Max Planck

Max Planck

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Max Planck, the son of a professor of law, was born in Kiel, Germany, on 23rd April, 1858. He studied physics at the University of Munich (1874-1877) and the University of Berlin (1877-78) before receiving his doctorate in 1879.

He taught at the University of Munich before being appointed associate professor at the University of Kiel. Planck researched into the manner in which heated bodies radiate energy led him to argue that energy is emitted only in indivisible amounts, called "quanta", the magnitudes of which are proportional to the frequency of the radiation.

Planck's theories were in conflict with classical physics and his work is said to have marked the commencement of modern science. Albert Einstein used Planck's quantum theory to explain photoelectricity and Niels Bohr successfully applied the quantum theory to the atom. In 1918 Planck was awarded the Nobel Prize for Physics.

In 1930 he was appointed as president of the Kaiser Wilhelm Institute in 1930. An opponent of Adolf Hitler, Planck resigned his position in 1937 in protest against the decision by Bernard Rust, the minister of education, to sack Jewish teachers from Germany's universities.

Planck refused to work on any of Germany's war research projects. In 1944 Planck's youngest son, Erwin Planck was arrested and charged with involvement in the July Plot against Adolf Hitler. He was killed while being tortured by the Gestapo in 1945. Max Planck died on 4th October, 1947.

A number of people who deserve to be taken seriously have independently warned me not to stay in Berlin for the time being and, especially, to avoid all public appearances in Germany. I am said to be among those whom the nationalists have marked for assassination. Of course, I have no proof, but in the prevailing situation it seems quite plausible.

The trouble is that the newspapers have mentioned my name too often, this mobilizing the rabble against me. I have no alternative but to be patient - and to leave the city. I do urge you to get as little upset over the incident as I myself.

An important scientific innovation rarely makes its way by gradually winning over and converting its opponents: it rarely happens that Saul becomes Paul. What does happen is that its opponents gradually die out, and that the growing generation is familiarized with the ideas from the beginning.

Max Planck

M ax Karl Ernst Ludwig Planck was born in Kiel, Germany, on April 23, 1858, the son of Julius Wilhelm and Emma (née Patzig) Planck. His father was Professor of Constitutional Law in the University of Kiel, and later in Göttingen.

Planck studied at the Universities of Munich and Berlin, where his teachers included Kirchhoff and Helmholtz, and received his doctorate of philosophy at Munich in 1879. He was Privatdozent in Munich from 1880 to 1885, then Associate Professor of Theoretical Physics at Kiel until 1889, in which year he succeeded Kirchhoff as Professor at Berlin University, where he remained until his retirement in 1926. Afterwards he became President of the Kaiser Wilhelm Society for the Promotion of Science, a post he held until 1937. The Prussian Academy of Sciences appointed him a member in 1894 and Permanent Secretary in 1912.

Planck’s earliest work was on the subject of thermodynamics, an interest he acquired from his studies under Kirchhoff, whom he greatly admired, and very considerably from reading R. Clausius’ publications. He published papers on entropy, on thermoelectric ity and on the theory of dilute solutions.

At the same time also the problems of radiation processes engaged his attention and he showed that these were to be considered as electromagnetic in nature. From these studies he was led to the problem of the distribution of energy in the spectrum of full radiation. Experimental observations on the wavelength distribution of the energy emitted by a black body as a function of temperature were at variance with the predictions of classical physics. Planck was able to deduce the relationship between the ener gy and the frequency of radiation. In a paper published in 1900, he announced his derivation of the relationship: this was based on the revolutionary idea that the energy emitted by a resonator could only take on discrete values or quanta. The energy for a resonator of frequency v is hv where h is a universal constant, now called Planck’s constant.

This was not only Planck’s most important work but also marked a turning point in the history of physics. The importance of the discovery, with its far-reaching effect on classical physics, was not appreciated at first. However the evidence for its validi ty gradually became overwhelming as its application accounted for many discrepancies between observed phenomena and classical theory. Among these applications and developments may be mentioned Einstein’s explanation of the photoelectric effect.

Planck’s work on the quantum theory, as it came to be known, was published in the Annalen der Physik. His work is summarized in two books Thermodynamik (Thermodynamics) (1897) and Theorie der Wärmestrahlung (Theory of heat radiat ion) (1906).

He was elected to Foreign Membership of the Royal Society in 1926, being awarded the Society’s Copley Medal in 1928.

Planck faced a troubled and tragic period in his life during the period of the Nazi government in Germany, when he felt it his duty to remain in his country but was openly opposed to some of the Government’s policies, particularly as regards the persecuti on of the Jews. In the last weeks of the war he suffered great hardship after his home was destroyed by bombing.

He was revered by his colleagues not only for the importance of his discoveries but for his great personal qualities. He was also a gifted pianist and is said to have at one time considered music as a career.

Planck was twice married. Upon his appointment, in 1885, to Associate Professor in his native town Kiel he married a friend of his childhood, Marie Merck, who died in 1909. He remarried her cousin Marga von Hösslin. Three of his children died young, leaving him with two sons.

He suffered a personal tragedy when one of them was executed for his part in an unsuccessful attempt to assassinate Hitler in 1944.

He died at Göttingen on October 4, 1947.

From Nobel Lectures, Physics 1901-1921, Elsevier Publishing Company, Amsterdam, 1967

This autobiography/biography was written at the time of the award and first published in the book series Les Prix Nobel. It was later edited and republished in Nobel Lectures. To cite this document, always state the source as shown above.

For more updated biographical information, see: Planck, Max, Scientific Autobiography and Other Papers. Philosophical Library, New York, 1949.

Copyright © The Nobel Foundation 1918

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Max Karl Ernst Ludwig Planck

Max Planck came from an academic family, his father Julius Wilhelm Planck being Professor of Constitutional Law in the University of Kiel at the time of his birth, and both his grandfather and great-grandfather had been professors of theology at Göttingen. His mother, Emma Patzig, was his father's second wife. Both Max's parents were relatively old when he was born, his father being 41 and his mother being 37 . He was born into a large family, being his father's sixth child ( two of the children were from his first marriage to Mathilde Voigt ) , and he was brought up in a tradition which greatly respected scholarship, honesty, fairness, and generosity. The values he was given as a young child quickly became the values that he would cherish throughout his life, showing the utmost respect for the institutions of state and church.

Max began his elementary schooling in Kiel. In the spring of 1867 his family moved to Munich when his father was appointed Professor there. This city provided a stimulating environment for the young boy who enjoyed its culture, particularly the music, and loved walking and climbing in the mountains when the family took excursions to Upper Bavaria. He attended secondary school there, entering the famous Maximilian Gymnasium in May 1867 . He did well at school, but not brilliantly, usually coming somewhere between third and eighth in his class. Music was perhaps his best subject and he was awarded the school prize in catechism and good conduct almost every year. One might have expected him to excel in mathematics and science, but certainly in his early school years, although he did well, there was no sign of outstanding talent in these subjects. However, towards the end of his school career, his teacher Hermann Müller raised his level of interest in physics and mathematics, and he became deeply impressed by the absolute nature of the law of conservation of energy. A school report for 1872 reads:-

In July 1874 , at the age of 16 , he passed his school leaving examination with distinction but, having talents for a wide variety of subjects particularly music ( he played piano and organ extremely well ) , he still did not have a clear idea of what he should to study at university. Before he began his studies at the University of Munich he discussed the possibility of a musical career with a musician who told him that if he had to ask the question he had better study something else.

He entered the University of Munich on 21 October 1874 and was taught physics by Philipp von Jolly and Wilhelm Beetz, and mathematics by Ludwig Seidel and Gustav Bauer. After taking mostly mathematics classes at the start of his course, he enquired about the prospects of research in physics from Philipp von Jolly, the professor of physics at Munich, and was told that physics was essentially a complete science with little prospect of further developments. Fortunately Planck decided to study physics despite the bleak future for research that was presented to him.

In [ 7 ] Planck describes why he chose physics:-

He was ill during the summer term of 1875 which caused him to give up studying for a while. It was customary for German students to move between universities at this time and indeed Planck moved to study at the University of Berlin from October 1877 where his teachers included Weierstrass, Helmholtz and Kirchhoff. He later wrote that he admired Kirchhoff greatly but found him dry and monotonous as a teacher. However it is likely to be the contrast between the research attitude of his teachers at Munich and those at Berlin which prompted the quote we gave above ( made many years later ) . One important part of his education at Berlin came, however, through independent study for at this stage he read Rudolf Clausius's articles on thermodynamics. Again the absolute nature of the second law of thermodynamics impressed him.

Planck returned to Munich and received his doctorate in July 1879 at the age of 21 with a thesis on the second law of thermodynamics entitled On the Second Law of Mechanical Theory of Heat. The award of the doctorate was made "summa cum laude'' on 28 July 1879 . Following this Planck continued to work for his habilitation which was awarded on 14 June 1880 , after he had submitted his thesis on entropy and the mechanical theory of heat, and he became a Privatdozent at Munich University. Such a teaching post was unpaid so Planck received no income to support himself. He lived with his parents during the five years that he held this post, but felt rather guilty that he was continuing to live at their expense. During this time he became friends with Carl Runge and it turned into a long lasting and academically fruitful friendship.

On 2 May 1885 Planck was appointed extraordinary professor of theoretical physics in Kiel and held this chair for four years. This now made him financially secure so he could now marry Marie Merck whom he had known for many years. She was the daughter of a Munich banker, and the pair were married on 31 March 1887 . He now worked on thermodynamics publishing three excellent papers on applications to physical chemistry and thermoelectricity.

After the death of Kirchhoff in October 1887 , the University of Berlin looked for a world leading physicist to replace him and to become a colleague of Helmholtz. They approached Ludwig Boltzmann but he was not interested, and the same proved true for Heinrich Hertz. In 1888 the appointment of Planck was proposed by the Faculty of Philosophy at the University of Berlin, strongly recommended by Helmholtz:-

Planck was appointed as an extraordinary professor of theoretical physics at the University of Berlin on 29 November 1888 , at the same time became director of the Institute for Theoretical Physics. He was promoted to ordinary professor on 23 May 1892 and held the chair until he retired on 1 October 1927 . His colleagues and friends included Émile du Bois-Reymond ( the famous physiologist and brother of Paul du Bois-Reymond ) , Helmholtz, Pringsheim, Wien, as well as theologians, historians, and philologists. He continued to indulge his passion for music having a harmonium built with 104 tones in each octave, and holding concerts in his own home.

While in Berlin Planck did his most brilliant work and delivered outstanding lectures. He studied thermodynamics, in particular examining the distribution of energy according to wavelength. By combining the formulae of Wien and Rayleigh, Planck announced in October 1900 a formula now known as Planck's radiation formula. Within two months Planck made a complete theoretical deduction of his formula renouncing classical physics and introducing the quanta of energy. On 14 December 1900 he presented his theoretical explanation involving quanta of energy at a meeting of the Physikalische Gesellschaft in Berlin. In doing so he had to reject his belief that the second law of thermodynamics was an absolute law of nature, and accept Boltzmann's interpretation that it was a statistical law. In a letter written a year later Planck described proposing the theoretical interpretation of the radiation formula saying:-

For many years, [ my aim ] was to solve the problem of energy distribution in the normal spectrum of radiating heat. After Gustav Kirchhoff has shown that the state of the heat radiation which takes place in a cavity bounded by any emitting and absorbing material at uniform temperature is totally independent of the nature of the material, a universal function was demonstrated which was dependent only on temperature and wavelength, but not in any way on the properties of the material. The discovery of this remarkable function promised deeper insight into the connection between energy and temperature which is, in fact, the major problem in thermodynamics and so in all of molecular physics. .

At that time I held what would be considered today naively charming and agreeable expectations, that the laws of classical electrodynamics would, if approached in a sufficiently general manner avoiding special hypotheses, allow us to understand the most significant part of the process we would expect, and so to achieve the desired aim. .

[ A number of different approaches ] showed more and more clearly that an important connecting element or term, essential to completely grasp the basis of the problem, had to be missing. .

I was busy. from the day I [ established a new radiation formula ] , with the task of finding a real physical interpretation of the formula, and this problem led me automatically to consider the connection between entropy and probability, that is, Boltzmann's train of ideas eventually after some weeks of the hardest work of my life, light entered the darkness, and a new inconceivable perspective opened up before me. .

Because [ a constant in the radiation law ] represents the product of energy and time . I described it as the elementary quantum of action. . As long as it was looked on as infinitely small . everything was fine but in the general case, however, a gap opened wide somewhere or other, which became more striking the weaker and faster the vibrations considered. That all efforts to bridge the chasm foundered soon left little doubt. Either the quantum of action was a fictional quantity, then the whole deduction of the radiation law was essentially an illusion representing only an empty play on formulas of no significance, or the derivation of the radiation law was based on a sound physical conception. In this case the quantum of action must play a fundamental role in physics, and here was something completely new, never heard of before, which seemed to require us to basically revise all our physical thinking, built as this was, from the time of the establishment of the infinitesimal calculus by Leibniz and Newton, on accepting the continuity of all causative connections. Experiment decided it was the second alternative.

Planck who was 42 years old when he made his historic quantum announcement, took only a minor part in the further development of quantum theory. This was left to Einstein with theories of light quanta, Poincaré who proved mathematically that the quanta was a necessary consequence of Planck's radiation law, Niels Bohr with his theory of the atom, Paul Dirac and others. Sadly his life was filled with tragedy in the years following his remarkable initiation of the study of quantum mechanics. His wife Marie died on 17 October 1909 . They had four children two sons Erwin and Karl, and twin daughters Margarete and Emma. Two years after the death of his first wife, Planck married again, to Marga von Hösslin the niece of Marie his first wife, on 14 March 1911 . They had one child, a son Hermann. Karl, the younger of Planck's sons from his first marriage, was killed in 1916 during World War I. Both his daughters died in childbirth, Margarete in 1917 and Emma in 1919 . His son Erwin became his best friend and advisor, but as we relate below Erwin died in even more terrible circumstances.

Planck always took on administrative duties, in addition to his research activities, such as Secretary of the Mathematics and Natural Science Section of the Prussian Academy of Sciences, a post he held from 1912 until 1943 . He had been elected to the Academy in 1894 . Planck was much involved with the German Physical Society, being treasurer and a committee member. He was chairman of the Society from 1905 to 1908 and then again from 1915 to 1916 . Planck was also honoured by being elected an honorary member in 1927 . Two years later an award, the Max Planck Medal, was established and Planck himself became the first recipient. He was on the committee of the Kaiser Wilhelm Gesellschaft, the main German research organisation, from 1916 and was president of the Society from 1930 until 1937 ( it was renamed the Max Planck Society ) . This was the time that the Nazis rose to power, and he tried his best to prevent political issues to take over from scientific ones. He could not prevent the reorganisation of the Society by the Nazis and refused to accept the presidency of the reorganised Society.

He remained in Germany during World War II through what must have been times of the deepest difficulty. In 1942 he explained why he was still in Berlin:-

History of Emotions

Do emotions have a history? And do they make history? These are the questions that the new Research Center “History of Emotions” seeks to answer. To explore the emotional orders of the past, historians work closely with psychologists and education specialists. In addition, they draw on the expertise of anthropologists, sociologists, musicologists and scholars working on literature and art. Our research rests on the assumption that emotions – feelings and their expressions – are shaped by culture and learnt/acquired in social contexts. What somebody can and may feel (and show) in a given situation, towards certain people or things, depends on social norms and rules. It is thus historically variable and open to change.

A central objective of the Research Center is to trace and analyse the changing norms and rules of feeling. We therefore look at different societies and see how they develop and organise their emotional regimes, codes, and lexicons. Research concentrates on the modern period (18th to 20th centuries).
Geographically, it includes both western and eastern societies (Europe, North America and South Asia).

Special attention is paid to institutions that have a strong impact on human behaviour and its emotional underpinnings, such as the family, law, religion, the military, the state.
Equally important to the Center&aposs research programme is the historical significance of emotions. Emotions are said to motivate human action and thus influence social, political, and economic developments.

In this capacity, they are and have been a privileged object of manipulation and instrumentalisation. Who appealed to what kind of emotions for what reasons? To what degree did emotions play a part in/contribute to the formation and dissolution of social groups, communities and movements? These and other questions open doors to a new field of research, one which aims to thoroughly historicise a crucial element of human development.


June 1975

The joint proposal submitted by the Biological-Medical and the Humanities Sections of the Max Planck Society to come up with a plan for a time-limited project group for psychology and language research was approved, provided a suitable project leader could be found.

June 1976

Following this joint proposal, in June 1976 the Senate decided to establish a Project Group for Psycholinguistics for a period of five years. The Dutch psychologist Willem Levelt was asked to organize and set up the group. At his request, Nijmegen was selected as the location.

April 1977

Plans progressed quickly, and in April 1977 the first twenty staff members (half of whom were scientists) were able to start their work in the Canisius building, a former Jesuit high school in Nijmegen.

The project group was supported by a very active Advisory Board under the leadership of Jerome Bruner, then professor of psychology at Oxford University.

March 1979

As early as 1979, the Senate of the Max Planck Society took the decision to transform the project group into a fully-fledged Institute for Psycholinguistics and to appoint Willem Levelt as a scientific member of the Max Planck Society and director of the Institute.

January 1980

The Institute was formally established in Nijmegen.

March 1980

The Institute was officially opened on 18 March by Professor Reimar Lüst, President of the Max Planck Society.
The Institute had three permanent research groups (rather than independent departments): Language Production, Language Comprehension, and Language Acquisition. Willem Levelt led the Institute's Language Production Research Group, Wolfgang Klein the Acquisition research group and both together the Comprehension research group.

July 1980

Wolfgang Klein was appointed as scientific member of the Max Planck Society and named co-director of the Institute.

1983 Nijmegen Lectures

Together with the Interfaculty Unit for Language and Speech of the Catholic University of Nijmegen (now Radboud University Nijmegen), the Institute organized two seminars as part of the new, yearly "Nijmegen Lectures" event.

  • In May, Barbara Hall Partee of the University of Massachusetts gave a week-long series on formal semantics.
  • In September, Albert M. Galaburda of Harvard Medical School gave a week-long series on the anatomy of brain structures that are required to support human linguistic capacity.

July 1984

The fifth year of the Max Planck Institute for Psycholinguistics saw the completion of its troika structure. The British psychologist William Marslen-Wilson was appointed as the Institute's third co-director, and took the helm of the Language Comprehension research group. His appointment represented a major expansion of the Institute's speech laboratory, both in terms of personnel and equipment.


Linguist Manfred Bierwisch of the Berlin Academy of Sciences, GDR is appointed as External Scientific Member of the Institute, the first such appointment in the whole Max Planck Society.

April 1986

The newly constructed Institute building on the Nijmegen University campus was officially opened in Nijmegen, on Wundtlaan 1, by the President of the Max Planck Society, Heinz Staab. The official opening addresses were followed by an open house researchers and technicians presented examples of their work and demonstrated some of the facilities.

“The Institute has now reached the shape which we hope it will essentially keep over the coming years. The end of its youth and the transition to a more sedate period was marked in April 1986 by the official opening of the new Institute building, to which we had already moved by the end of the preceding year and in which after the usual initial disturbance, everything and everybody is working again.”
Wolfgang Klein, Managing Director.

The main hall of the new building also harbors a "Scientists Gallery", displaying in bronze some of the pioneers of psycholinguistics including in front of a slab with quotes from their writings.

July 1987

William Marslen-Wilson returned to the University of Cambridge but stayed closely involved with the Institute through a series of research projects.


Uli Frauenfelder is appointed leader of a newly established Max Planck Junior Research Group on Lexical processing in language comprehension.

December 1993

Anne Cutler accepted the appointment as scientific member of the Max Planck Society. She was also appointed director of the Institute, taking responsibility for research into speech and language comprehension.

July 1994

Stephen C. Levinson was appointed scientific member and director at the Institute, leading the new Cognitive Anthropology Research Group. Its programme of field research institutionalised the long-standing interest of the Institute in how human language capacity copes with the huge variety of natural languages.

In this year, the Institute consolidated its new structure. It now had four permanent research areas: language production, language comprehension, language acquisition and cognitive anthropology.


Early 1997, a group of PhD students took the initiative to launch a series in which they could publish their theses the "MPI Series in Psycholinguistics". This became the Institute's standard platform for publishing PhD theses it makes the quality and diversity of dissertation research conducted at the Institute more visible to the outside world.

September 1997

The substantially enlarged Institute’s building was reopened by Dr Bludau, Secretary General of the Max Planck Society, following a full year’s reconstructionwork.


The Cognitive Anthropology Research Group led by Stephen C. Levinson was transformed into the Department of Language and Cognition at the Institute.

June 1999

The F.C. Donders Centre for Cognitive Neuroimaging was established. This Centre is a joint venture of the Max Planck Institute for Psycholinguistics and the Universities of Nijmegen (Radboud University), Utrecht, Maastricht and Brabant. Its founding director is Peter Hagoort, Professor of Cognitive Neuroscience at Radboud University.


Pieter Muysken was appointed as External Scientific Member.

July 2002

Start of Michael Dunn's Research Group on Evolutionary Processes in Language and Culture.

November 2005

The Institute marked its 25th anniversary by hosting the “Reimar Lust Lecture”, presented by Peter Hagoort, in the presence of the former Max Planck President.


The Institute’s founding director, Willem Levelt, retired as head of the Language Production Group. Peter Hagoort succeeded him as scientific member of the Max Planck Society and director at the Institute. Hagoort also continued to head the Donders Centre for Cognitive Neuroimaging at Radboud University Nijmegen.

June 2008

Start of Andrea Weber's Research Group on Adaptive Listening.

October 2008

Robert Van Valin started his Max Planck Fellowship Group on Syntax, Typology and Information Structure.

Start of Daniel Haun's research group on Comparative Cognitive Anthropology.


Antje Meyer was appointed as Scientific Member and Max Planck Director, directing the newly established department on individual differences in language processing.

September 2009

The International Max Planck Research School (IMPRS) for Language Sciences is established as a joint venture of the Max Planck Institute for Psycholinguistics and two Radboud University partner institutes - the Donders Institute for Brain, Cognition and Behaviour and the Centre for Language Studies. The IMPRS offers a wide range of courses, training programmes, and networking opportunities to doctoral students of the participating organisations.


A new department on Language and Genetics, was founded, devoted to the study of genetic infrastructure that provides the brain with the capacity to support our language and communication skills. Simon Fisher was appointed as its Director and as Scientific Member of the Max Planck Society.


This year the MPI celebrated its 30th anniversary. To mark the occasion, Willem Levelt presented a preview of his book on the history of psycholinguistics, demonstrating that the history of our field goes back much further than is often assumed.


Anne Cutler, head of the Comprehension Department, retired as director of the Institute, taking up a research chair at the University of Western Sydney.

Researchers and staff at the MPI were deeply saddened to learn of the death of Melissa Bowerman, senior scientist emerita of the MPI's Language Acquisition Department. Melissa passed away unexpectedly on 31 October 2011 after a brief illness.


David Norris was appointed as External Scientific Member.

February 2015

Wolfgang Klein, co-founder of the Institute, retires as Director of the Language Acquisition Department.

June 2015

The new wing of the MPI building was opened by Princess Laurentien of the Netherlands. To celebrate the occasion, she planted the "Tree of Language". This brand-new wing is home to an extended auditorium, extra office space, new server rooms, a virtual reality suite, experiment rooms (including baby labs and EEG facilities) and, for the first time at our Institute, in-house molecular biology laboratories.

Following on from this official opening event, an open house for the general public attracted more than 600 visitors.

January 2016

Sonja Vernes was appointed as Max Planck Research Group leader.
Her research group “Neurogenetics of Vocal Communication” focuses on the study of vocal communication in mammals as a way to understand the biological basis of human speech and language and how this trait evolved.

September 2016

Caroline Rowland succeeded Wolfgang Klein as Max Planck Director and as Scientific Member, establishing a new Language Development Department, which addresses a central question in our field: How do infants acquire the intricate and highly complex system of natural language?

December 2017

Director Stephen C. Levinson retired as director of the Language and Cognition Department.


Peter Indefrey was appointed as Neural Dynamics of Language Production Research Group leader.

January 2020

Andrea Martin was appointed as Max Planck Research Group leader.
Her research group “Language and Computation in Neural Systems” is interested in how language is represented and processed in the mind and brain, and in discovering the computational mechanisms and principles that underlie language processing.

Andrea Ravignani was appointed as Max Planck Research Group leader.
His research group “Comparative Bioacoustics” investigates why humans and some other species are so skilled at vocal learning and rhythm, and how these capacities underlying speech and music may have evolved.

April 2021

Researchers and staff at the MPI were deeply saddened to learn of the death of Pieter Muysken on April 6th 2021. He was an External Scientific Member for the Max Planck Institute for Psycholinguistics, appointed to supplement our linguistic expertise.
Link to obituary

Max Planck: The Nature of Light

The Kaiser-Wilhelm-Gesellschaft ( Kaiser Wilhelm Society ) was founded on 11 January 1911 by August von Trott zu Solz, the Prussian Secretary of Cultural Affairs. Max Planck was on the committee of the Society from 1916 and it was this Society he addressed on the topic The Nature of Light. His address was published in English translation by Methuen & Co in 1925 .

The address was given at an interesting time in the development of ideas on the nature of light at just the time when quantum theory was being proposed and the lecture considers both the traditional and quantum-mechanical view.

Before giving the text of Planck's lecture, however, let us note that Planck was president of the Society from 1930 until 1937 and after his death, the Society was renamed the Max Planck Society for the Advancement of Science in 1948 .

The Nature of Light

One of the most important branches of work of this society ( the Kaiser-Wilhelm-Gesellschaft ) is the maintenance of a research laboratory for natural science. The society has, however, discovered the old truism that in its own sphere, as in all spheres of work, knowledge must precede application, and the more detailed our knowledge of any branch of physics, the richer and more lasting will be the results which we can draw from that knowledge.

In this respect, of all the branches of physics, there is no doubt that it is in optics that research work is most advanced, and, therefore, I am going to speak to you about the Nature of Light. I shall doubtless mention much that is familiar to each of you, but I shall also deal with newer problems still awaiting solution.

The first problem of physical optics, the condition necessary for the possibility of a true physical theory of light, is the analysis of all the complex phenomena connected with light, into objective and subjective parts. The first deals with those phenomena which are outside, and independent of, the organ of sight, the eye. It is the so-called light rays which constitute the domain of physical research. The second part embraces the inner phenomena, from eye to brain, and this leads us into the realms of physiology and psychology. It is not at all self-evident, from first principles, that the objective light rays can be completely separated from the sight sense, and that such a fundamental separation involves very difficult thinking cannot better be proved than by the following fact. Johann Wolfgang von Goethe was gifted with a very scientific mind ( though little inclined to consider analytical methods ) , and would never see a detail without considering the whole, yet he definitely refused, a hundred years ago, to recognize this difference. Indeed, what assertion could give a greater impression of certainty to the unprejudiced than to say that light without the perceptive organ is inconceivable? But, the meaning of the word light in this connection, to give it an interpretation that is unassailable, is quite different from the light ray of the physicist. Though the name has been retained for simplicity, the physical theory of light or optics, in its most general sense, has as little to do with the eye and light perceptions as the theory of the pendulum has to do with sound perception. This ignoring of the sense-perceptions, this restricting to objective real phenomena, which doubtless, from the point of view of immediate interest, means a considerable sacrifice made to pure knowledge, has prepared a way for a great extension of the theory. This theory has surpassed all expectations, and yielded important results for the practical needs of mankind.

A very significant discovery relating to the physical nature of light rays was that light, emanating from stars or terrestrial sources, takes a certain measurable time to travel from the position of the source to the place at which it is observed. What is this something which spreads through empty space and moves through the atmosphere at the enormous, speed of 300 , 000 kilometres per second? Isaac Newton, the founder of classical mechanics, made the most simple and obvious assumption that there are certain infinitesimally small corpuscles which are sent out in all directions with that velocity from a source of light, e.g. a glowing body. These particles are different for different colours. This provides a striking proof that a high authority can exercise a hindrance to the development of even this most exact of all natural sciences, for Newton's emanation theory was able to hold the field for a whole century, although another distinguished investigator, Christian Huygens, had from the first opposed it with his much more suitable undulation theory. Huygens did not place the velocity of light on a par with that of wind, as Newton did, but on a par with the velocity of sound, in which the velocity of propagation is something quite different from that of air movements. Consider the air surrounding a sounding instrument or the surface of water into which a stone has been thrown. It is not the air or water particles themselves that spread out in all directions with equal velocity, but the intensification and rarefaction, or wave crests and troughs in other words, it is not with matter itself, but with a certain state of matter that we are concerned. To this end, Huygens formulated an ideal substance, uniformly occupying all space, as a foundation for his theory. This is the light-ether, the waves of which produce light perceptions in the eye, as air waves give rise to sound perceptions in the ear. The wave-length or frequency determines the colour in the same manner as it determines the pitch in sound. After a bitter controversy, Huygens's theory ultimately superseded that of Newton. This was due to the fact, amongst many others, that when two light rays of the same colour are superposed and made to travel on the same path, the intensities are not always simply additive, but under certain conditions the intensity is decreased and may even vanish. This last phenomenon, interference, can be straightway explained on Huygens's assumption that in every case the wave crests of one ray coincide with the wave troughs of the other ray. Newton's emanation theory naturally contradicts this, since it is impossible for two similar corpuscles travelling with the same speed in the same direction to neutralize one another.

A more significant fundamental view of the nature of light was obtained through the discovery of the identity of light and heat rays, and this was the first step on the way towards the complete separation of the science from the sense-perceptions. The cold light rays of the moon are physically of exactly the same nature as the black heat rays emitted from a stove, except that they are of much shorter wavelength. It is only natural that this assertion at first excited much discussion, and it is characteristic that Melloni, who played a great part in the verification of this fact, set out originally to disprove it. It must be remembered that here, as in all inductive results, a logical and conclusive proof cannot be given it can only be shown that all laws which hold for light rays, namely those of reflection, refraction, interference, polarization, dispersion, emission, and absorption, are also true for heat rays. Whoever refuses to admit the identity of the two kinds of rays in spite of this, could certainly never be accused on this account of a logical fallacy for he would always maintain that it is still possible in the future for an essential difference to be discovered. The practical weakness of his position is that he is, consequently, compelled to renounce a series of important conclusions, immediately deduced from the theory of identity. He cannot, for example, maintain that moonbeams also carry heat, though this fact would, at present, appear indubitable to all rational physicists, though it has not been specifically proved.

Having accepted the identity of light and heat rays, there is no difficulty in connecting the infra-red rays with the chemically active ultra-violet rays at the other end of the spectrum. It was some time later that it was realized that this connection of different kinds of rays was capable of great extension, on both sides of the spectrum. Before such an advance could come about, as a preliminary, a transition from the mechanical to the electromagnetic theory of light was necessary.

In spite of diversity of view, Newton, Huygens, and all their immediate successors were agreed that the clear understanding of the nature of light must be sought in the fundamentals of mechanical science, and this point of view was greatly stimulated by the strengthening of the mechanical theory of heat due to the discovery of the principle of conservation of energy. It is necessary for the explanation of polarization that ether oscillations are not longitudinal, moving in the direction of propagation, like air movements in a pipe, but are transversal, perpendicular to the direction of propagation, like those of a violin string. But one could get no nearer the nature of these oscillations from the laws of mechanics and elasticity. The more elaborate the hypotheses founded on the mechanical theory of light, whether ether was assumed to be continuous or atomic, the more evident became this inadequacy. At this stage, in the middle of the last century, came James Clerk Maxwell, with his bold hypothesis that light was electro-magnetic. His theory of electricity led him to the conclusion that every electrical disturbance moved from its source through space in waves with a velocity of 300 , 000 kilometres per second, and the coincidence of this figure, obtained from purely electrical measurements, with the magnitude of the velocity of light, led him to consider light as an electro-magnetic disturbance. The only proof of the correctness of this view lies in the fact that all deductions made from it agree with observation. The fundamental advance associated with his suggestion lies in the enormous simplification of the theory and in the number of results that can be immediately derived from it.

Now, the nature of electro-magnetic phenomena is no more intelligible than that of optical phenomena. To belittle the electro-magnetic theory of light, on the ground that it simply replaces one riddle by another, is to misunderstand the meaning of the theory. For its importance rests on the fact that it unites two branches of physics, which previously had to be treated as independent, and that, therefore, all theorems which are valid for one branch, are applicable to the other - a result which the mechanical theory of light did not, and could not, give. Before the introduction of the electro-magnetic theory, physics was divided into three separate branches - mechanics, optics, and electro-dynamics, and the unification of these is the ultimate and greatest aim of physical research. Though optics cannot be associated with mechanics, it combines completely with electrodynamics, and thus the number of independent branches has been reduced to two - the penultimate step towards the unification of the physical world picture. When and how the last step will be made, the linking up of mechanics and electro-dynamics, cannot be said, and though many clever physicists are at present occupied with this question, the time does not yet seem ripe for the solution. However, the original mechanical comprehension of Nature, which will allow the coalescing of mechanics and electro-dynamics, has now been thrust into the background in the minds of most physicists, since it regards ether, or, if ether is not sufficient, some substitute as the medium of all electrical phenomena. That which has harmed it most is the result, deduced from Einstein's theory of relativity, that there can be no objective substantial ether, i.e. one independent of the observer. For, if that were not so, then when we consider two observers moving relative to one another in space, one at most could correctly assert that he was at rest relative to the ether, whereas, by the theory of relativity, each of the two could do so equally correctly.

What Maxwell could only prophecy, Heinrich Hertz was able to verify a generation later, when he showed how to produce the electro-magnetic waves calculated by Maxwell, and thereby ensured the final acceptance of the electro-magnetic theory of light, according to which electric waves only differ from heat and light rays in that they have very much greater wave-length. If the optical spectrum were extended on the side of the slow oscillations in a manner undreamt of at one time, the extension would be of equal importance with that made on the other side of the spectrum through the discovery of the Röntgen rays and the appreciably faster so-called Gamma rays of radio-active substances. These rays, too, have the character of light waves, and are electro-magnetic oscillations, but have a very much shorter wave-length. Laue's very recent discovery of interference phenomena with Röntgen rays has confirmed the belief that they obey the same laws. It is remarkable how simply and quietly the transition from the mechanical to the electro-magnetic theory was made in physical literature. This is a good example of the fact that the kernel of a physical theory is not the observations on which it is built, but the laws to which they give rise. The fundamental equations of optics remain unaltered: they have always been in agreement with observation, but they are no longer to be interpreted mechanically ( although they were thus derived ) but electro-magnetically, and this has increased enormously their range of application.

This is not the first time that an important goal has been reached by a path which has afterwards been proved to be untrustworthy. It would have been possible to seek a solution by supposing that the theory would have been better had it abstained, in general, from making special hypotheses, based on immediate observations, and to limit oneself to the pure facts, i.e. to the results of measurements. However, the theory would thus surrender the most important aid, absolutely necessary to its development, namely, the setting up and consistent expansion of ideas which lead to progress. For this, not only understanding, but also imagination is necessary. As it is, the mechanical theory of light has done its duty. Without it the present brilliant results of optics would not have been obtained so quickly.

Huygens's undulation theory has not been essentially altered by the electro-magnetic hypothesis, when it states that any disturbance spreads out from its source in concentric spherical waves. But it is electro-magnetic energy and not mechanical energy that is sent out, for an oscillating electric and magnetic field of force appears in place of periodic vibrations of the ether.

Considered from this advanced point of view, the study of light, or, as it is often more exactly called, the study of radiant energy, gives us a picture of a gigantic co-ordinated structure, unified and completed. In this, all electro-magnetic oscillations, though apparently of very different kinds, find their proper positions, and all are governed by the same laws of propagation, following Huygens's wave theory. On the one hand, we have the Hertzian waves a kilometre long on the other, the hard Gamma rays, with many milliards of waves to the centimetre. The human eye has no place in this, it appears merely as an accidental and, although very delicate, a very limited piece of apparatus, for it can only perceive rays within a small spectral range of hardly an octave. Instead of the eye, special pieces of apparatus have been devised for receiving and measuring the different wave-lengths of the remainder of the spectrum. Such instruments are the wave detector, thermocouple, bolometer, radiometer, photographic plate, and the ionic cell. Thus, in optics, the separation of the physical foundations from the sense-perceptions has been accomplished in exactly the same way as in mechanics, where the conception of force has long lost its connection with the idea of muscular strength.

If I had delivered my lecture twenty years ago, I could have stopped here, for no further fundamental discoveries had then been made, and the imposing picture described above would have been a good conclusion which would have made modern physics famous. But probably I should not then have delivered this lecture, fearing that I should be able to present to you too little that was new. Today it has become quite otherwise, for, since that time, the picture has been essentially changed. The proud structure, which I have just described to you, has recently revealed certain fundamental weaknesses, and not a few physicists maintain that new foundations are required already. The electro-magnetic theory must always remain untouched, but Huygens's wave theory is seriously threatened, at least in one essential detail, due to the discovery of certain new facts. Instead of collecting as many as possible of the multifarious facts available, I shall simple examine one of them in detail.

When ultra-violet rays fall on a piece of metal in a vacuum, a large number of electrons are shot off from the metal at a high velocity, and since the magnitude of this velocity does not essentially depend on the state of the metal, certainly not on its temperature, it is concluded that the energy of the electrons is not derived from the metal, but from the light rays which fall on the metal. This would not be strange in itself it would even be assumed that the electro-magnetic energy of light waves is transformed into the kinetic energy of electronic movements. An apparently insuperable difficulty from the view of Huygens's wave theory is the fact ( which was discovered by Philipp Lenard and others ) , that the velocity of the electrons does not depend on the intensity of the beam, but only on the wavelength, i.e. on the colour of light used. The velocity increases as the wave-length diminishes. If the distance between the metal and the source of light is continuously increased, using, for example, an electric spark as the source of light, the electrons continue to be flung off with the same velocity, in spite of the weakening of the illumination the only difference is that the number of electrons thrown off per second decreases with the intensity of the light.

The difficulty is to state whence the electron obtains its energy, when the distance of the source of light becomes ultimately so great that the intensity of the light almost vanishes, and yet the electrons show no sign of diminution in their velocity. This must evidently be a case of a kind of accumulation of light energy at the spot from which the electron is flung out - an accumulation which is quite contrary to the uniform spreading out in all directions of electro-magnetic energy according to Huygens's wave theory. However, if it is assumed that the light source does not emit its rays uniformly but in impulses, something like an intermittent light, it follows that the energy of such a flash, spreading outwards in all directions in uniform waves, would finally be distributed over the surface of a sphere so large that the metal considered would receive but little of it. It is easy to calculate that under certain circumstances radiation extending for minutes, even hours, would be necessary for the liberation of one electron with the velocity corresponding to the colour of the light, while, in fact, no limiting condition can be determined, for the duration of radiation necessary to produce the effects the action certainly takes place with great rapidity. Like ultra-violet rays, Röntgen rays and Gamma rays give us the same effect, though, owing to the very much shorter wave-lengths of these rays, the velocities of the liberated electrons are much greater.

The only possible explanation for these peculiar facts appears to be that the energy radiated from the source of light remains, not only for all time, but also throughout all space, concentrated in certain bundles, or, in other words, that light energy does not spread out quite uniformly in all directions, becoming continuously less intense, but always remains concentrated in certain definite quanta, depending only on the colour, and that these quanta move in all directions with the velocity of light. Such a light-quantum, striking the metal, communicates its energy to an electron, and the energy always remains the same, however great the distance from the source of light.

Here we have Newton's emanation theory resurrected in another and modified form. But interference, which was a bar to the further development of Newton's emanation theory, is also an enormous difficulty in the quantum theory of light, for it is difficult at present to see how two exactly similar light quanta, moving independently in space, and meeting on a common path, can neutralize each other, without violating the principle of energy.

From this state of affairs arose the pressing need of the radiation theory for an investigation to find some way out of this dilemma, difficult from all sides. A natural assumption to try is that the energy of the electrons driven off comes from the metal itself and not from the radiation, and, therefore, that the radiation acts merely as a liberator in the same manner that a small spark liberates any amount of energy in a powder cask. But the further assumption would be necessary that the amount of the energy freed depends solely on the manner in which it is freed. It is not difficult to point out somewhat analogous phenomena in other branches of physics. As an example, I will consider in greater detail a convenient illustration used by Max Born. Imagine a tall apple tree, all its branches weighed down with ripe fruit, all of the same size, but with stalks of different lengths the apples are arranged so that those with short stalks are higher than those with long stalks. If an extremely weak, uniform wind blows through the branches, all the apples will oscillate slightly, without any one dropping, and the higher apples will oscillate more rapidly than the lower ones. If, now, the tree is shaken very gently with a definite rhythm, resonance will increase the oscillations of those apples whose period agrees with the period of the shaking, and a certain number of these will fall, the number increasing the longer and more forcibly the tree is shaken. These apples will reach the ground with a certain definite velocity determined only by their original height, i.e. by the lengths of their stalks all the other apples remain on the tree.

It must be understood that this comparison, like every other, fails in many respects, since, in this illustration, the source of energy is not internal kinetic energy but gravitation. But the essential point is realized that the final velocity of the particles liberated depends solely on the period of the disturbance, while the intensity of the disturbance determines only the number of these particles.

Can one attribute, however, such a complicated structure and such a wealth of energy to a tiny piece of metal? This question is less awkward than would perhaps appear at first. For we have long known that the chemical atom is not by any means the simple invariable element of which all matter is constituted, but rather that every single atom, particularly one of a heavy metal, must be considered as a world in itself, and the farther one penetrates, the richer and more varied the structure appears. The energy contained in every gram of a substance, according to the theory of relativity, amounts to over 20 billion calories, quite independently of its temperature - more than sufficient to liberate countless electrons.

Whether this presentation gives a possible way of saving the compromised wave theory, or simply leads ultimately to a blind alley, can only be settled by following the methods of research already outlined and seeing where they end. At this stage we must make use of theory. We must first of all examine more closely each of the two opposing hypotheses, without considering whether or not we have confidence in either of them, and must work out the results and reduce them to a form suitable for experimental verification. For this purpose, in addition to a training in physics and the requisite mathematical ability, it is necessary to have a discriminating judgment of the measure of the reliability that can be placed on the accuracy of the measurements for the effects sought for are mostly of the same order as the errors of observation. It is not possible today to predict with certainty when any definite solution to this problem will be obtained.

What I have tried to set before you here about the action of light, holds in an exactly similar manner with regard to the cause of light, that is, to the phenomena of generation of light rays. In this also we have new riddles, difficult to unravel, which are at variance with certain surprisingly deep glimpses recently obtained into the laws governing natural phenomena. The only thing that can be said with certainty, is that the quanta, already referred to, play a characteristic part in connection with the origin of light.

According to the bold hypothesis of the Danish physicist Niels Bohr, the consequences of which have been astonishingly multiplied recently, electrons oscillate in every atom of an illuminated gas. These electrons circle about the nucleus in a greater or smaller number and at different distances, in certain definite paths and obey the same laws as those governing the motions of the planets about the sun. But light, arising from these oscillations, is not sent out from the atom into surrounding space uninterruptedly and uniformly, as are the sound waves from the prongs of a vibrating tuning-fork. The emission of light always takes place abruptly, by impulses, for it is not determined by the regular oscillations of the electrons themselves but is only emitted when these electron oscillations receive a sudden change and a certain disruption in themselves a kind of internal catastrophe, which throws the electrons out of their original paths into others more stable but associated with less energy. It is the surplus amount of energy liberated by the atom which travels out into space as a light quantum.

Indeed, the most remarkable thing about this phenomenon is that the period of the emitted light, and therefore its colour, does not, in general, agree with the period of oscillation of the electrons, either in their original or in their final paths. It is definitely determined by the amount of energy emitted, since the more rapid the oscillations, the greater is the light quantum. It follows that a short wave-length corresponds to a large amount of energy, considered as a light quantum. If, therefore, for example, much energy is emitted, we get ultra-violet or even Röntgen rays if, however, but little energy is emitted, red or infra-red rays result. It is at present a complete mystery why the oscillations of light produced in this way are, with the utmost regularity, strictly monochromatic.

Indeed, we might be inclined to consider all these ideas as the play of a vivid but empty imagination. When, on the other hand, we consider that these hypotheses help us to elucidate the mysterious structure of the spectra of the different chemical elements and, in particular, the complicated laws governing the spectral lines, not only as a whole but, as Arnold Sommerfeld first showed, partly even in minute details, with an exactness equal to, and even surpassing, that of the most accurate measurements-when we consider this we must, for good or ill, make up our minds to assign a real existence to these light quanta, at least at the instant of their origin.

What becomes of them later as light disperses - whether the energy of a quantum remains concentrated as in Newton's emanation theory or whether, as in Huygens's wave theory, it spreads out in all directions and gets less dense indefinitely - is another question of a very fundamental character, to which I have referred above.

So the present lecture on our knowledge of the physical nature of light ends not in a proud proclamation, but in a modest question. In fact, this question, whether light rays themselves consist of quanta, or whether the quanta exist only in matter, is the chief and most difficult dilemma before which the whole quantum theory halts, and the answer to this question will be the first step towards further development.

Max Planck - History

The Max Planck Society and Freie Universität Berlin (Department of History and Cultural Studies and Department of Philosophy and Humanities) are seeking to appoint a Professor of the History of Science/the History of Knowledge who would also be responsible for leading a Max Planck Research Group (at the Max Planck Institute for the History of Science).

Salary grade W2 fixed-term appointment for five years (public employee)


The candidate will lead a Max Planck Research Group at the Max Planck Institute for the History of Science (MPIWG) and is expected to teach and conduct research in the History of Science and the History of Knowledge at the Freie Universität Berlin. The standard teaching load is one course (two hours per week) per semester.

Appointment requirements:

Appointment requirements are governed by section 100 of the Berlin Higher Education Act (Berliner Hochschulgesetz).

Further requirements for appointment:

Early-career researchers are sought who demonstrate an excellent research record in the area of the history of science/the history of knowledge. The applicant’s qualifications in a specific subject area should relate to one of the following disciplines: humanities, social sciences, human sciences, natural sciences, engineering, or an interdisciplinary field such as area studies, cultural studies, digital humanities etc. The candidate’s research interests in the history of science/the history of knowledge should align with those of the participating institutions.

The candidate will possess a clear ability to lead a research group and have international experience in research and teaching in an academic context. Experience in obtaining external funding such as grants is desirable.

The candidate will be expected to participate in collaborative projects between the Max Planck Institute for the History of Science and the universities of Berlin on the history of knowledge and to contribute to other ongoing and planned collaborations.

The Max Planck Institute for the History of Science will offer funding for a research fellow or for visiting scholars as well as secretarial support and grant funding.

Candidates should submit their application with a CV, a list of publications and a research proposal (750 words max.) no later than August 21, 2020 (23:59 CET). Please submit application materials online through the application portal of the Max Planck Institute for the History of Science.

University and a Ph.D. at age 21

In 1874, age 17, and now a freshman at the University of Munich, Planck spoke to Professor Philipp von Jolly about the merits physics. Jolly famously replied:

“In this field [physics] almost everything is already discovered, and all that remains is to fill a few insignificant gaps.”

Undeterred, Planck chose to study physics. One day he was destined to find evidence to prove the absurdity of his professor’s beliefs. In fairness to Philipp von Jolly – and although it’s hard to believe today given the rapid march of science and technology – many physicists of that era shared Jolly’s view: they believed they had already discovered and understood most of what there was in the universe to be discovered and understood!

At university Planck discovered he did not enjoy experimental work. His mathematical talent found its natural home in the world of theoretical physics.

He continued to enjoy music. He sang in the university choir and composed a mini-opera.

An Important Vacation

During the spring vacation of 1877, close to his twentieth birthday, Planck embarked on a hiking tour in northern Italy with university friends including the mathematician Carl Runge. While walking, the students discussed science, mathematics, and their views of the world.

Lake Como in northern Italy, one of the places Max Planck and his friends walked. Hiking amid spectacular scenery became one of Planck’s lifelong pleasures.

Runge raised a question about whether Christianity and religion did more harm than good – a question that shocked Planck, who had received a traditional Lutheran upbringing. Planck began to question his personal view of the world. He remained a Lutheran throughout his life and rejected atheism, but became very tolerant of alternative philosophies and religions.

Berlin and Thermodynamics

In the winter semester of 1877, age 20, Planck transferred for a year to Berlin’s Friedrich Wilhelms University where he was taught by two of the giants of physics – Hermann von Helmholtz and Gustav Kirchhoff.

In Planck’s opinion, each of these renowned men of science delivered lectures distinguished only by their dreariness.

Nevertheless, he and Helmholtz became great friends. Planck admired – indeed almost worshiped – Helmoltz for his scientific integrity, honesty, kindness, modesty, and tolerance.

One of Helmholtz’s passions in physics was thermodynamics – the study of the relationships between temperature, heat, energy, and work. Planck grew increasingly fascinated by thermodynamic theory.

He began his own program of work in the field, spending endless hours poring over papers written by Rudolf Clausius, one of thermodynamics’ founders.

Unlike the lectures he attended, he found Clausius’s work to be interesting, well-delivered, and clear.

The Highest Honors and a First Job

After his year in Berlin, Planck returned to Munich in late 1878 where he passed his state exam allowing him to teach physics in high schools.

A few months later, in February 1879, he submitted a doctoral thesis concerning the second law of thermodynamics. Three months later he defended his thesis in an oral examination and – age 21 – was awarded a Ph.D. in physics with the highest honors – summa cum laude.

Funnily enough, from the questions he was asked during his thesis defense, Planck drew the conclusion that none of the professors who interrogated him understood his thesis!

A year later Planck successfully submitted a further thermodynamics thesis for his habilitation – a much more demanding qualification than the Ph.D., which allowed its holder to become a professor if such a job became available.

At age 22, Planck became a physics lecturer (unpaid) at the University of Munich. Without any salary, he continued living with his parents. His research focused on entropy – a quantity sometimes defined in a loose sense as a measure of the amount of disorder at the atomic level.

A Return to his Birthplace, then back to Berlin

Finally, almost on his 27th birthday, Planck became an associate professor of theoretical physics at the University of Kiel, where he probed ever more deeply into thermodynamics. He continued making progress in this difficult field, but made no major breakthroughs.

At age 31, in April 1889, Planck returned to Berlin to take over the lecturing duties of Gustav Kirchhoff, who had died in the fall of 1887.

In 1892 Planck became a full professor of theoretical physics. By all accounts his students found his lectures much more interesting than Planck had found his predecessor’s. One of his students, the British chemist James Partington, described Planck’s lectures:

“using no notes, never making mistakes, never faltering the best lecturer I ever heard. 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”.

Two of Planck’s Ph.D. students would later win Nobel Prizes in physics: Max von Laue and Walther Bothe.

The scene was now set for Planck’s momentous discovery – quantum theory.

Left: Original colour drawing by Brodmann, showing cortical areas in the European ground squirrel Spermopilus citellus [Archive of the MPI for Brain Research]. Right: Cortical areas in the human brain, from Brodmann (1909) Vergleichende Lokalisationslehre der Großhirnrinde.

In the 1920s Oskar Vogt became interested in the potential morphological correlates of mental abilities, and hence in the neuroanatomical study of &aposelite brains&apos. When Lenin died of a brain hemorrhage in 1924, his brain was preserved in formaldehyde, where it remained for two years. In 1926, Vogt was recruited by the Soviet government to help establish Lenin&aposs genius via histological investigation of his brain. He was given some space in Moscow to carry out this work and two years later, a spacious and representative brick building that had been confiscated from an American business (Fig. 4). In it, he helped establish and then headed the Moscow Brain Institute (Fig. 5). Between 1926 and 1930, Vogt travelled to Moscow several times to supervise the work on Lenin&aposs brain (Fig. 6) by the Russian collaborators who had been trained at Vogt&aposs KWI for Brain Research in Berlin.

In 1927, Vogt gave a preliminary report on his findings in Moscow, concluding from his histological observations that Lenin must have been an athlete in associative thinking ("Assoziationsathlet") - a conclusion deemed farfetched by some of his neurologist colleagues and adversaries. Lenin&aposs brain was, for a time, on display in the Lenin Mausoleum and now rests at Moscow&aposs Brain Institute [4].

Planck constant introduces the discontinuity in the description of elementary phenomena, which constitutes the basis of quantum physics.

This is how the importance of Planck’s discovery does not consist in a formal operation or in his mathematical ability. In reality, the transcendence of his proposal resides above all in the revolutionary interpretation of the physical sense of the constant h.

From the beginning, Planck attributed to the constant the name of “action elemental quantum” because it possesses the dimensions of an action (energy multiplied by a time) and because it only intervenes by multiple wholes. Thus, Planck introduced the idea of a granular composition when all physicists thought that continuity reigned.

To conclude, it can be appreciated that thanks to Planck’s formula the energy of a radiation can be measured, not only in a unit of energy, but also in units of length and frequency. Also, by using the law of the black body, one can determine the temperature of an object whose emission is centered on a certain frequency.