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.
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,
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.
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.
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.
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.
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.
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.
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
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.
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
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.
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.
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.
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.
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.
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:
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?
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
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.
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 ..."
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.
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.
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.
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."
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."
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.
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 ..."
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.
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.
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.
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.
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
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.
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".
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.
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."
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.
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.
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."
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.
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.
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."
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.
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.
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.