# Max Planck

Max Karl Ernst Ludwig Planck (April 23, 1858October 4, 1947) 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.

## Origin 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 on April 23, 1858 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. In 1867 the family moved to Munich, where Planck attended high school, graduating early at age 16.

## Education

Planck was musically gifted: he took voice 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 didn't wish to discover new things, only to understand the known fundamentals of the field and began his studies in 1874 in Munich. With Jolly, Planck performed the only experiment of his entire intellectual 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). In June 1880 he presented his habilitation thesis, "Gleichgewichtszustände isotroper Körper in verschiedenen Temperaturen" (Equilibrium states of isotropic bodies at different temperatures).

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 January 10, 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 (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 scientist 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, 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 into 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 society 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". He did not establish an actual "school", the number of his graduate students was only about 20 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)

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. Wilhelm Wien proposed Wien's law, which correctly predicted the behaviour at high frequencies, but failed at low frequencies.

Planck interpolated between the laws of Wein and Rayleigh Jeans but found that he could only derive a satisfactory equation using what he thought was merely a mathematical trick, namely that light is only emitted in "packets". He did not for many years believe that these packets, known as quanta, corresponded with reality.

His famous Planck black-body radiation law 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.

A commemoration plaque for Max Planck on his discovery of Planck's constant, in front of Humboldt University, Berlin

By December 14, 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 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 ${\displaystyle E=h\nu }$, where ${\displaystyle h}$ is Planck's constant, also known as Planck's action quantum (introduced already in 1899), and ${\displaystyle \nu }$ 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 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), harbingered by Philipp Lenard's 1902 discovery of the photoelectric effect, was initially rejected by Planck; he was unwilling to sacrifice also 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 at 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, 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 Fritz Haber and he 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-Society (Kaiser-Wilhelm-Gesellschaft zur Förderung der Wissenschaften (KWG, Emperor-Wilhelm-Society for the advancement of science, which in 1948 became the Max-Planck-Gesellschaft); under such conditions he was hardly able to conduct anymore 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 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 Twenties 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 74; 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 "persevere and continue working" and asked scientists who were considering emigration to stay in Germany. In some cases he was successful with this request, such as in the case of Heisenberg.

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 other will come and speak against it, because they are eager to take over the positions of the others." In the case of Fritz Haber (who was the father of German chemical warfare in WWI, but Jewish), however, Planck went so far as to try to discuss the issue with 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 to enable 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 80th 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 5 years later he was sufficiently fit to climb to 3000-meter peaks in the Alps mountains.

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 went to London on the occasion of the 300th birthday of Isaac Newton. He was the only German invited.

On April 1st of 1946, Planck was succeded as president of the KWG by Otto Hahn.

On October 4th, 1947, he died from the consequences of a fall and several strokes.

## Honours and medals

• "Pour le Mérite" for Science and Arts 1915 (in 1930 Planck became chancellor of this order)
• Nobel prize for physics 1918 (awarded 1919)
• Adlerschild des Deutschen Reiches (1928)
• Max-Planck medal (1928, together with Einstein)
• Planck received honorary doctorates from the universities of Frankfurt, München (TH), Rostock, Berlin(TH), Graz, Athen, Cambridge, London and Glasgow
• The asteroid 1069 was given the name "Stella Planckia" (1938)