorsed the general excitement of the public, writing that, "Besides much that is horrible, there is also much that is unexpectedly great and beautiful: the smooth solution of the most difficult domestic political problems by the unification of all parties (and) ... the extolling of everything good and noble."<ref>Heilbron, 2000, [https://books.google.com/books?id=d5zKH2Bx2AwC&pg=PA72 p. 72] {{Webarchive|url=https://web.archive.org/web/20150320070521/http://books.google.com/books?id=d5zKH2Bx2AwC&pg=PA72 |date=20 March 2015 }}</ref><ref>
{{cite book
 |title=Quantum mechanics at the crossroads: new perspectives from history, philosophy and physics
 |first1=James |last1=Evans
 |first2=Alan S. |last2=Thorndike
 |publisher=Springer
 |year=2007
 |isbn=978-3-540-32663-2
 |page=31
 |url=https://books.google.com/books?id=S3FOuMYHcqIC
 |access-date=14 October 2016
 |archive-url=https://web.archive.org/web/20150320051740/http://books.google.com/books?id=S3FOuMYHcqIC
 |archive-date=20 March 2015
 |url-status=live
}} [https://books.google.com/books?id=S3FOuMYHcqIC&pg=PA31 Extract of page 31] {{Webarchive|url=https://web.archive.org/web/20150320054630/http://books.google.com/books?id=S3FOuMYHcqIC&pg=PA31 |date=20 March 2015 }}</ref> Planck also signed the infamous "[[Manifesto of the Ninety-Three|Manifesto of the 93 intellectuals]]", a pamphlet of polemic war propaganda (while Einstein retained a strictly pacifistic attitude which almost led to his imprisonment, only being spared thanks to his [[Swiss (people)|Swiss]] citizenship).

In 1915, when Italy was still a neutral power, Planck voted successfully for a scientific paper from Italy, which received a prize from the [[Prussian Academy of Sciences]], where Planck was one of four permanent presidents.

=== Post-war and the Weimar Republic ===
In the turbulent post-war years, Planck, now the highest authority of German physics, issued the slogan "persevere and continue working" to his colleagues.

In October 1920, he and [[Fritz Haber]] established the ''[[Notgemeinschaft der Deutschen Wissenschaft]]'' (Emergency Organization of German Science), aimed at providing financial support for scientific research. A considerable portion of the money the organization would distribute was raised abroad.

Planck held leading positions at Berlin University, the Prussian Academy of Sciences, the German Physical Society, and the [[Kaiser Wilhelm Society]] (which became the [[Max Planck Society]] in 1948). During this time economic conditions in Germany were such that he was hardly able to conduct research. In 1926, Planck became a foreign member of the [[Royal Netherlands Academy of Arts and Sciences]].<ref>{{cite web |url=http://www.dwc.knaw.nl/biografie/pmknaw/?pagetype=authorDetail&aId=PE00002335 |title=Max Karl Ernst Ludwig Planck (1858–1947) |publisher=Royal Netherlands Academy of Arts and Sciences |access-date=4 August 2015 |archive-url=https://web.archive.org/web/20150910170739/http://www.dwc.knaw.nl/biografie/pmknaw/?pagetype=authorDetail&aId=PE00002335 |archive-date=10 September 2015 |url-status=live }}</ref>

During the interwar period, Planck became a member of the Deutsche Volks-Partei ([[German People's Party]]), the party of Nobel Peace Prize laureate [[Gustav Stresemann]], which aspired to liberal aims for domestic policy and rather revisionistic aims for politics around the world.

Planck disagreed with the introduction of [[universal suffrage]] and later expressed the view that the Nazi dictatorship resulted from "the ascent of the rule of the crowds".<ref>
{{cite book
 |title=The demon and the quantum: from the pythagorean mystics to Maxwell's demon and quantum mystery
 |first1=Robert J. |last1=Scully
 |first2=Marlan O. |last2=Scully
 |publisher=Wiley-VCH
 |year=2007
 |isbn=978-3-527-40688-3
 |page=90
 |url=https://books.google.com/books?id=mQHswDNUbvMC
 |access-date=14 October 2016
 |archive-url=https://web.archive.org/web/20150320043330/http://books.google.com/books?id=mQHswDNUbvMC
 |archive-date=20 March 2015
 |url-status=live
}}, [https://books.google.com/books?id=mQHswDNUbvMC&pg=PA90 Chapter 7, p 90] {{Webarchive|url=https://web.archive.org/web/20150320084201/http://books.google.com/books?id=mQHswDNUbvMC&pg=PA90 |date=20 March 2015 }}</ref>

=== Quantum mechanics ===
[[File:Nernst, Einstein, Planck, Millikan, Laue in 1931.jpg|thumb|330px|From left to right: [[Walther Nernst|W. Nernst]], [[Albert Einstein|A. Einstein]], Planck, [[Robert Andrews Millikan|R. A. Millikan]], and [[Max von Laue|von Laue]] at a dinner given by von Laue in Berlin on 11 November 1931]]
At the end of the 1920s, [[Niels Bohr]], [[Werner Heisenberg]], and [[Wolfgang Pauli]] had worked out the [[Copenhagen interpretation]] of quantum mechanics, but it was rejected by Planck, and by Schrödinger, Laue, and Einstein as well. Planck expected that [[Schrödinger equation#Particles as waves|wave mechanics]] would soon render quantum theory {{snd}}his own child{{snd}} unnecessary. This was not to be the case, however. Further work only served to underscore the enduring central importance of quantum theory, even against his and Einstein's philosophical revulsions. Here Planck experienced the truth of his own earlier observation from his struggle with the older views during his younger years: "A new scientific truth does not triumph by convincing its opponents and making them see the light, but rather because its opponents eventually die, and a new generation grows up that is familiar with it."<ref>Quoted in Thomas Kuhn, ''[[The Structure of Scientific Revolutions]]'' (1970 ed.): p. 150.</ref>

=== Emergence of Planck-scale Physics ===
Planck laid a foundation for new physics in his 1901 paper with his derivation of new "natural units of measurement." What became known as the Planck length was far smaller than the atomic scale which at that time was seen as smaller than the reach of physics. Ninety years later, [[Peter Bergmann]] said, "The [[Planck length]] is 1.6×10-33 cm, very much smaller
than any known nuclear dimension."<ref>P. G. Bergmann, The Riddle of Gravitation (New York: Dover Publications, 1992), p. x.</ref> It was slowly accepted as maybe setting a minimum possible size, but even in 2003 physicist Yee Jack Ng would say that "it takes a certain amount of foolhardiness to even mention Planck-scale physics".<ref>Y. J. Ng, "Selected Topics in Planck-scale Physics," Mod. Phys. Lett., (2003) A18, p. 1073.</ref>

=== Nazi dictatorship and the Second World War ===
When the Nazis came to power in 1933, Planck was 74 years old. He witnessed many Jewish friends and colleagues expelled from their positions and humiliated, and hundreds of scientists emigrate from [[Nazi Germany]]. Again he tried to "persevere and continue working" and asked scientists who were considering emigration to remain in Germany. Nevertheless, he did help his nephew, the economist [[Hermann Kranold]], to emigrate to [[London]] after his arrest.<ref name="IJ">{{cite web|title=Johanna Kranold Stein|url=http://www.legacy.com/obituaries/theithacajournal/obituary.aspx?n=johanna-kranold-stein&pid=87920213|website=Ithaca Journal|publisher=Legacy.com|access-date=10 October 2016|archive-url=https://web.archive.org/web/20161011142403/http://www.legacy.com/obituaries/theithacajournal/obituary.aspx?n=johanna-kranold-stein&pid=87920213|archive-date=11 October 2016|url-status=live}}</ref> He hoped the crisis would abate soon and the political situation would improve.

[[Otto Hahn]] asked Planck to gather well-known German professors in order to issue a public proclamation against the treatment of Jewish professors, but Planck replied, "If you are able to gather today 30 such gentlemen, then tomorrow 150 others will come and speak against it, because they are eager to take over the positions of the others."<ref>In a slightly different translation, Hahn remembers Planck saying: "If you bring together 30 such men today, then tomorrow 150 will come to denounce them because they want to take their places." This translated quote is found in: Heilbron, 2000, p. 150. Heilbron, at the end of the paragraph, on p. 151, cites the following references to Hahn's writings: Otto Hahn ''Einige persönliche Erinnerungen an Max Planck'' MPG, ''Mitteilungen'' (1957) p. 244, and Otto Hahn ''My Life'' (Herder and Herder, 1970) p. 140.</ref> Under Planck's leadership, the [[Kaiser Wilhelm Society]] (KWG) avoided open conflict with the Nazi regime, except concerning the Jewish Fritz Haber. In May 1933 Planck requested and received an interview with the recently appointed Chancellor of Germany [[Adolf Hitler]] to discuss the issue, telling him that the "forced emigration of Jews would kill German science and Jews could be good Germans", to which the chancellor replied "but we don't have anything against the Jews, only against communists". Planck was therefore unsuccessful, since this reply "took from him every basis for further negotiation",<ref>{{cite book |last1=Clary |first1=David |title=Schrödinger in Oxford |date=2022 |page=54|bibcode=2022scox.book.....C }}</ref> as to Hitler "the Jews are all Communists, and these are my enemies." In the following year, 1934, Haber died in exile.<ref>{{Cite journal|last=O'Flaherty|first=James C.|date=1956|title=Max Planck and Adolf Hitler|url=https://www.jstor.org/stable/40222051|journal=AAUP Bulletin|volume=42|issue=3|pages=437–444|doi=10.2307/40222051|jstor=40222051|issn=0001-026X}}</ref>

One year later, Planck, having been the president of the KWG since 1930, organized in a somewhat provocative style an official commemorative meeting for Haber. He also succeeded in secretly enabling a number of Jewish scientists to continue working in institutes of the KWG for several years. In 1936, his term as president of the KWG ended, and the Nazi government pressured him to refrain from seeking another term.

As the political climate in Germany gradually became more hostile, [[Johannes Stark]], prominent exponent of the ''[[Deutsche Physik]]'' ("German Physics", also called "Aryan Physics") attacked Planck, [[Arnold Sommerfeld]], and Heisenberg for continuing to teach the theories of Einstein, calling them "white Jews". The "Hauptamt Wissenschaft" (Nazi government office for science) started an investigation of Planck's ancestry, claiming that he was "1/16 Jewish", but Planck denied it.<ref>Heilbron, 2000, [https://books.google.com/books?id=d5zKH2Bx2AwC&pg=PA191 p. 191] {{Webarchive|url=https://web.archive.org/web/20150320084452/http://books.google.com/books?id=d5zKH2Bx2AwC&pg=PA191 |date=20 March 2015 }}</ref>

In 1938, Planck celebrated his 80th birthday. The DPG held a celebration, during which the Max-Planck medal (founded as the highest medal by the DPG in 1928) was awarded to French physicist [[Louis de Broglie]]. At the end of 1938, the Prussian Academy lost its remaining independence and was taken over by Nazis, as part of their process of ''[[Gleichschaltung]]''. Planck protested by resigning his presidency. He continued to travel frequently, giving numerous public talks, such as his talk on Religion and Science and, five years later, he was sufficiently fit to climb 3,000-metre peaks in the [[Alps]].

During the [[Second World War]], the increasing number of Allied bombing missions against Berlin forced Planck and his wife to temporarily leave the city and live in the countryside. In 1942, he wrote: "In me an ardent desire has grown to persevere this crisis and live long enough to be able to witness the turning point, the beginning of a new rise." In February 1944, his home in Berlin was completely destroyed by an air raid, annihilating all his scientific records and correspondence. His rural retreat was threatened by the rapid advance of the Allied armies from both sides.

In 1944, Planck's son [[Erwin Planck|Erwin]] was arrested by the [[Gestapo]] following the attempted assassination of Hitler in the {{awrap|[[20 July plot]]}}. He was tried and sentenced to death by the [[People's Court (Germany)|People's Court]] in October 1944. Erwin was hanged at Berlin's [[Plötzensee Prison]] in January 1945. The death of his son destroyed much of Planck's will to live.<ref>{{cite web|url=http://physics.nobel.brainparad.com/max_karl_ernst_ludwig_planck.html |title=Max Karl Ernst Ludwig Planck |access-date=17 June 2010 |url-status=dead |archive-url=https://web.archive.org/web/20080512151051/http://physics.nobel.brainparad.com/max_karl_ernst_ludwig_planck.html |archive-date=12 May 2008 }}</ref>