He studied theoretical physics with Hendrik Antoon Lorentz at University of Leiden. He subsequently carried out Ph.D. research under the supervision of Lorentz, concerning an application of the statistical mechanics of Gibbs to molecular problems. In 1914, he was appointed professor of physics, as successor of Peter Debye, at University of Utrecht. Among his doctoral students was Jan Frederik Schouten. In 1922, he became director of Physical Laboratory and extended his research interests to experimental subjects. His precision measurements concerning intensities of spectral lines brought Physical Laboratory in the international limelight. He is also remembered for the Ornstein-Zernike theory concerning correlation functions, and the Ornstein-Uhlenbeck process, a stochastic process. Together with Gilles Holst, director of Philips Research Laboratories, he was the driving force behind establishing the Dutch Physical Society in 1921. From 1939 until November 1940 he was Chairman of this Society. From 1918 until 1922 Ornstein was Chairman of the Dutch Zionist Society. In 1929, he became member of the Royal Netherlands Academy of Arts and Sciences. Immediately after the involvement of the Netherlands in the World War II, a friend from the United States of America, the astronomerPeter van de Kamp, offered to bring Ornstein and his family to America. However, Ornstein did not accept this offer, since, as he put it, he would not leave his laboratory in Utrecht. Owing to his Jewish heritage, Ornstein was summarily dismissed from the University in September, 1940. He was even barred from entering his own laboratory. In November, 1940, he was officially dismissed from the University. On November 29, 1940, Ornstein willingly withdrew his membership of the Dutch Physical Society. During this period he increasingly distanced himself from public life, to the degree that he no longer wished to receive guests at home. Ornstein died on May 20, 1941, six months after being barred from University. One of the five buildings of Department of Physics of University of Utrecht, the Ornstein Laboratorium, is named in his honor.
Publications
Toepassing der statistische mechanica van Gibbs op moleculair-theoretische vraagstukken, Phd Thesis 26 March 1908
Problemen der kinetische theorie van de stof, 1915
Strahlungsgesetz und Intensität von Mehrfachlinien, 1924
