The Governmental Film Archive of the German Democratic Republic presents
Film Archive of Personalities
Privy Councilor: Max Planck
December 15, 1942
If you are like me and in your 85th year of life, you will probably feel more strongly than ever the need to understand how life has gone, to take a look back at your life so far, and to give a comprehensive summary to yourself and others about what you have achieved and what you have strived for. Finally, you will also take a last look into the future.
I come from a family of Swabians. My great-grandfather moved from Nürtingen near Stuttgart to Göttingen. From that point on, his children, grandchildren, and great-grandchildren moved to North Germany and spread out. Most of them were theologians or philologists, and I am probably the only one who stepped out of line and became a natural scientist. I attribute this to environmental influences. In school, I was encouraged by my math teacher, Hermann Müller. He was a very humorous and strict man who was enthusiastic about everything related to mathematics and natural science. On one hand, the strictness of the mathematical method, and on the other hand, the beauty of the laws that govern nature. Even then, my ideal for scientific activity, which I still follow today, was to feel the harmony between the strictness of mathematics and the fullness of the laws of nature that surround us. This corresponds to the possibility of mastering nature with the sharpness of our logic and human thought by seeking the harmony of these logical laws with the laws of nature. In this way, we can achieve domination over nature, which always seemed to me to be the most beautiful goal of all scientific pursuits.
As I mentioned, we grew up in North Germany, although we were originally Swabians. I was born in Kiel and later moved to Munich, where my father was appointed a professor at the university. My father was a strict Augustenburger, a Schleswig-Holsteiner, and was not at all satisfied with the fact that Schleswig-Holstein was annexed by Prussia. This essentially led him to move from Holstein to Munich and the University there. Later, peace was established between the Augustenburgers and the Prussians, which led to our princess, the daughter of the Duke of Augustenburg, becoming a German Empress. I still have the memory of this time in Kiel and how she received me there one time as an Empress. It was very nice. There was a ceremony at the royal court, and I had to pass by her in a large parade and bow to her. Then, I mentioned something about Kiel to her and she pulled me into a conversation that lasted so long that the court ceremony master later approached me, curiously asking what was going on and what the Empress had spoken to me about. I had to laugh about this little episode.
So we came to Munich, and I studied there. There was my mathematics teacher who made me enthusiastic about mathematics, and I started my studies there. Later, I went to the University, and during my last year, I studied in Berlin and started my career as a private lecturer. In this way, my life went on in a very ordinary way. I was a private lecturer and later became a professor in Kiel and Berlin. Since 1889, I have been here in Berlin at the University, first as an Extraordinary, then as an Ordinary, and now I am an Emeritus professor. So I am actually an old Berliner. But actually, there are no real old Berliners who were born here. It all goes back and forth in academic circles. You come from one university to another, and if one considers that I am actually very sedentary, but once you have moved to Berlin, you can hardly get away from here because it is the center of all the spiritual movements in all of Germany. It has become stronger and stronger with time. In a certain sense, it is regrettable that it has become more and more centralized.
I used to be very fond of the noble aspirations of the different royal courts of Darmstadt, Dresden, Munich and Weimar and whatever they are all called. There are many others: Karlsruhe, Dresden, Leipzig... But all of this has now been faded in favor of Berlin. That is actually a pity. But that is probably due to the course of time.
Now I would like to tell you something scientific about myself. That is probably what most people are interested in, but maybe not everyone will understand what I am about to say now. It will be quite technical. As I mentioned earlier, I began my university studies in Munich, where I had also attended school. There, Professors Ludwig Seidel and Gustav Bauer introduced me to mathematics, and Philipp von Jolly introduced me to physics. I have fond memories of all three of them, but I will never forget the words of Mr. von Jolly when I said goodbye to him to study my last semester in Berlin, covering mainly theoretical physics. He told me that theoretical physics is a very interesting subject, but currently, there are no available professorships for it. He also added that I would hardly be able to discover anything new in theoretical physics because the foundations of theoretical physics were already completely set with the discovery of the principle of the conservation of energy. He said that one might find a few small things here and there if one looked hard enough, but one would never be able to find anything new in principle. However, these words did not discourage me from pursuing the exciting path towards theoretical physics. I did not expect to discover something new, but I had a desire to research the laws of nature more deeply.
Coming to Berlin, I turned to the main topic of my entire work, namely the study of the main laws of thermodynamics. As classical mechanics seemed to me to be completely exhausted, the main laws of thermodynamics, which have to be valid for all natural processes, regardless of whether one thinks of matter atomistically or continuously, seemed more intriguing to me. When I arrived in Berlin, Professor Hermann Helmholtz shone as an experimentalist, and Gustav Kirchhoff as a theoretician in physics. I attended both of their lectures, but I was not very interested in them as they did not cover anything about heat. It was much more the writings of Rudolf Clausius that made me enthusiastic through their simplicity, clarity, and preciseness, which led me to dedicate myself entirely to the study of these laws of heat theory. In particular, I was interested in the concept of entropy introduced by Clausius.
Entropy is a quantity that, in addition to energy, probably corresponds to one of the most important variables in nature. While energy remains constant, entropy always grows and can never be reduced. This is the essence of the second main law of thermodynamics, meaning that the entropy of a system of bodies can always only increase. In the limiting case, it remains the same. If it increases, then the process is irreversible. If it remains the same, then the process is reversible, which means that you can also let it run backwards. This is the big difference between all laws of nature. Thermodynamic equilibrium is present when the entropy has reached its maximum value. If entropy can no longer grow, then no change can occur. It is from this statement that all properties of a thermodynamic equilibrium emerge. I applied this statement to physical-chemical as well as radiation equilibria.
Regarding physical-chemical equilibria, an American was faster than me. That was Josiah Willard Gibbs, a professor in Baltimore. He thus took away a success from me. However, regarding radiation equilibrium, it turned out that I was the one who laid the foundations, and my main achievement lies in this area. However, I did not find the entropy of heat radiation, i.e., the dependency of the entropy on radiation intensity, purely by theoretical means at first. I only found it by referring to experimental measurements conducted on the one hand by Otto Lummer and Ernst Pringsheim from the Physical-Technical Reichsanstalt, and on the other hand by Heinrich Rubens and Friedrich Kohlrausch from the Technical University. In order to interpret these experimentally found laws, I was guided by the great thoughts of Ludwig Boltzmann. It was he who was able to interpret the entropy of a gas from the standpoint of atomic theory as the logarithm of the probability of the state of the gas.
The application of Boltzmann's procedure to black body radiation could only succeed if one considered radiation as atomistic - a combination of individual, special quanta. This requirement unavoidably led to the assumption of radiation atoms or radiation quanta of a very specific size, known exactly from previous measurements. At first, I did not like this hypothesis because it contradicted all the ideas of classical atomistics, but there was no other way. Later, thanks to the work of numerous colleagues, it was shown to correspond to reality. However, it took several years for the physics community to take notice of my theory, as it was misunderstood by many and initially completely ignored, as often happens with new ideas. Nevertheless, I did not allow myself to be sad or afraid of this lack of recognition because I was too sure of my work to doubt its final success. I could relax and observe the development without worry, which occurred much faster than I anticipated. More precise measurements made it very clear, very soon, that the values of an important physical constant, namely the electric elementary quantum (now known as Planck's constant), was getting closer to the value I had predicted from the radiation measurements. This demonstrated that the radiation measurement and subsequent calculation were the right way to obtain these values.
So far, I have mainly spoken about thermodynamics. I would like to say one more word about mechanics. I had always considered mechanics, like most physicists, as the basis of all physical sciences. And in a sense, it still is important today, because mechanics does not deal with any other variables than those of space and time, which are indispensable in all areas of nature. Because of this, the theory of relativity aroused my special attention right from the beginning. And it filled me with lively satisfaction when I found out that the theory of relativity, in which energy and momentum are set equal, actually represents the completion and culmination of the whole foundation of theoretical physics. I have also devoted some of my work to it, at a time when it had not been completed yet.
I have generally always turned my interest to questions that possibly lead to a simplification of the whole system of theoretical physics. Currently, the latest development in physics is the introduction of quantum theory. Quantum theory is a new step on the path to knowledge about nature. Many excellent physicists are inclined to consider the current state of physics as a new final point, meaning the final conclusion of our investigation into the laws of nature. I do not belong to them. I believe that quantum theory has not yet reached its full maturity, its full final point. I believe that we still have to make some generalizations, some abstractions, in order to reach a similar understanding of the laws of nature as the classical theory had once permitted, which we have had to give up now. I believe that research still has to go much further before we reach such a conclusion. This is somewhat unsatisfactory, but on the other hand, it is also very appropriate and joyful because we will never reach the final conclusion about nature.
Scientific pursuit will never stop. It would be terrible if it stopped. If there were no more problems, one would sit back and take things easy, turn one's head off, and not work anymore. Such calmness is stagnation and death in a scientific sense. The happiness of the scientist lies not in possessing the truth, but in discovering the truth. And in this continuous, successful search for truth, lies satisfaction. The search itself is not satisfactory, of course. It has to be successful. But this successful work represents the source of every effort and the source of every spiritual pleasure. If this source dries up, that is if the truth is found, then it is over. Then one can go to sleep mentally and physically. But it is certain that we will not experience this, and that is the foundation of our happiness.
Now I would like to say one last word, which should clear my conscience. I have followed a friendly invitation here today on the 15th of December 1942, here in Berlin, on Jägerstraße 26. I have spoken a few words for the film archive, so that later generations, if they are interested, can take note of my personality. But I would like to emphasize that what you hear and see here is not a final image of my personality. What I said here was mostly improvised and said directly just as I thought of it. I could not prepare myself in the way I would have done otherwise, if I had known, prior to arriving, how to appear here. So I ask you to view this as an attempt to express myself such that at least something will remain of me. For those who want to find out more about me, I ask you to stick to what I wrote and what I printed as I can stand up for that. The older you get, the more you feel the responsibility for what you say. And that is why I got used to, in the latest years, carefully preparing myself and thinking through everything I will be saying or presenting beforehand. That did not happen today. Therefore, I have to put a question mark behind everything I have said here today, and I ask you kindly to consider this.