Max Planck was a German theoretical physicist, considered to be the initial founder of quantum theory, and one of the most important physicists of the 20th Century. Around the turn of the century, he realized that light and other electromagnetic waves were emitted in discrete packets of energy that he called "quanta" - "quantum" in the singlular - which could only take on certain discrete values (multiples of a certain constant, which now bears the name the “Planck constant”). This is generally regarded as the first essential stepping stone in the development of quantum theory, which has revolutionized the way we see and understand the sub-atomic world.
Karl Ernst Ludwig Marx Planck, better known as Max, was born in Kiel in Holstein, northern Germany on 23 April 1858. His family was traditional and intellectual (his father was a law professor and his grandfather and great-grandfather had been theology professors). In 1867, the family moved to Munich, where Planck attended the Ludwig Maximilians gymnasium school. There, he came under the tutelage of Hermann Müller, who taught him astronomy and mechanics as well as mathematics, and awoke Planck’s early interest in physics.
Although a talented musician (he sang, played the piano, organ and cello, and composed songs and even operas), he chose to study physics at the University of Munich in 1874, soon transferring to theoretical physics, before going on to Berlin for a year of further study in 1877. Having completed his habilitation thesis on heat theory in 1880, Planck became an unpaid private lecturer in Munich, waiting until he was offered an academic position. In April 1885, the University of Kiel appointed him as an associate professor of theoretical physics, and he continued to pursue work on heat theory and on Rudolph Clausius’ ideas about entropy and its application in physical chemistry.
In 1889, Planck moved to the University of Berlin, becoming a full professor in 1892. He had married Marie Merck in 1887, and they went on to have four children, Karl (1888), the twins Emma and Grete (1889) and Erwin (1893), of whom only Erwin was to survive past the First World War. The Planck home in Berlin became a social and cultural center for academics, and many well-known scientists, including Albert Einstein, Otto Hahn and Lise Meitner, were frequent visitors.
In 1894, Planck turned his attention to the problem of black body radiation, the observation that the greatest amount of energy being radiated from a “black body” (or any perfect absorber) falls near the middle of the electromagnetic spectrum, rather than in the ultraviolet region as classical theory would suggest. In particular, he investigated how the intensity of the electromagnetic radiation emitted by a black body depends on the frequency of the radiation (e.g. the color of the light) and the temperature of the body. After some initial frustrations, he derived the first version of his black body radiation law in 1900. However, although it described the experimentally observed black body spectrum well, he realized that it was not perfect.
The previous year, though, in 1899, he had noted that the energy of photons could only take on certain discrete values which were always a full integer multiple of a certain constant, which is now known as the “Planck constant”. Thus, light and other waves were emitted in discrete packets of energy that he called "quanta". Defining the Planck constant enabled him to go on to define a new universal set of physical units or Planck units (such as the Planck length, the Planck time, the Planck temperature, etc), all based on five fundamental physical constants: the speed of light in a vacuum, the gravitational constant, the Coulomb force constant, the Boltzmann constant and his own Planck constant.
Later in 1900, then, he revised his black body theory to incorporate the supposition that electromagnetic energy could be emitted only in “quantized” form, so that the energy could only be a multiple of an elementary unit E = hv (where h is the Planck constant, previously introduced by him in 1899, and v is the frequency of the radiation). Although quantization was a purely formal assumption in Planck’s work at this time and he never fully understood its radical implications (which had to await Albert Einstein’s interpretations in 1905), its discovery has come to be regarded as effectively the birth of quantum physics, and the greatest intellectual accomplishment of Planck's career. It was in recognition of this accomplishment that he was awarded the Nobel Prize in Physics in 1918.
Planck was among the few who immediately recognized the significance of Einstein’s 1905 Special Theory of Relativity, and he used his influence in the world of theoretical physics (he was president of the newly formed German Physical Society from 1905 to 1909) to ensure that the theory was soon widely accepted in Germany, as well as making his own contributions to extending the theory. After Planck had been appointed dean of Berlin University, it became possible for him to call Einstein to Berlin and to establish a new professorship specifically for him in 1914, and the two scientists soon became close friends and met frequently to play music together.
Planck's wife Marie died in 1909, possibly from tuberculosis, and, in 1911, he married his second wife, Marga von Hoesslin, who bore him a third son, Hermann, the same year. By the time of the German annexation and the First World War in 1914 (which Planck initially welcomed, but later argued against), he was effectively the highest authority of German physics, as one of the four permanent presidents of the Prussian Academy of Sciences, and a leader in the influential umbrella body, the Kaiser Wilhelm Society. By the end of the 1920s, Niels Bohr, Werner Heisenberg and Wolfgang Pauli had worked out the so-called "Copenhagen interpretation" of quantum mechanics, and the quantum theory which Planck’s work had triggered became ever more established, even if Planck himself (like Einstein) was never quite comfortable with some of its philosophical implications.
When the Nazis seized power in 1933, Planck was an old man of 74, and he generally avoided open conflict with the Nazi regime, although he did organize a somewhat provocative official commemorative meeting after the death in exile of fellow physicist Fritz Haber. He also succeeded in secretly enabling a number of Jewish scientists to continue working in institutes of the Kaiser Wilhelm Society for several years.
The “Deutsche Physik” movement attacked Planck, Arnold Sommerfeld and Werner Heisenberg among others for continuing to teach the theories of Einstein, calling them "white Jews". When his term as president of the Kaiser Wilhelm Society ended in 1936, the Nazi government pressured him to refrain from seeking another term. At the end of 1938, the Prussian Academy of Sciences lost its remaining independence and was taken over by Nazis, and Planck protested by resigning his presidency. He steadfastly refused to join the Nazi party, despite being under significant political pressure to do so.
Allied bombing campaigns against Berlin during the Second World War forced Planck and his wife to leave the city temporarily to live in the countryside, and his house in Berlin was completely destroyed by an air raid in 1944. He continued to travel frequently, giving numerous public lectures, including talks on Religion and Science (he was a devoted and persistent adherent of Christianity all his life), and at the advanced age of 85 he was still sufficiently fit to climb 3,000-meter peaks in the Alps.
At the end of the Second World War (during which his youngest son Erwin was implicated in the attempt on Hitler's life in 1944 and hanged), Planck, his second wife and his remaining son moved to Göttingen. He died there on 4 October 1947, aged 89, from the consequences of a fall and several strokes.
Max Planck Books
See the additional sources and recommended reading list below, or check the physics books page for a full list.