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Planck, Max (1858- 1941)
"The history of all times and nations teaches us that exactly in the naïve, unshakable belief, furnished by religion in active life of believers, originate the most intense motives for the most significant creative performance, not only in the field of arts and sciences but also in politics."

"Religion belongs to that realm that is inviolable before the law of causation and therefore is closed to science."

-- Max Planck


Max Karl Ernst Ludwig Planck was one of the most important German physicists of the late 19th and early 20th century; he is considered to be the inventor of quantum theory.

Childhood and youth
Planck came from a traditional, intellectual family. His paternal great-grandfather and grandfather were both theology professors in Göttingen, his father was a law professor in Kiel and Munich, and his paternal uncle was a judge.

Max Planck was born in Kiel to Johann Julius Wilhelm Planck and his second wife, Emma Patzig. He was the sixth child in the family, though two of his siblings were from his father's first marriage. Among his earliest memories was the marching-in of Prussian and Austrian troups into Kiel during the Danish-Prussian war 1864. In 1867 the family moved to Munich, and Planck was enrolled in Munich's Königliches Maximiliansgymnasium, where he came under the tutelage of Hermann Müller, a mathematician who took an interest in the youth and taught him astronomy and mechanics as well as mathematics. It was from Müller that Planck first learned the principle of conservation of energy. Planck graduated early, at age sixteen.

Education
Planck was very gifted when it came to music: he took singing lessons in addition to playing the piano, organ and cello, and composing songs and operas. However, instead of music he chose to study physics.

The Munich physics professor Philipp von Jolly advised him against going into physics, saying, "in this field, almost everything is already discovered, and all that remains is to fill a few holes." Planck replied that he did not wish to discover new things, only to understand the known fundamentals of the field, and began his studies in 1874 at the University of Munich. Under Jolly's supervision, Planck performed the only actual experiments of his scientific career (studying the diffusion of hydrogen through heated platinum), but soon transferred to theoretical physics.

In 1877 he went to Berlin for a year of study with the famous physicists Hermann von Helmholtz and Gustav Kirchhoff and also the mathematician Karl Weierstrass. He wrote that Helmholtz was never quite prepared, spoke slowly, miscalculated endlessly, and bored his listeners, while Kirchhoff spoke in carefully prepared lectures, which were, however, dry and monotonous. Despite this he soon became close friends with Helmholtz. While there he mostly undertook a program of self-study of Clausius's writings, which led him to choose heat theory as his field.

In October 1878 Planck passed his qualifying exams and in February of 1879 defended his dissertation, Über den zweiten Hauptsatz der mechanischen Wärmetheorie (On the second fundamental theorem of the mechanical heat theory). He briefly taught mathematics and physics at his former school in Munich.

In June 1880 he presented his habilitation thesis, Gleichgewichtszustände isotroper Körper in verschiedenen Temperaturen (Equilibrium states of isotropic bodies at different temperatures).

Academic career
With the completion of his habilitation thesis, Planck became an unpaid private lecturer in Munich, waiting until he would be offered an academic position. Although he was initially ignored by the academic community, he furthered his work on the field of heat theory and discovered one after the other the same thermodynamical formalism as 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 an associate professor of theoretical physics. Further work on entropy and its treatment, especially as applied in physical chemistry, followed. He proposed a thermodynamic basis for Arrhenius's theory of electrolytic dissociation.

Within four years he was named the successor to Kirchhoff's position in Berlin — presumably thanks to Helmholtz's intercession — and by 1892 became a full professor. In 1907 Planck was appointed to Boltzmann's position in Vienna, but turned it down to stay in Berlin. He retired on 10 January 1926, and the successor to his position was Erwin Schrödinger.

Family
In March 1887 Planck married Marie Merck (1861-1909), sister of a school fellow, and moved with her into a sublet apartment in Kiel. Four children were born to the couple: Karl (1888-1916), the twins Emma (1889-1919) and Grete (1889-1917), and Erwin Planck (1893-1945).

After the appointment to Berlin the Planck family lived in a villa in Berlin-Grunewald, Wangenheimstraße 21. In the vicinity of this address several other professors of Berlin University were living, among them the famous theologian Adolf von Harnack, who became a close friend of Planck. Soon the Planck home became a social and cultural centre; numerous well-known scientists were frequent visitors, such as Albert Einstein, Otto Hahn and Lise Meitner. The tradition of jointly playing music had already been established in the home of Helmholtz.

After several happy years the Planck family was struck by a series of disasters: in October 1909 Marie Planck died, possibly from tuberculosis. In March 1911 Max Planck married his second wife, Marga von Hoesslin (1882-1948); in December his third son, Herrmann, was born.

During the First World War Planck's oldest son, Karl, was killed in action in Verdun, and Erwin had already in 1914 been taken prisoner by the French. Grete died in 1917 while giving birth to her first child; her sister lost her life two years later under the same circumstances, after marrying Grete's widower. Both granddaughters survived and were named after their mothers. Planck endured all these losses with stoic submission to fate.

Finally in January 1945 Erwin, to whom Max Planck had been particularly close, was executed by the Nazis because of his participation in the failed attempt to assassinate Hitler in July 1944.

Professor at Berlin University
In Berlin Planck joined the local Physical Society: later he 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". 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 of the DPG.

Planck started a six-semester course of lectures on Theoretical Physics, "dry, somewhat impersonal" according to Lise Meitner, "using no notes, never making mistakes, never faltering; the best lecturer I ever heard" according to an English participant, 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". Planck did not establish an actual "school", the number of his graduate students was only about twenty all together, among them the following:

Max Abraham 1897 (1875 - 1922)
Moritz Schlick 1904 (1882 - 1936)
Walther Meißner 1906 (1882 - 1974)
Max von Laue 1906 (1879 - 1960)
Fritz Reiche 1907 (1883 - 1960)
Walter Schottky 1912 (1886 - 1976)
Walther Bothe 1914 (1891 - 1957)

Black-body radiation
In 1894 Planck turned his attention to the problem of black-body radiation. He had been commissioned by electric companies to discover how to create the most light from lightbulbs with the minimum energy. The problem had already 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 (e.g., the colour of the light) and the temperature of the body?

The question had been explored experimentally, but the Rayleigh-Jeans law, derived from classical physics, failed to explain the observed behaviour at high frequencies, where it predicted an unphysical divergence of the energy density towards infinity (the ultraviolet catastrophe). Wilhelm Wien proposed Wien's law, which correctly predicted the behaviour at high frequencies, but failed at low frequencies.

By interpolating between the laws of Wien and Rayleigh-Jeans Planck found the famous Planck black-body radiation law which described the experimentally observed black-body spectrum very well; it was first proposed in a meeting of the DPG on October 19, 1900 and published in 1901.

By 14 December 1900, he was already able to present a theoretical derivation of the law, but this required him to use ideas from statistical mechanics, as introduced by Boltzmann. So far he had been holding a strong aversion against any statistical interpretation of the Second law of thermodynamics which he regarded as of an axiomatic nature: "... an act of despair ... I was ready to sacrifice any of my previous convictions about physics ..."

The central assumption behind his derivation was the supposition that the electromagnetic energy could be emitted only in quantized form, in other words, the energy could only be a multiple of an elementary unit E = h?, where h is Planck's constant, also known as Planck's action quantum (introduced already in 1899), and ? is the frequency of the radiation.

At first Planck considered that the quantisation was only as "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 (however, Ludwig Boltzmann had already in 1877, in a theoretical paper, been discussing the possibility that the energy states of a physical system could be discrete). It was in recognition of this accomplishement that Planck was awarded the Nobel prize for physics in 1918.

The discovery of Planck's constant enabled him to define a new universal set of physical units (such as the Planck length and the Planck mass), all based on fundamental physical constants.

Subsequently, Planck tried to grasp the meaning of the 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 like Rayleigh, Jeans, and Lorentz set Planck's 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 are wrong."

Max Born wrote about Planck: "He was by nature and by the tradition of his family conservative, averse to revolutionary novelties and sceptical towards speculations. But his belief in the imperative power of logical thinking based on facts was so strong that he did not hesitate to express a claim contradicting to all tradition, because he had convinced himself that no other resort was possible."

Einstein and the Theory of Relativity
In 1905 the three epochal papers of the hitherto completely unknown 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.

However, Einstein's hypothesis of light quanta (photons), based on Philipp Lenard's 1902 discovery of the photoelectric effect, was initially rejected by Planck; he was unwilling to discard completely Maxwell's theory of electrodynamics. "The theory of light would be thrown back not by decades, but by centuries, into the age when Christian Huygens dared to fight against the mighty emission theory of Newton ..."

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 Nernst, in order to clarify the increasing number of contradictions, organised the First Solvay Conference (Brussels 1911); at this meeting Einstein was finally 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 for jointly playing music.

World War and Weimar Republic
At the onset of the First World War Planck was not immune to the general excitement of the public: "... besides of much horrible also much unexpectedly great and beautiful: the swift solution of the most difficult issues of domestic policy through arrangement of all parties... the higher esteem for all that is brave and truthful..." Admittedly, he refrained from the extremes of nationalism, e.g., he voted successfully for a scientific paper from Italy receiving a prize from the Prussian Academy of Sciences in 1915 (Planck was one of its four permanent presidents), although at that time Italy was about to join the Allies; nevertheless the infamous "Manifesto of the 93 intellectuals", a polemic pamphlet of war propaganda, was also signed by Planck, while Einstein retained a strictly pacifistic attitude which almost led to his imprisonment (from which he was only saved by his Swiss citizenship). But already in 1915 Planck revoked (after several meetings with Dutch physicist Lorentz) parts of the Manifesto, and in 1916 he signed a declaration against German annexionism.

In the turbulent post-war years, Planck, by 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 Organisation of German Science), which aimed at providing support for the destitute scientific research; the funds they could distribute were raised to a considerable part abroad. In this time Planck held leading positions also at Berlin University, the Prussian Academy of Sciences, the German Physical Society and the Kaiser-Wilhelm-Gesellschaft (which in 1948 became the Max-Planck-Gesellschaft); under such conditions he was hardly able to conduct any more research himself.

He became a member of the Deutsche Volks-Partei (German People's Party), the party of peace Nobel prize laureate Gustav Stresemann, which aspired to liberal aims for domestic policy and rather revisionistic aims for international politics. He disagreed with the introduction of universal suffrage and expressed later the view that the Nazi dictatorship was the result of "the ascent of the rule of the crowds".

Quantum mechanics
At the end of the 1920s Bohr, Heisenberg and Pauli had worked out the Copenhagen interpretation of quantum mechanics, but it was rejected by Planck, as also by Schrödinger and Laue; even Einstein had turned into a conservative. Heisenberg's matrix mechanics he called "disgusting", the Schrödinger equation he welcomed like salvation. He expected that wave mechanics would soon render quantum theory - his own child - unnecessary.

Scientific progress ignored his concerns. He experienced the truth of his own earlier observation from his struggle with the older views in his younger years: "A new scientific truth does not establish itself by its enemies being convinced and expressing their change of opinion, but rather by its enemies gradually dying out and the younger generation being taught the truth from the beginning".

Nazi dictatorship and Second World War
When the Nazis seized power in 1933, Planck had already reached the age of seventy-four; he had to witness how many Jewish friends and colleagues were expelled from their positions and humiliated, and how hundreds of scientists emigrated from Germany. Again he tried the "persevere and continue working" slogan and asked scientists who were considering emigration to stay in Germany. He still hoped that the crisis would abate soon and that the political situation would improve again. There was also a deeper argument against immigration: Emigrating German scientist would need to look for academic positions abroad, but these positions should better be reserved for those scientists with Jewish background, who had no chance of continuing to work in Germany.

Hahn asked Planck whether they should gather a number of 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."

Under Planck's leadership the Kaiser-Wilhelm-Gesellschaft (KWG) generally tried to avoid any open conflict with the Nazi regime; only in the case of Fritz Haber (who was the father of German chemical warfare in World War I, but Jewish) Planck went so far as to try to discuss the issue with Adolf Hitler himself, naturally without any success. In the following year of 1934, Haber died in exile.

One year later, Planck, being the president of the KWG since 1930, organised in a somewhat provocative style an official commemorative meeting for Haber. He also succeeded in secretely 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 put pressure on him to refrain from running for another term.

As the political climate in Germany gradually became more hostile, Johannes Stark, prominent exponent of Deutsche Physik ("German Physics", also called "Aryan Physics") attacked Planck, 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, but all they could find out was that he was "1/16 Jewish."

In 1938, Planck celebrated his eightieth birthday; the DPG held an official 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 - one year before the outbreak of a new war between France and Germany.

At the end of the same year the Prussian Academy lost its remaining independence and was taken over by loyal Nazis (Gleichschaltung); Planck protested by resigning his presidency. He continued to travel frequently, giving numerous public talks, such as his famous talk on Religion and Science, and five years later he was still sufficiently fit to climb to 3,000-meter peaks in the Alps.

During the Second World War, the increasing number of Allied bombing campaigns at Berlin forced Planck and his wife to leave the city temporarily 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 his entire scientific records and correspondence. Finally, he got into a dangerous situation in his rural retreat due to the rapid advance of the Allied armies from both sides. After the end of the war he was brought to a relative in Göttingen.

Final years
After the war, a number of German physicists assembled in Göttingen in order to reestablish the Kaiser-Wilhelm-Gesellschaft. In July of 1945, Planck agreed to act formally as its president, again. The British occupation authorities insisted on changing the name, and therefore in February 1948 the Max-Planck-Gesellschaft was established. Despite his deteriorating health, Planck resumed travelling in order to give public talks.

In 1946, he travelled to London on the occasion of the 300th birthday of Isaac Newton. He was the only German invited. On 1 April 1946, Planck was succeeded as president of the KWG by Otto Hahn. On 4 October 1947, he died, aged 89, from the consequences of a fall and several strokes.

 
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