Wednesday, April 23, 2008

Max Planck: 150th birthday

Max Karl Ernst Ludwig Planck was born in Kiel (near Hamburg) on April 23rd, 1858, to a family of theologians and lawyers with many children. He spent some early years in Munich.

A devoted yet moderate Christian, he is the father of quantum theory. How did it happen? Well, in 1894, he was hired by electric companies to create energy-efficient light bulbs. ;-) So he had to study their spectrum.

Looking at the black body

In 1900, he was able to interpolate in between two formulae (the low frequency classical Rayleigh-Jeans law and the high frequency Wien's law) describing the radiation of a black body. A few months later, he was even able to derive the resulting formula from a funny assumption that the energy of an electromagnetic wave is not quantized but rather a multiple of E=hf.

(The factors of 2.pi are arranged in the old way because it is much easier to type it, even including this explanatory sentence.)

Textbooks often say that Planck wanted to solve the ultraviolet catastrophe, i.e. the high-energy divergences of the Rayleigh-Jeans law. While it is a theoretically natural story, it is historically misleading because Planck's goals were different. Planck has been looking for heuristic ways to justify the novel, quantum, exponential Wien's law since 1899. Finally and happily, Planck came to "an act of despair ... [he] was ready to sacrifice any of [his] previous convictions about physics." And he introduced the quanta ("a purely formal assumption"...) at the end of 1900.

Einstein gave Planck's assumption about quanta a more real meaning when he explained the photoelectric effect in 1905. Only in the 1920s, Arthur Compton convinced the people that photons were real when he observed his photon-electron scattering.

Relativity and playing dice

Incidentally, in 1905, Planck was also a referee of an article written by a Niemand from a patent office. Of course, he almost instantly understood that relativity was correct and recommended the article. Later, Einstein was younger and slightly "more quantum" than Planck but both Gentlemen found it too difficult to get rid of the classical way of thinking. Planck himself was always hoping that both probabilities as well as his own quanta would fully disappear and all of their results would be derived from some kind of classical waves.

Strange advise to a smart kid

When Planck was 16, physicist Philipp von Jolly told Planck that physics was not a good field to join because everything had been discovered. Well, this recommendantion was not made at the right time, in the right place, or addressed to the right recipient. ;-) But it is true that Planck was a "reluctant revolutionary", indeed. That agrees with the conservative traditions of his family.

Disciplines and units

Max Planck was interested in electrodynamics and thermodynamics and he has achieved quite a lot. He was also a master of dimensional analysis - that's why discovered the natural units - units independent of any culture and "valid for all times and even extraterrestrial and non-human civilizations", using his words - for all quantities, the so-called Planck units. They only started to be viewed as "very physical" very recently, with progress in quantum gravity.

Planck's house was a good place for frequent visitors such as Albert Einstein, Otto Hahn, Lise Meitner. Planck's beloved second son, Erwin, was executed in 1945 because of their failed attempt to assassinate the Führer in 1944. Planck had various other negative - but not militant - interactions with the Nazis. He died in 1947 in Göttingen, the physically powerful Western German city.

Happy birthday, Max Karl Ernst Ludwig!

See also Stefan Scherer's post.

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