Niessen began his studies in physics at the
University of Utrecht in 1914. In 1922, he received his doctorate under L. S. Ornstein. In 1922, Niessen’s doctoral thesis, as well as
Wolfgang Pauli’s extended doctoral thesis, dealt with the
hydrogen molecule ion in the
Bohr–Sommerfeld framework. Their work is referred to as the Pauli-Niessen model. Their works helped to show the inadequacy of the old quantum mechanics, which gave physicists the impetus to explore new paths which led to the
matrix mechanics formulation of
quantum mechanics by
Werner Heisenberg and
Max Born in 1925 and the
wave mechanics formulation by
Erwin Schrödinger in 1926, which were shown to be equivalent. ==Career==