Einstein, Albert

Autograph letter signed ("A. Einstein"), in German, 1 page (11 3/8 x 8 3/4 in.; 290 x 225 mm), [Berlin], 6 June 1926, to George Yuri Rainich at Johns Hopkins Unversity; splitting at center horizontal fold, wear and fraying to top margin and right margin; with typed envelope addressed to Rainich, apparently from an earlier communication.

From one physicist to another. A leading mathematical physicist of the early twentieth century, George Yuri Rainich studied mathematics in Odessa and Munich, eventually obtaining his doctorate in 1913 from the University of Kazan. In 1922, he emigrated to the United States and spent three years at Johns Hopkins University before joining the faculty of the University of Michigan where he remained until his retirement in 1956.

Rainich's research centered around general relativity and a unified field theory. In 1924, while at Johns Hopkins University, Rainich found a set of equivalent conditions for a Lorentzian manifold to admit an interpretation as an exact non-null electrovacuum solution in general relativity now known as the Rainich conditions. No doubt, Einstein took great interest in Rainich's work. Herewith Einstein discusses an aspect of electron theory and explains that any theory which postulates a solution representing two electrons at rest is unacceptable. He admits he was mistaken in his earlier views and states it would be an enormous step forward to be able to prove through differential equations the need for electrons to be identical. He concludes his latter praising Rainich for his  tenacious research on the problem which he finds far more valuable than the search for instant success.