The theoretical determination of the fine structure constant is certainly the most important of the unsolved problems of modern physics. We believe that any regression to the ideas of classical physics (as, for instance, to the use of the classical field concept)cannot bring us nearer to this goal. To reach it, we shall, presumably, have to pay with further revolutionary changes of the fundamental concepts of physics with a still farther digression from the concepts of the classical theories.

The bridge between the electron and the other elementary particles is provided by the fine structure constant. ... An expanded form of the constant leads to equations that define the transformation of electromagnetic energy into electron mass/energy, ...

I introduce the subject of fine structure with a mini-calendar of events. ...Winter 1914-15. Sommerfeld computes relativistic orbits for hydrogen-like atoms. Pashcen, aware of these studies, carefully investigates fine structures, ....January 6, 1916. Sommerfeld announces his fine structure formula, citing results to be published by Paschen in support of his answer.February 1916. Einstein to Sommerfeld: "A revelation!"March 1916. Bohr to Sommerfeld: "I do not believe ever to have read anything with more joy than your beautiful work."September 1916. Paschen publishes his work, acknowledging Sommerfeld's "indefatigable efforts.