Stern, Otto (1888-1969)

German physicist who demonstrated by means of the Stern-Gerlach apparatus that elementary particles have wavelike properties as well as the properties of matter that had been demonstrated. He was awarded the Nobel Prize for Physics in 1943 for his development of the molecular-ray method of investigating elementary particles, and for the discovery of the magnetic moment of the proton.

Stern was born in Sohrau, Upper Silesia (now Zory in Poland), and studied at a number of German universities. He then worked with Albert Einstein in Prague and Zürich. In 1923 Stern became professor at Hamburg and director of the Institute of Physical Chemistry. With the rise of the Nazis, he emigrated to the USA in 1933 and set up a department for the study of molecular beams at the Carnegie Technical Institute in Pittsburgh.

In 1920 Stern and Walther Gerlach (1899-1979) carried out their experiment, which consisted of passing a narrow beam of silver atoms through a strong magnetic field. Classical theory predicted that this field would cause the beam to broaden, but quantum theory predicted that the beam would split into two separate beams. The result, showing a split beam, was the first clear evidence for space quantization - the phenomenon that, in a magnetic field, certain atoms behave like tiny magnets which can only take up particular orientations with respect to the direction of the field. Stern went on to improve this molecular-beam technique and in 1931 was able to detect the wave nature of particles in the beams.

In 1933, Stern measured the magnetic moment of the proton and the deuteron, and demonstrated that the proton's magnetic moment was 2.5 times greater than expected.