Arnold Johannes Sommerfeld (December 5, 1868 - April 26, 1951) was a German physicist who introduced the fine-structure constant in 1919.

Arnold Sommerfeld was born in Königsberg, where he also studied mathematics. After receiving doctorate in 1891 he changed to the University of Göttingen, where he received professorship in 1896. 1897 he became professor of mathematics at the University of Clausthal-Zellerfeld, in 1900 for technical engineering at the University of Aachen, where he developed the theory of Hydrodynamic lubrication .

In 1906 he finally became professor of physics at the University of Munich. There he came in contact with the special theory of relativity by Albert Einstein, which was not yet widely accepted at that time. His mathematical contributions to the theory helped its accepting among the sceptic physicists. Later he became one of the founders of the quantum mechanics, many of his students became famous of their own - most notably Werner Heisenberg and Wolfgang Pauli.

In 1927 Sommerfeld applied Fermi-Dirac statistics to the Drude model of electrons in metals. The new theory solved many of the problems predicting thermal properties that model had.

Sommerfeld died in 1951 in Munich from injuries after a traffic accident.