Planck, Max (1858–1947)

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The German physicist Max Planck was the discoverer of the quantum of action, also called Planck's constant. Born in Kiel, he studied physics and mathematics at the University of Munich under Philipp von Jolly and at the University of Berlin under Hermann von Helmholtz and Gustav Kirchhoff. After receiving his Ph.D. at Munich (1879), he taught theoretical physics, first in Kiel, then (starting in 1889) in Berlin, as Kirchhoff's successor. "In those days," he wrote later, "I was the only theoretician, a physicist sui generis, as it were, and this circumstance did not make my début so easy." At this time Planck made important, and indeed quite fundamental, contributions to the understanding of the phenomena of heat, but he received hardly any attention from the scientific community: "Helmholtz probably did not read my paper at all. Kirchhoff expressly disapproved of its contents." The spotlight was then on the controversy between Ludwig Boltzmann and the Wilhelm OstwaldGeorg HelmErnst Mach camp, which supported a purely phenomenological theory of heat. It was via this controversy, and not because of the force of his arguments, that Planck's ideas were finally accepted. "This experience," he wrote, "gave me an opportunity to learn a remarkable fact: a new scientific truth does not triumph by convincing its opponents and making them see the light, but rather because its opponents eventually die."

Nevertheless, the discovery of the quantum of action in 1900, for which Planck received the Nobel Prize in physics (1918), was a direct result of these earlier studies. In 1912 Planck became permanent secretary of the (then) Prussian Academy of Sciences, a post that he retained with only minor interruptions for the rest of his life. He used this position with excellent judgment for furthering the international collaboration of all scientists. From 1930 to 1935 he was president of the Kaiser-Wilhelm-Institut, which later became the Max-Planck-Institut.

Politically Planck was conservative, loyal to the Prussian ideas of the state and of honor, and loyal to Wilhelm II. During World War I he more than once expressed his devotion to the cause of the German people united in battle, and he received the order of "pour le mérite," one of the highest orders of Wilhelm's Germany. However, he opposed the Nazi regime. He defended Albert Einstein, first against his scientific opponents, then against his political enemies. Despite severe criticism by Johannes Stark, Phillip Lenard, and Ernst Müller, he continued to defend Einstein and other Jewish scientists (such as Walther Nernst) even after 1933. He later personally demanded of Adolf Hitler that those scientists who had been imprisoned be freed; as a consequence he was removed as president of the Physical Society, was refused the Goethe Prize of the city of Frankfurt (he was awarded it after the war, in 1946), and finally was forced to witness the execution of his only son, who had been connected with the German resistance. Antiquated as some of his political ideas may have been, he nevertheless put individual justice above all and defended it even at the risk of his own life. At the end of the war he was rescued by the Allied forces. He spent the last years of his life in Göttingen.

Approach to Science

Planck's research was guided by his belief "of the existence in nature of something real, and independent of human measurement." He considered "the search for the absolute" to be the highest goal of science. "Our everyday starting point," he explained, "must necessarily be something relative. The material that goes into our instruments varies according to our geographical source; their construction depends on the skill of the designers and toolmakers; their manipulation is contingent on the special purposes pursued by the experimenter. Our task is to find in all these factors and data, the absolute, the universally valid, the invariant that is hidden in them."

This point of view was not allowed to remain a philosophical luxury, without influence upon the procedures of physics. One of the main objections that Planck raised against the positivistic creed was its sterility in the promotion of theory. "Positivism lacks the driving force for serving as a leader on the road of research. True, it is able to eliminate obstacles, but it cannot turn them into productive factors. For its glance is directed backwards. But progress, advancement requires new associations of ideas and new queries, not based on the results of measurement alone."

Scientific Discoveries

Of new ideas Planck himself produced essentially two. He recognized and clearly formulated those properties of heat that separate it from purely mechanical processes, and he introduced and applied to concrete problems the idea of an atomistic structure not only of matter but of radiation also. In his doctoral dissertation he had already separated thermodynamic irreversibility from mechanical processes and had interpreted Rudolf Clausius's entropy as its measure. Later he showed (independently of Willard Gibbs) that "all the laws of physical and chemical equilibrium follow from a knowledge of entropy."

His conviction that the principle of the increase of entropy was a genuine and independent physical law and his belief in the universal (or, to use his term, absolute ) validity of all physical laws led him to apply thermodynamic reasoning in domains that until then had been regarded as inaccessible to it. For example, he determined that the lowering of the freezing point of dilute solutions could be explained only by a dissociation of the substances dissolved, thus extending the science of thermodynamics to electrically charged particles. This tendency to strain laws to the limit rather than to restrict them to the domain of their strongest evidence caused a temporary clash with Boltzmann, who was quite unperturbed by the fact that in his approach the entropy of a system could both increase and decrease. But it also led to Planck's greatest triumphhis discovery of the quantum of action.

Planck was the only one to correlate the relevant features of radiation with the entropy, rather than the temperature, of the radiant body. "While a host of outstanding physicists worked on the problem of spectral energy distribution, both from the experimental and theoretical aspect, every one of them directed his efforts solely towards exhibiting the dependence of the intensity of radiation on the temperature. On the other hand I suspected that the fundamental connexion lies in the dependence of entropy upon energy. As the significance of the concept of entropy had not yet come to be fully appreciated, nobody paid attention to the method adopted by me, and I could work out my calculations completely at my leisure." These calculations furnished a formula that agreed with experiment and contained the existing theoretical results (Wien's formula and the Raleigh-Jeans law) as limiting cases. In the attempt to find a rationale for this result, Planck used Boltzmann's statistical interpretation of entropy and was thus led to the discovery of the "atomic," or discontinuous, structure of action (energy).

Realism, Determinism, and Religion

The discovery of the quantum of action was brought about not only by the specific physical arguments used but also by the philosophical belief in the existence of a real world behaving in accordance with immutable laws. The intellectual climate of the late nineteenth century was opposed to such a belief (Boltzmann was almost the only other figure to uphold it). This climate not only found expression in the philosophical superstructure but influenced physical practice itself. Laws were regarded as summaries of experimental results and were applied only where such results were available. However, it was the "metaphysics" of Planck, Boltzmann, and, later on, Einstein (whom Planck interpreted as a realist from the very beginning) that made possible many of the theories that are now frequently used to attack realism and other "metaphysical" principles.

Planck never accepted the positivistic interpretation of the quantum theory. He distinguished between what he called the "world picture" of physics and the "sensory world," identifying the former with the formalism of the ψ waves, the latter with experimental results. The fact that the ψ-function obeys the Schrödinger equation enabled him to say that while the sensory world might show indeterministic features, the world picture, even of the new physics, did not. His belief in the existence of objective laws also provided him with an important steppingstone to religious belief. Planck argued that the laws of nature are not invented in the minds of men; on the contrary, external factors force us to recognize them. Some of these laws, such as the principle of least action, "exhibit a rational world order" and thereby reveal "an omnipotent reason which rules over nature." He concluded that there is no contradiction between religion and natural science; rather, they supplement and condition each other.

See also Quantum Mechanics.


works by planck

Theory of Heat Radiation. Translated by Morton Masius. Philadelphia: P. Blakiston's Son, 1914; 2nd ed., New York: Dover, 1959.

Eight Lectures on Theoretical Physics. Translated by A. P. Wills. New York: Columbia University Press, 1915. Lectures given at Columbia University in 1909.

The Origin and Development of the Quantum Theory. Translated by H. T. Clarke and L. Silberstein. Oxford: Clarendon Press, 1922. Nobel Prize address.

A Survey of Physics; A Collection of Lectures and Essays. Translated by R. Jones and D. H. Williams. London: Methuen, 1925. Reissued as A Survey of Physical Theory. New York: Dover, 1960.

Treatise on Thermodynamics. Translated by Alexander Ogg. London: Longmans, Green, 1927; 3rd rev. ed., New York, 1945.

Introduction to Theoretical Physics. Translated by Henry L. Brose, 5 vols. London, 19321933; New York, 1949. Includes General Mechanics, The Mechanics of Deformable Bodies, Theory of Electricity and Magnetism, Theory of Light, and Theory of Heat.

Scientific Autobiography and Other Papers. Translated by Frank Gaynor. New York: Philosophical Library, 1949.

The New Science. Translated by James Murphy and W. H. Johnston. New York: Meridian, 1959. Includes Where Is Science Going? (a defense of determinism with a preface by Albert Einstein), The Universe in the Light of Modern Physics, and The Philosophy of Physics.

works on planck

Schlick, Moritz. "Positivism and Realism." In Logical Positivism, edited by A. J. Ayer. Glencoe, IL: Free Press, 1959. This essay was a direct reply to the criticisms of positivism that Planck expressed in Positivismus und reale Aussenwelt. Leipzig, 1931.

Vogel, H. Zum philosophischen Wirken Max Plancks. Berlin: Akadamie, 1961. Excellent biography with detailed bibliography.

Paul K. Feyerabend (1967)