After the death of his wife in 1963, he returned to the University of Michigan at Ann Arbor and organized there a seminar on general relativity theory for physicists and mathematicians.

Rainich's research centered on general relativity and early work toward a unified field theory.

According to some sources, Peter Gabriel Bergmann brought Rainich's suggestion that algebraic topology (and knot theory in particular) should play a role in physics to the attention of John Archibald Wheeler, which shortly led to the Ph.D. thesis of Charles W. Misner.

According to the Editor of The American Mathematical Monthly,[1][2] Rainich is the inventor of the Rabinowitsch trick, a clever argument to deduce the Hilbert Nullstellensatz from an easier special case.

[5][6] Rainich was an Invited Speaker at the ICM in 1928 at Bologna (with talk On a Spacetime possessing the symmetry properties of radiation) and in 1932 at Zürich.