Adriaan Daniël Fokker (17 August 1887 – 24 September 1972) was a Dutch physicist and musician.Fokker was born in Buitenzorg, Dutch East Indies (now Bogor, Indonesia); he was a cousin of the aeronautical engineer Anthony Fokker. He studied mining engineering at the Delft University of Technology and physics at the University of Leiden with Hendrik Lorentz, where he earned his doctorate in 1913. He continued his studies with Albert Einstein, Ernest Rutherford and William Bragg. In his 1913 thesis, he derived the Fokker–Planck equation along with Max Planck. After his military service during World War I he returned to Leiden as Lorentz' and Ehrenfest's assistant.In 1928 Fokker succeeded Hendrik Lorentz as director of research at Teylers Museum in Haarlem.Fokker made several contributions to special relativity, and some less well-known contributions to general relativity, particularly in the area of geodetic precession, the phenomena of precession of a freely falling gyroscope in a gravitational field.Fokker began to study music theory during the Second World War, when the University of Leiden was closed; partly this was due to a desire to convince the Nazis he would be of no use to the war effort, and partly it was a response to reading the work of Christiaan Huygens on the 31 equal temperament.In 1938, Fokker – along with Dirk Coster and Otto Hahn – helped Jewish-born physicist Lise Meitner escape from Austria to the Netherlands. "Fokker and Coster both knew that university positions were virtually unavailable for foreigners. Laboratory space was not a problem, however, either in Groningen or Haarlem. 'Perhaps we can tap colleagues for regular contributions,' Coster suggested. Fokker set a goal of f.20,000, enough to support Meitner for five years, and immediately began contacting colleagues for advice and donations." They were unsuccessful in obtaining funding, but Fokker succeeded in getting official permission for Meitner to leave, although he was unable to telegraph that to her due to secrecy. She escaped barely in time to evade arrest. The year 1942 consequently marked a turning point in his life; after then he wrote many pieces in 31-equal, which are notable for using the 7th harmonic as a consonant interval (31-equal has a much better approximation of the 7th harmonic than the ubiquitous 12-equal). He also made notable contributions to music theory, such as the Fokker periodicity block.He died at Beekbergen (near Apeldoorn).