Adriaan Fokker Meaning and Mystery of Einstein’s Theory

  • Per Chr. Hemmer
  • Ola Kai Ledang


Adriaan Daniel Fokker (1887-1972) was born in Buitenzorg (now Bogor) on Java in Indonesia, then Dutch East India, where his father was a successful businessman. Fokker was educated in the Netherlands as a mining engineer from Delft University of Technology, and as a physicist from the Leiden University. In Leiden he earned his doctorate in 1913. In physics Fokker’s name is best known through the Fokker-Planck equation. The Fokker-Planck equation is a partial differential equation of second order, which describes the time evolution of the probability distribution of a physical variable subjected to a stochastic force, in addition to friction and possibly other driving forces. The prototypical example is Brownian motion. The Fokker-Planck equation was contained in Fokker’s thesis, and was independently derived by Max Planck.

During 1913-14 Fokker worked in Zürich as Albert Einstein’s assistant, and published an article in general relativity with Einstein as coauthor. He kept a lifelong interest in relativity, as witnessed by his 1963 article in the proceedings of the DKNVS. His best known contribution in this field is a determination of the change of direction when a gyroscope (like the Earth) moves in a closed orbit in a gravitational field.

In 1923 Fokker was appointed professor of physics at Delft University of Technology. However, after five years he preferred to succeed Lorentz as curator at the Teyler Museum in Haarlem, a position that was combined with a special professorship at Leiden University.

During reduction of academic activity under the German occupation, Fokker turned to music theory, tuning practice and microtonality, in particular issues related to just intonation and the compromises embodied in equal temperament. His chief interests were the theories of Euler and Huygens. During the war, he constructed and had built a 12-key pipe organ with mean-tone tuning according to the principles of Euler’s Generibus musicis. Later on, he had a 31-key organ built that realised an approximate pure tuned scale, based on Huygens’s microtonal scale of 5th-tones. Concerts involving the Fokker organ were given regularly since 1951. An electronic version (called Archiphone) with essentially the same keyboard layout, was introduced in 1970. Now two Archiphones are at the Huygens-Fokker Foundation (Haarlem), one in Australia and one in USA.

Through his sharp and discerning contributions to musicology, Fokker has explored and demonstrated some interesting perspectives and possibilities of microtonality, of major interest to composers and performers. The complexity of his writings is perhaps the main reason why he is relatively little known to musicologists.