# Max Planck

Max Planck

Max Karl Ernst Ludwig Planck
Born

April 23, 1858
Kiel, Germany

Died October 4, 1947

Göttingen, Germany

Residence Germany
Nationality German
Field Physicist
Institutions University of Kiel
Humboldt-Universität zu Berlin
Georg-August-Universität Göttingen
Kaiser-Wilhelm-Gesellschaft
Alma mater Ludwig-Maximilians-Universität München
Notable students  Gustav Ludwig Hertz

Erich Kretschmann
Walther Meißner
Walter Schottky
Max von Laue
Max Abraham
Moritz Schlick
Walther Bothe

Known for Planck's constant, quantum theory
Notable prizes Nobel Prize in Physics (1918)
He was the father of Erwin Planck.

Max Karl Ernst Ludwig Planck (April 23, 1858 – October 4, 1947) was a German physicist who is widely regarded as one of the most significant scientists in history. He developed a simple but revolutionary concept that was to become the foundation of a new way of looking at the world, called quantum theory.

In 1900, to solve a vexing problem concerning the radiation emitted by a glowing body, he introduced the radical view that energy is transmitted not in the form of an unbroken (infinitely subdivisible) continuum, but in discrete, particle-like units. He called each such unit a quantum (the plural form being quanta). This concept was not immediately accepted by physicists, but it ultimately changed the very foundations of physics. Planck himself did not quite believe in the reality of this concept—he considered it a mathematical construct. In 1905, Albert Einstein used that concept to explain the photoelectric effect, and in 1913, Niels Bohr used the same idea to explain the structures of atoms. From then on, Planck's idea became central to all of physics. He received the Nobel Prize in 1918, and both Einstein and Bohr received the prize a few years later.

Planck was also a deeply religious man who believed that religion and science were mutually compatible, both leading to a larger, universal truth. By basing his convictions on seeking the higher truth, not on doctrine, he was able to stay open-minded when it came to formulating scientific concepts and being tolerant toward alternative belief systems.

## Life and work

### Early childhood

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.

Planck was born in Kiel to Johann Julius Wilhelm Planck and his second wife, Emma Patzig. He was the sixth child in the family, including two siblings from his father's first marriage. Among his earliest memories was the marching of Prussian and Austrian troops into Kiel during the Danish-Prussian War in 1864. In 1867, the family moved to Munich, and Planck enrolled in the Maximilians gymnasium. There 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 16. This is how Planck first came in contact with the field of physics.

### Education

Planck was extremely gifted when it came to music: He took singing lessons and played the piano, organ, and cello, and composed songs and operas. However, instead of music, he chose to study physics.

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. In 1874, he began his studies at the University of Munich. Under Jolly's supervision, Planck performed the only experiments of his scientific career: Studying the diffusion of hydrogen through heated platinum. He 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 the mathematician Karl Weierstrass. He wrote that Helmholtz was never quite prepared (with his lectures), spoke slowly, miscalculated endlessly, and bored his listeners, while Kirchhoff spoke in carefully prepared lectures, which were, however, dry and monotonous. Nonetheless, he soon became close friends with Helmholtz. While there, he mostly undertook a program of self-study of Rudolf Clausius's writings, which led him to choose heat theory as his field.

In October 1878, Planck passed his qualifying exams and in February 1879, defended his dissertation, Über den zweiten Hauptsatz der mechanischen Wärmetheorie (On the second fundamental theorem of the mechanical theory of heat). 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).

With the completion of his habilitation thesis, Planck became an unpaid private lecturer 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 heat theory and discovered one after the other the same thermodynamical formalism as Josiah Willard 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 at the University of Berlin—presumably thanks to Helmholtz's intercession—and by 1892 became a full professor. In 1907, Planck was offered Boltzmann's position in Vienna, but turned it down to stay in Berlin. During 1909, he was the Ernest Kempton Adams Lecturer in Theoretical Physics at Columbia University in New York City. He retired from Berlin on January 10, 1926, and was succeeded by 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. They had four children: 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. Several other professors of Berlin University lived nearby, among them the famous theologian Adolf von Harnack, who became a close friend of Planck. Soon the Planck home became a social and cultural center. Numerous well-known scientists—such as Albert Einstein, Otto Hahn, and Lise Meitner—were frequent visitors. 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 July 1909, Marie Planck died, possibly from tuberculosis. In March 1911, 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 son Erwin was taken prisoner by the French in 1914, and his son Karl was killed in action at Verdun in 1916. His daughter 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.

During World War II, Planck's house in Berlin was completely destroyed by bombs in 1944, and his youngest son, Erwin, was implicated in the attempt made on Hitler's life on July 20, 1944. Consequently, Erwin died a horrible death at the hands of the Gestapo in 1945.

### Professor at Berlin University

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." Thanks to his initiative, the various local Physical Societies of Germany merged in 1898 to form the German Physical Society (Deutsche Physikalische Gesellschaft, DPG), and Planck was its president from 1905 to 1909.

Planck started a six semester course of lectures on theoretical physics. Lise Meitner described the lectures as "dry, somewhat impersonal." An English participant, James R. Partington, wrote, "using no notes, never making mistakes, never faltering; the best lecturer I ever heard." He 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 20 altogether. Among his students were the following individuals. The year in which each individual achieved the highest degree is indicated after the person's name (outside the parentheses); the individual's year of birth and year of death are given within parentheses.

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)

This question had been explored experimentally, but the Rayleigh-Jeans law, derived from classical physics, failed to explain the observed behavior at high frequencies, where it predicted a divergence of the energy density toward infinity (the "ultraviolet catastrophe"). Wilhelm Wien proposed Wien's law, which correctly predicted the behavior at high frequencies but failed at low frequencies. By interpolating between the laws of Wien and Rayleigh-Jeans, Planck formulated the now-famous Planck's law of black-body radiation, 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 December 14, 1900, Planck 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 held a strong aversion to any statistical interpretation of the second law of thermodynamics, which he regarded as having an axiomatic nature. Compelled to use statistics, he noted: "… 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 electromagnetic energy could be emitted only in quantized form. In other words, the energy could only be a multiple of an elementary unit. Mathematically, this was expressed as:

$E = h \nu$

where $h$ is a constant that came to be called Planck's constant (or Planck's action quantum), first introduced in 1899, and $\nu$ is the frequency of the radiation. Planck's work on quantum theory, as it came to be known, was published in the journal Annalen der Physik. His work is summarized in two books Thermodynamik (Thermodynamics) (1897) and Theorie der Wärmestrahlung (theory of heat radiation) (1906).

At first, Planck considered that quantization was only "a purely formal assumption … actually I did not think much about it…" This assumption, incompatible with classical physics, is now regarded as the birth of quantum physics and the greatest intellectual accomplishment of Planck's career. (However, in a theoretical paper published in 1877, Ludwig Boltzmann had already been discussing the possibility that the energy states of a physical system could be discrete.) In recognition of this accomplishment, 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 Planck length and Planck mass—all based on fundamental physical constants.

Subsequently, Planck tried to integrate the concept of energy quanta with classical physics, 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—including Lord Rayleigh, James Jeans, and Hendrik Lorentz—set Planck's constant to zero, in an attempt 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 skeptical 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.

To explain the photoelectric effect (discovered by Philipp Lenard in 1902), Einstein proposed that light consists of quanta, which he called photons. Planck, however, initially rejected this theory, as he was unwilling to completely discard Maxwell's theory of electrodynamics. Planck wrote, "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 Isaac Newton …"

In 1910, Einstein pointed out the anomalous behavior of specific heat at low temperatures as another example of a phenomenon that defies explanation by classical physics. To resolve the increasing number of contradictions, Planck and Walther Nernst organized the First Solvay Conference in Brussels in 1911. At this meeting, Einstein was finally able to convince Planck.

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

### World War I and the 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…"

He refrained from the extremes of nationalism. For instance, he voted successfully for a scientific paper from Italy to receive 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. Einstein, on the other hand, retained a strictly pacifist attitude, which almost led to his imprisonment, from which he was saved only by his Swiss citizenship. But already in 1915, Planck revoked parts of the Manifesto, (after several meetings with Dutch physicist Lorentz), and in 1916, he signed a declaration against the German policy of annexation.

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 Organization of German Science), which aimed at providing support for the destitute scientific research. They obtained a considerable portion of their funds from 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 (KWG, which in 1948 became the Max Planck Gesellschaft). Under such circumstances, he himself could hardly conduct any more research.

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 revisionist 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, Werner Heisenberg, and Wolfgang Pauli had worked out the Copenhagen interpretation of quantum mechanics. It was, however, rejected by Planck, as well as Schrödinger and Laue. Even Einstein had rejected Bohr's interpretation. Planck called Heisenberg's matrix mechanics "disgusting," but he gave the Schrödinger equation a warmer reception. He expected that wave mechanics would soon render quantum theory—his own brainchild—unnecessary.

Nonetheless, scientific progress ignored Planck's concerns. He experienced the truth of his own earlier concept, after his struggle with the older views. He wrote, "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 World War II

When the Nazis seized power in 1933, Planck was 74. He witnessed many Jewish friends and colleagues expelled from their positions and humiliated, and 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 hoped the crisis would abate soon and the political situation would improve again. There was also a deeper argument against emigration: Emigrating non-Jewish scientists would need to look for academic positions abroad, but these positions better served Jewish scientists, who had no chance of continuing to work in Germany.

Hahn asked Planck to gather well-known German professors, to issue a public proclamation against the treatment of Jewish professors. Planck, however, 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." Although, 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." Under Planck's leadership, the KWG avoided open conflict with the Nazi regime. One exception was Fritz Haber. Planck tried to discuss the issue with Adolf Hitler but was unsuccessful. In the following year, 1934, Haber died in exile.

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 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, 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, 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. At the end of 1938, the Prussian Academy lost its remaining independence and was taken over by 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." Five years later, he was still sufficiently fit to climb 3,000-meter peaks in the Alps.

During the Second World War, the increasing number of Allied bombing campaigns against 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 all his scientific records and correspondence. Finally, he was in a dangerous situation in his rural retreat during the rapid advance of Allied armies from both sides. After the end of the war, Planck, his second wife, and their son Herrmann moved to Göttingen, where he died on October 4, 1947.

Max Planck commemorated on the German 2 Mark Coin

## Religious views

Max Planck was a devoted Christian from early life to death. As a scientist, however, he was very tolerant toward other religions and alternate views, and was discontent with the church organization's demands for unquestioning belief. He noted that "natural laws … are the same for men of all races and nations."

Planck regarded the search for universal truth as the loftiest goal of all scientific activity. Perhaps foreseeing the central role it now plays in current thinking, Planck made great note of the fact that the quantum of action retained its significance in relativity because of the relativistic invariance of the Principle of Least Action.

Max Planck's view of God can be regarded as pantheistic, with an almighty, all-knowing, benevolent but unintelligible God who permeates everything, manifest by symbols, including physical laws. His view may have been motivated by an opposition—like that of Einstein and Schrödinger—to the positivist, statistical, subjective universe of scientists such as Bohr, Heisenberg, and others. Planck was interested in truth and the Universe beyond observation, and he objected to atheism as an obsession with symbols.[1]

## Legacy

Planck was the very first scientist to contradict the physics established by Newton. This is why all physics before Planck is called "classical physics," while all physics after him is called "quantum physics." In the classical world, energy is continuous; in the quantum world, it is discrete. On this simple insight of Planck's was constructed all of the new physics of the twentieth century.

Planck had the firm conviction that religion and science are mutually compatible, both leading to a higher, universal truth that embraces everything. His convictions were based on seeking that higher truth, not on doctrine, and he was aware that science itself had just started on the quest. This allowed him to keep an open mind when young, in terms of scientific theory and to be tolerant toward alternative belief systems. His scientific views were, of course, in the classical mode of solids and forces—the quantum view of a much more sophisticated reality was not available to him. For he had just started the revolution and had second thoughts about the "reality" of his own concept of particle-like energy.

Unlike religion with its great leaps, science proceeds by baby steps. The small step taken by Planck was the first of the many needed to reach the current "internal wave and external particle" view of modern physics a century later.

## Honors and medals

• "Pour le Mérite" for Science and Arts 1915 (in 1930 he became chancellor of this order)
• Nobel Prize in Physics 1918 (awarded 1919)
• Lorentz Medal 1927
• Adlerschild des Deutschen Reiches (1928)
• Max Planck medal (1929, together with Einstein)
• Planck received honorary doctorates from the universities of Frankfurt, Munich (TH), Rostock, Berlin(TH), Graz, Athens, Cambridge, London, and Glasgow
• The asteroid 1069 was given the name "Stella Planckia" (1938)

## Planck units

• Planck time
• Planck length
• Planck temperature
• Planck current
• Planck power
• Planck density
• Planck mass

## Notes

1. Adherents.com, The Religious Affiliation of Physicist Max Planck. Retrieved July 16, 2007.

## References

• Gamow, George. 1966. Thirty Years That Shook Physics: The Story of Quantum Theory. Garden City, NY: Doubleday.
• Heilbron, J. L. 2000. The Dilemmas of an Upright Man: Max Planck and the Fortunes of German Science. Cambridge, MA: Harvard University Press. ISBN 0-674-00439-6
• Rosenthal-Schneider, Ilse. 1980. Reality and Scientific Truth: Discussions with Einstein, von Laue, and Planck. Wayne State University. ISBN 0-8143-1650-6