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Planck, Max (Karl Ernst Ludwig)

theory physics quantum classical

(1858–1947) German physicist: originated quantum theory, making 1900 the transition between classical and modern physics.

The son of a professor of civil law, Planck attended university at Berlin and Munich, finishing his doctorate in 1880. He then went to Kiel, becoming a professor there in 1885. A move to Berlin in 1888 followed. In 1930 he became president of the Kaiser Wilhelm Institute; he resigned in 1937 in protest at the behaviour of the Nazis towards Jewish scientists. At the end of the Second World War the Institute was moved to Göttingen and renamed the Max Planck Institute and Planck was reappointed president.

In 1900 Planck published a paper which, together with paper of 1905, initiated quantum theory. Had studied the distribution of radiation emitted by a black body as a function of frequency and temperature. Wien found a formula that would agree with experiments at high frequencies, while Rayleigh and found one for low frequencies. Planck discovered one that worked at all frequencies v, but this needed the assumption that radiation is emitted or received in energy packets (called quanta); these have an energy E = hv where h is the Planck constant (6.626×10 –34 J Hz –1 ). This assumption is counter to classical physics and its adoption began the modern age of using quantum theory in physics. The conservative Planck reluctantly recognized how revolutionary the result was, and on a New Year’s Day walk in 1900 with his small son told him how the age of classical physics had just passed away. Rapid acceptance of the idea came with its use in Einstein’s prediction of the photoelectric effect (1905) and in successful theory of the electronic structure of atoms (1913). A full quantum theory arrived in the 1920s, when Planck and others had shown how to express all the new concepts consistently. Planck was awarded the Nobel Prize in 1919 for his discovery of the energy quanta.

Planck bore the tragedies of his second son dying in the First World War, his twin daughters both dying in childbirth and finally his first son Erwin being executed for his part in the plot against Hitler of July 1944. He was always anti-Nazi, but the other founder of 20th-c physics, his friend Einstein, never forgave Planck for not showing firmer opposition; it is not clear that he could have achieved more. Planck is one of the very few scientists to be immortalized on a coin (the German DM 2 piece of 1958).

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