Planck, Max (1858-1947)

Max Planck was the originator of quantum theory, one of the cornerstones of twentieth century physics. He studied science at Munich, but it was as Professor of Theoretical Physics in Berlin that he made his greatest contribution to physics. Planck was well versed in thermodynamics and, stimulated by the work of Kirchhoff and Helmholtz, began in the mid-1890s theoretical investigations the radiation emitted by a black body. Such a body would absorb and emit radiation of all frequencies, but classical physics was unable to account for the energy distribution of the radiation. A classical theoretical model of the emission that agreed with experiment at low frequencies, predicted that far too much energy would be emitted at high frequencies (the ultraviolet catastrophe); another model that worked at high frequencies failed at low frequencies. In 1900 Planck announced his resolution of the problem, and thus launched the 'quantum century'. Using mainly thermodynamic methods Planck was able to derive a function of temperature and frequency that agreed with the observed black-body spectrum at all frequencies, and was also consistent with Josef Stefan's discovery that the total energy emitted by a black body was proportional to the fourth power of its absolute temperature. However, in order to achieve this, Planck had to assume that the radiation (or, at least, the energy that it carried) is not emitted and absorbed continuously (as predicted by classical physics), but in 'packets' that he called quanta, with an energy that depends on the frequency of the radiation where is Planck's constant). This was an entirely non-classical assumption that seemed rather ad hoc to some and caused them to initially undervalue Planck's achievement.

In 1905, Albert Einstein used Planck's law to explain the photoelectric effect and the success of this explanation gave considerable support to the quantum theory of radiation. The law was also used by Niels Bohr in his formulation of a quantum model of the atom. These developments eventually led to the formulation of quantum mechanics in the 1920s, marking the birth of modern physics. For his seminal contribution, Planck was awarded the Nobel Prize for Physics in 1918. He was very strongly opposed to the Nazi regime, relinquishing his presidency at the Kaiser Wilhelm Institute in protest at the Nazi treatment of Jewish scientists. He suffered a personal tragedy when his son was executed for his involvement in the plot to kill Hitler.