Planck, Max (1858-1947)

German physicist

Max Planck is best known as one of the founders of the quantum theory of physics. As a result of his research on heat radiation, Planck concluded that energy can sometimes be described as consisting of discrete units, later given the name quanta. This discovery was important because it made possible, for the first time, the use of matter-related concepts in an analysis of phenomena involving energy. Planck also made important contributions in the fields of thermodynamics, relativity, and the philosophy of science. He was awarded the 1918 Nobel Prize in physics for his discovery of the quantum effect.

Max Karl Ernst Ludwig Planck was born in Kiel, Germany. His parents were Johann Julius Wilhelm von Planck, originally of Göttingen, and Emma Patzig, of Griefswald. Max was the couple's fourth child.

Johann von Planck was descended from a long line of lawyers, clergyman, and public servants and was himself Professor of Civil Law at the University of Kiel. Young Max began school in Kiel, but moved at the age of nine with his family to Münich. There he attended the Königliche Maximillian Gymnasium until his graduation in 1874.

Planck entered the University of Münich in 1874 with plans to major in mathematics. He soon changed his mind, however, when he realized that he was more interested in practical problems of the natural world than in the abstract concepts of pure mathematics. Although his course work at Münich emphasized the practical and experimental aspects of physics, Planck eventually found himself drawn to the investigation of theoretical problems. It was, biographer Hans Kango points out in Dictionary of Scientific Biography, "the only time in [his] life when he carried out experiments."

Planck's tenure at Münich was interrupted by illness in 1875. After a long period of recovery, he transferred to the University of Berlin for two semesters in 1877 and 1878. At Berlin, he studied under a number of notable physicists, including Hermann Helmholtz and Gustav Kirchhoff. By the fall of 1878, Planck was healthy enough to return to Münich and his studies. In October of that year, he passed the state examination for higher-level teaching in math and physics. He taught briefly at his alma mater, the Maximillian Gymnasium, before devoting his efforts full time to preparing for his doctoral dissertation. He presented that dissertation on the second law of thermodynamics in early 1879 and was granted a Ph.D. by the University of Münich in July of that year.

Planck's earliest field of research involved thermodynamics, an area of physics dealing with heat energy. He was very much influenced by the work of Rudolf Clausius, whose work he studied by himself while in Berlin. He discussed and analyzed some of Clausius's concepts in his own doctoral dissertation. Between 1880 and 1892, Planck carried out a systematic study of thermodynamic principles, especially as they related to chemical phenomena such as osmotic pressure, boiling and freezing points of solutions, and the dissociation of gases. He brought together the papers published during this period in his first major book, Vorlesungen über Thermodynamik, published in 1897.

During the early part of this period, Planck held the position of Privat-Dozent at the University of Münich. In 1885, he received his first university appointment as extraordinary professor at the University of Kiel. His annual salary of 2,000 marks was enough to allow him to live comfortably and to marry his childhood sweetheart from Münich, Marie Merck. They eventually had three children.

Planck's research on thermodynamics at Kiel soon earned him recognition within the scientific field. Thus, when Kirchhoff died in 1887, Planck was considered a worthy successor to his former teacher at the University of Berlin. Planck was appointed to the position of assistant professor at Berlin in 1888 and assumed his new post the following spring. In addition to his regular appointment at the university, Planck was also chosen to head the Institute for Theoretical Physics, a facility that had been created especially for him. In 1892, Planck was promoted to the highest professorial rank, ordinary professor, a post he held until 1926.

Once installed at Berlin, Planck turned his attention to an issue that had long interested his predecessor, the problem of black body radiation. A black body is defined as any object that absorbs all frequencies of radiation when heated and then gives off all frequencies as it cools. For more than a decade, physicists had been trying to find a mathematical law that would describe the way in which a black body radiates heat.

The problem was unusually challenging because black bodies do not give off heat in the way that scientists had predicted that they would. Among the many theories that had been proposed to explain this inconsistency was one by the German physicist Wilhelm Wien and one by the English physicist John Rayleigh. Wien's explanation worked reasonably well for high frequency black body radiation, and Rayleigh's appeared to be satisfactory for low frequency radiation. But no one theory was able to describe black body radiation across the whole spectrum of frequencies. Planck began working on the problem of black body radiation in 1896, and by 1900, had found a solution to the problem. That solution depended on a revolutionary assumption, namely that the energy radiated by a black body is carried away in discrete "packages" that were later given the name quanta (from the Latin, quantum, for "how much"). The concept was revolutionary because physicists had long believed that energy is always transmitted in some continuous form, such as a wave. The wave, like a line in geometry, was thought to be infinitely divisible.

Planck's suggestion was that the heat energy radiated by a black body be thought of as a stream of "energy bundles," the magnitude of which is a function of the wavelength of the radiation. His mathematical expression of that concept is relatively simple: E = h 6,50 − υ, where E is the energy of the quantum, υ is the wavelength of the radiation, and h 6,50 − is a constant of proportionality, now known as Planck's constant. Planck found that by making this assumption about the nature of radiated energy, he could accurately describe the experimentally observed relationship between wavelength and energy radiated from a black body. The problem had been solved.

The numerical value of Planck's constant, h 6,50 −, can be expressed as 6.62 × 10−27 erg second, an expression that is engraved on Planck's headstone in his final resting place at the Stadtfriedhof Cemetery in Göttingen. Today, Planck's constant is considered to be a fundamental constant of nature, much like the speed of light and the gravitational constant. Although Planck was himself a modest man, he recognized the significance of his discovery. Robert L. Weber in Pioneers of Science: Nobel Prize Winners in Physics writes that Planck remarked to his son Erwin during a walk shortly after the discovery of the quantum concept, "Today I have made a discovery which is as important as Newton's discovery." That boast

has surely been confirmed. The science of physics today can be subdivided into two great eras, classical physics, involving concepts worked out before Planck's discovery of the quantum, and modern physics, ideas that have been developed since 1900, often as a result of that discovery. In recognition of this accomplishment, Planck was awarded the 1918 Nobel Prize in physics.

After completing his study of black body radiation, Planck turned his attention to another new and important field of physics: relativity. Albert Einstein's famous paper on the theory of general relativity, published in 1905, stimulated Planck to look for ways on incorporating his quantum concept into the new concepts proposed by Einstein. He was somewhat successful, especially in extending Einstein's arguments from the field of electromagnetism to that of mechanics. Planck's work in this respect is somewhat ironic in that it had been Einstein who, in another 1905 paper, had made the first productive use of the quantum concept in his solution of the photoelectric problem.

Throughout his life, Planck was interested in general philosophical issues that extended beyond specific research questions. As early as 1891, he had written about the importance of finding large, general themes in physics that could be used to integrate specific phenomena. His book Philosophy of Physics, published in 1959, addressed some of these issues. He also looked beyond science itself to ask how his own discipline might relate to philosophy, religion, and society as a whole. Some of his thoughts on the correlation of science, art, and religion are presented in his 1935 book, Die Physik im Kampf um die Weltanschauung.

Planck remained a devout Christian throughout his life, often attempting to integrate his scientific and religious views. Like Einstein, he was never able to accept some of the fundamental concepts of the modern physics that he had helped to create. For example, he clung to the notion of causality in physical phenomena, rejecting the principles of uncertainty proposed by Heisenberg and others. He maintained his belief in God, although his descriptions of the Deity were not anthropomorphic but more akin to natural law itself.

By the time Planck retired from his position at Berlin in 1926, he had become the second most highly respected scientific figure in Europe, if not the world, behind Einstein. Four years after retirement, he was invited to become president of the Kaiser Wilhelm Society in Berlin, an institution that was then renamed the Max Planck Society in his honor. Planck's own prestige allowed him to speak out against the rise of Nazism in Germany in the 1930s, but his enemies eventually managed to have him removed from his position at the Max Planck Society in 1937. During an air raid on Berlin in 1945, Planck's home was destroyed with all of his books and papers. During the last two and a half years of his life, Planck lived with his grandniece in Göttingen, where he died at the age of 89.

See also Quantum electrodynamics (QED)